Comparative study between Mobile WiMAX (IEEE802.16e based) and 3GPP LTE

Transcription

1 Comparative study between Mobile WiMAX (IEEE802.16e based) and 3GPP LTE Karim Ahmed Samy Banawan, Mohammed Salaheldin Abdullah, and Mohamed Abdel Ghani Mohammed El-Gharabawy. Abstract: In this paper we present a comparative study between Mobile WiMAX (IEEE802.16e based) and 3GPP LTE, we present the key technologies that are utilized in both systems, then PHY layers are presented,besides Network Architectures. Our conclusions and result are also introduced. Key terms: Mobile WiMAX, IEEE802.16e, LTE, PHY layer, OFDMA,SC- FDMA,MIMO, system architecture. I.INTRODUCTION The demand for high data rate wireless multi-media applications has increased significantly in the past few years. The wireless user s pressure towards faster communications, no matter whether mobile, nomadic, or fixed positioned, without extra cost is nowadays a reality. Finding an optimal solution for this dilemma is a challenge, not only for manufacturers but also for network operators. The recent strategy followed within ETSI 3GPP LTE and the WiMAX Forum was a new and evolutionary concept, especially for mobile applications. Both have adopted a new PHY layer multi-carrier transmission with a MIMO scheme, a promising combination offering a high data rate at low cost. The 3GPP LTE is acronym for long term evolution of UMTS.The multiple access scheme in LTE downlink uses Orthogonal Frequency Division Multiple Access (OFDMA) and uplink uses Single Carrier Frequency Division Multiple Access (SC-FDMA). These multiple access solutions provide orthogonality between the users, reducing the interference and improving the network capacity. The resource allocation in the frequency domain takes place with a resolution of 180 khz resource blocks both in uplink and in downlink. The frequency dimension in the packet scheduling is one reason for the high LTE capacity. The uplink user specific allocation is continuous to enable single carrier transmission while the downlink can use resource blocks freely from different parts of the spectrum. The uplink single carrier solution is also designed to allow efficient terminal power amplifier design, which is relevant for the terminal battery life. LTE solution enables spectrum flexibility where the transmission bandwidth can be selected between 1.4 MHz and 20 MHz depending on the available spectrum. The 20 MHz bandwidth can provide up to 150 Mbps downlink user data rate with 2 2 MIMO, and 300 Mbps with 4 4 MIMO. The uplink peak data rate is 75 Mbps. The high network capacity also requires efficient network architecture in addition to the advanced radio features. The target in 3GPP Release 8 is to improve the network scalability for traffic increase and to minimize the end-to-end latency by reducing the number of network elements. All radio protocols, mobility management, header compression and all packet

2 retransmissions are located in the base stations called enodeb. enodeb includes all those algorithms that are located in Radio Network Controller (RNC) in 3GPP Release 6 architecture. Also the core network is streamlined by separating the user and the control planes. The Mobility Management Entity (MME) is just the control plane element while the user plane bypasses MME directly to System Architecture Evolution (SAE) Gateway (GW). The architecture evolution is This Release 8 core network is also often referred to as Evolved Packet Core (EPC) while for the whole system the term Evolved Packet System (EPS) can also be used. WiMAX is the commonly used name for broadband wireless access based on the IEEE family of standards.wimax stands for worldwide interoperability for microwave access. The WiMAX forum is an industry-led, nonprofit corporation formed to promote and certify compatibility and interoperability of broadband wireless products. IEEE is an IEEE Standard for Wireless MANs (WMANs). The most recent addition to the WiMAX family of standards is e, which is also called Mobile WiMAX. The IEEE standards specify the physical layer (PHY) and the Medium Access Layer (MAC), with no definition of higher layers. For IEEE , those are addressed in the WiMAX Forum Network Working Group. There is a range of options specified in IEEE , making the standards much more fragmented than what is seen in 3GPP and 3GPP2 standards. The standard defines four different physical layers, of which two are certified by the WiMAX forum: OFDM-PHY: based on an FFT size of 256 and aimed at fixed networks. OFDMA-PHY (scalable): based on an FFT size from 128 to 2048 for e. In addition to the multiple physical layers, the standards support a range of options, including: TDD, FDD, and half-duplex FDD (H- FDD) operation, TDM access with variable frame size (2 20 ms),ofdm with a configurable cyclic prefix length, A wide range of bandwidths supported ( MHz), Multiple modulation and coding schemes: QPSK, 16QAM, and 64QAM combined with convolutional codes, convolutional Turbo codes, block Turbo codes, and LDPC (Low-Density Parity Check) codes, Hybrid ARQ and Adaptive antenna system (AAS) and MIMO. There is also a range of Radio Resource Management (RRM) options, MAC features and enhancements in the standards. The WiMAX forum defines system profiles that reduce all the optional features to a smaller set to allow interoperability among different vendors. This is done through an industry selection of features for MAC, PHY, and RF from specifications and forms the basis for testing conformance and interoperability. Products certified by the WiMAX forum adhere to a Certification Profile that is based on a combination of band of operation, duplexing option and bandwidth. The intended applications with the original standard were fixed access and backhaul, mainly for line-of-sight operation. The addition of a physical layer for non-line-of-sight applications in IEEE and support for mobility in IEEE e opens up the standard for nomadic and mobile use. In addition, provisions for multicast and broadcast services (MBS) are also included. This makes the standard more similar to the evolved 3G

3 standards, but coming from a completely different direction. The IEEE standards such as are driven by the datacom industry as Layer 1 and 2 standards, starting with line-of-sight use for limited mobility, targeting best-effort data applications and now moving to higher mobility and encompassing also other applications such as conversational services. The evolved 3G standards are driven by the telecom industry, targeting non-line-of-sight use and mobility from the beginning, optimized end-to-end standards for voice and later also data services, now moving to broader data applications including best-effort services. II. KEY TECHNOLOGIES 1- Common key technologies: (a) Multiple Antenna support Multi-antenna techniques can be seen as a joint name for a set of techniques with the common theme that they rely on the use of multiple antennas at the receiver and/or the transmitter, in combination with more or less advanced signal processing. Multi-antenna techniques can be used to achieve improved system performance, including improved system capacity (more users per cell) and improved coverage (possibility for larger cells), as well as improved service provisioning, for example, higher per-user data rates.both systems support Multiple antenna systems. For the LTE Multiple antenna systems are integral part of its specifications. Multiple antennas at the transmitter and/or the receiver can be used to provide additional diversity against fading on the radio channel. In this case, the channels experienced by the different antennas should have low mutual correlation, implying the need for a sufficiently large inter-antenna distance (spatial diversity). Multiple antennas at the transmitter and/or the receiver can be used to shape the overall antenna beam in a certain way, for example, to maximize the overall antenna gain in the direction of the target receiver/transmitter or to suppress specific dominant interfering signals. Such beam-forming can be based either on high or low fading correlation between the antennas. The simultaneous availability of multiple antennas at the transmitter and the receiver can be used to create what can be seen as multiple parallel communication channels over the radio interface. This provides the possibility for very high bandwidth utilization without a corresponding reduction in power efficiency or, in other words, the possibility for very high data rates within a limited bandwidth without an un-proportionally large degradation in terms of coverage. Herein we will refer to this as spatial multiplexing. It is often also referred to as MIMO (Multi-Input Multi- output) antenna processing. (b) OFDMA transmission scheme: Both systems use OFDMA as multiple access scheme, for Mobile WiMAX it is the multiple access scheme for both UL and DL and for LTE it is the multiple access scheme in the DL only. The OFDM is used to mitigate the multipath fading which will result in delay spread and frequency selective

4 fading instead of using complex equalizers or high complexity rake receivers. OFDM systems break the available bandwidth into many narrower sub-carriers and transmit the data in parallel streams; each OFDM symbol is preceded by a cyclic prefix (CP), which is used to effectively eliminate ISI. In practice, the OFDM signal can be generated using IFFT with a CP of sufficient duration, preceding symbols do not spill over into the FFT period, Also, Once the channel impulse response is determined (by periodic transmission of known reference signals), distortion can be corrected by applying an amplitude and phase shift on a subcarrier-by-subcarrier basis. Problems of OFDM are: susceptibility to carrier frequency errors and a large signal peak-toaverage power ratio (PAPR). OFDMA is an excellent choice of transmission scheme for the 3GPP LTE downlink and Mobile WiMAX which allows the access of multiple users on the available bandwidth. Each user is assigned a specific timefrequency resource. (C) Hybrid ARQ with soft combining Fast hybrid ARQ with soft combining is used in LTE and WiMAX to allow the terminal to rapidly request re-transmissions of erroneously received transport blocks and to provide a tool for implicit rate adaptation. The underlying protocol is also similar to the one used for HSPA multiple parallel stop-and-wait hybrid ARQ processes. Retransmissions can be rapidly requested after each packet transmission, thereby minimizing the impact on end-user performance from erroneously received packets. Incremental redundancy is used as the soft combining strategy and the receiver buffers the soft bits to be able to do soft combining between transmission attempts. 2-Key technologies for 3GPP LTE system (a) Spectrum flexibility A high degree of spectrum flexibility is considered one of the main characteristics of the LTE radio access. The aim of this spectrum flexibility is to allow for the deployment of the LTE radio access in diverse spectrum with different characteristics, including different duplex arrangements, different sizes of the available spectrum and different frequency-bands-of-operation. (i) Flexibility in duplex arrangement One important part of the LTE requirements in terms of spectrum flexibility is the possibility to deploy LTE-based radio access in both paired and unpaired spectrum. Therefore, LTE supports both frequencydivision-based and time-division-based duplex arrangements. Frequency Division Duplex (FDD) as illustrated to the left in Fig. 1 implies that downlink and uplink transmission take place in different, sufficiently separated, frequency bands. Time Division Duplex (TDD), as illustrated to the right in Fig. 1, implies that downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum. Support for both paired and unpaired spectrum is part of the 3GPP specifications already from Release 99 through the use of FDD-based WCDMA/HSPA radio access in paired allocations and TDD-based TD- CDMA/TD-SCDMA radio access in

5 unpaired allocations. However, this is achieved by means of relatively different radio-access technologies and, as a consequence, terminals capable of both FDD and TDD operations are fairly uncommon. LTE, on the other hand, supports both FDD and TDD within a single radio-access technology, leading to a minimum of deviation between FDD and TDD for LTE-based radio access. LTE also supports half-duplex FDD at the terminal (illustrated in the middle of Fig. 1). In half-duplex FDD, transmission and reception at a specific terminal are separated in both frequency and time. The base station still uses full duplex as it simultaneously may schedule different terminals in uplink and downlink; this is similar to, for example, GSM operation. The main benefit with halfduplex FDD is the reduced terminal complexity as no duplex filter is needed in the terminal, which is especially beneficial in case of multiband terminals which otherwise would need multiple sets of duplex filters. Fig. 1. Frequency- and time-division duplex. (ii) Flexibility in frequency-band-ofoperation LTE is envisioned to be deployed on a per-need basis when and where spectrum can be made available, either by the assignment of new spectrum for mobile communication, such as the 2.6 and 3.5 GHz band, or by the migration to LTE of spectrum currently used for other mobilecommunication technologies, such as GSM or cdma2000 systems, or even non-mobile radio technologies such as in current broadcast spectrum. As a consequence, it is required that the LTE radio access should be able to operate in a wide range of frequency bands, from as low as 450 MHz band up to, at least, 3.5 GHz. (iii) Bandwidth flexibility Related to the possibility to deploy the LTE radio access in different frequency bands is the possibility of being able to operate LTE with different transmission bandwidths on both downlink and uplink. The main reason for this is that the amount of spectrum being available for LTE may vary significantly between different frequency bands and also depending on the exact situation of the operator. Furthermore, the possibility to operate in different spectrum allocations gives the possibility for gradual migration of spectrum from other radio access technologies to LTE.

6 LTE supports operation in a wide range of spectrum allocations, achieved by a flexible transmission bandwidth being part of the LTE specifications. To efficiently support very high data rates when spectrum is available, a wide transmission bandwidth is necessary. However, a sufficiently large amount of spectrum may not always be available, either due to the band-of-operation or due to a gradual migration from another radioaccess technology, in which case LTE can be operated with a more narrow transmission bandwidth. Obviously, in such cases, the maximum achievable data rates will be reduced correspondingly. The LTE physical-layer specifications are bandwidth-agnostic and do not make any particular assumption on the supported transmission bandwidths beyond a minimum value. The basic radioaccess specification including the physical-layer and protocol specifications, allows for any transmission bandwidth ranging from roughly 1 MHz up to around 20 MHz. At the same time, at an initially stage, radio-frequency requirements are only specified for a limited subset of transmission bandwidth, corresponding to what is predicted to be relevant spectrum-allocation sizes and relevant migration scenarios. Thus, in practice LTE radio access supports a limited set of transmission bandwidths, but additional transmission bandwidths can easily be supported by updating only the RF specifications. (b) Channel-dependent scheduling and rate adaptation At the core of the LTE transmission scheme is the use of shared-channel transmission, with the overall time-frequency resource dynamically shared between users. This is similar to the approach taken in HSDPA, although the realization of the shared resource differs between the two time and frequency in case of LTE vs. time and channelization codes in case of HSDPA. The use of sharedchannel transmission is well matched to the rapidly varying resource requirements posed by packet data and also enables several of the other key technologies used by LTE. The scheduler controls, for each time instant, to which users the shared resources should be assigned. The scheduler also determines the data rate to be used for each link, that is, rate adaptation can be seen as a part of the scheduler. The scheduler is thus a key element and to a large extent determines the overall downlink system performance, especially in a highly loaded network. Both downlink and uplink transmissions are subject to tight scheduling. A substantial gain in system capacity can be achieved if the channel conditions are taken into account in the scheduling decision, socalled channel-dependent scheduling. This is exploited already in HSPA, where the downlink scheduler transmits to a user when its channel conditions are advantageous to maximize the data rate, and is, to some extent, also possible for the Enhanced Uplink. However, LTE has, in addition to the time domain, also access to the frequency domain, due to the use of OFDM in the downlink and DFTS- OFDM in the uplink. Therefore, the scheduler can, for each frequency region, select the user with the best channel conditions. In other words, scheduling in LTE can take channel variations into account not only in the time domain, as HSPA, but also in the frequency domain. This is illustrated in Fig. 2.

7 The possibility for channeldependent scheduling in the frequency domain is particularly useful at low terminal speeds, in other words when the channel is varying slowly with time. Channel-dependent scheduling relies on channel-quality variations between users to obtain a gain in system capacity. For delay-sensitive services, a time-domain only scheduler may be forced to schedule a particular user, despite the channel quality not being at its peak. In such situations, exploiting channel-quality variations also in the frequency domain will help improving the overall performance of the system. For LTE, scheduling decisions can be taken as often as once every 1 ms and the granularity in the frequency domain is 180 khz. This allows for relatively fast channel variations to be tracked and utilized by the scheduler. Fig. 2. Downlink channel-dependent scheduling in time and frequency domains. (i) Downlink scheduling To support downlink scheduling, a terminal may provide the network with channel-status reports indicating the instantaneous downlink channel quality in both the time and frequency domain. The channel status can, for example, be obtained by measuring on a reference signal transmitted on the downlink and used also for demodulation purposes. Based on the channel-status report, the downlink scheduler can assign resources for downlink transmission to different mobile terminals, taking the channel quality into account in the

8 scheduling decision. In principle, a scheduled terminal can be assigned an arbitrary combination of 180 khz wide resource blocks in each 1 ms scheduling interval. (ii) Uplink scheduling The LTE uplink is based on orthogonal separation of different uplink transmissions and it is the task of the uplink scheduler to assign resources in both time and frequency domain (combined TDMA/FDMA) to different mobile terminals. Scheduling decisions, taken once per 1 ms, control what set of mobile terminals are allowed to transmit within a cell during a given time interval and, for each terminal, on what frequency resources the transmission is to take place and what uplink data rate (transport format) to use. Channel conditions can also be taken into account in the uplink scheduling process, similar to the downlink scheduling. However, obtaining information about the uplink channel conditions is a non trivial task. Therefore, different means to obtain uplink diversity are important as a complement in situations where uplink channel-dependent scheduling is not suitable. (c) Inter-cell interference coordination LTE provides orthogonality between users within a cell in both uplink and downlink, that is, at least in principle, there is no interference between transmissions within one cell (no intra-cell interference). Hence, LTE performance in terms of spectrum efficiency and available data rates is, relatively speaking, more limited by interference from other cells (inter-cell interference) compared to WCDMA/HSPA. Means to reduce or control the inter-cell interference can therefore, potentially, provide substantial benefits to LTE performance, especially in terms of the service (data rates, etc.) that can be provided to users at the cell edge. Inter-cell interference coordination is a scheduling strategy in which the cell-edge data rates are increased by taking inter-cell interference into account. Basically, inter-cell interference coordination implies certain (frequency-domain) restrictions to the uplink and downlink schedulers in order to control the intercell interference. By restricting the transmission power of parts of the spectrum in one cell, the interference seen in the neighboring cells in this part of the spectrum will be reduced. This part of the spectrum can then be used to provide higher data rates for users in the neighboring cell. In essence, the frequency reuse factor is different in different parts of the cell (Fig. 3).

9 Fig. 3 Example of inter-cell interference coordination. Inter-cell interference coordination is mainly a scheduling strategy, taking the situation in neighboring cells into account. Thus, inter-cell interference coordination is to a large extent an implementation issue and hardly visible in the specifications. This also implies that interference coordination can be applied to only a selected set of cells, depending on the requirements set by a particular deployment. To aid the implementation of various inter-cell interference coordination schemes, LTE supports exchange of interference indicators between base stations. (d) Multicast and broadcast support Multi-cell broadcast implies transmission of the same information from multiple cells as described. By exploiting this at the terminal, effectively using signal power from multiple cell sites at the detection, a substantial improvement in coverage (or higher broadcast data rates) can be achieved. LTE takes this one step further to provide highly efficient multi-cell broadcast. By transmitting not only identical signals from multiple cell sites (with identical coding and modulation), but also synchronize the transmission timing between the cells, the signal at the mobile terminal will appear exactly as a signal transmitted from a single cell site and subject to multi-path propagation. Due to the OFDM robustness to multi-path propagation, such multi-cell transmission, also referred to as Multicast Broadcast Single-Frequency Network (MBSFN) transmission, will then not only improve the received signal strength, but also eliminate the inter-cell interference. Thus, with OFDM, multi-cell broadcast/multicast throughput may eventually be limited by noise only and can then, in case of small cells, reach extremely high values. (e) New transmission scheme for the uplink: SC-FDMA(DFTS-OFDM) While many of the requirements for the design of the LTE uplink physical layer and multiple-access scheme are similar to those of the downlink, the uplink also poses some unique challenges. Some of the desirable attributes for the LTE uplink include: Orthogonal uplink transmission by different User Equipment (UEs), to minimize intra-cell interference and maximize capacity. Flexibility to support a wide range of data rates, and to enable data rate to be adapted to the SINR (Signal-to-Interference plus Noise Ratio). Sufficiently low Peak-to-Average Power Ratio (PAPR) of the transmitted waveform, to avoid excessive cost, size and power consumption of the UE Power Amplifier (PA). Ability to exploit the frequency diversity afforded by the wideband channel (up to 20 MHz), even when transmitting at low data rates. Support for frequency-selective scheduling. Support for advanced multipleantenna techniques, to exploit spatial diversity and enhance uplink capacity. The multiple-access scheme selected for the LTE uplink so as to fulfill these principle characteristics is Single- Carrier Frequency Division Multiple Access (SC-FDMA). A major advantage of SC- FDMA over the Direct-Sequence Code

10 Division Multiple Access (DS-CDMA) scheme used in UMTS is that it achieves intra-cell orthogonality even in frequency-selective channels. SC- FDMA avoids the high level of intracell interference associated with DS- CDMA which significantly reduces system capacity and limits the use of adaptive modulation. A codemultiplexed uplink also suffers the drawback of an increased PAPR if multi-code transmission is used from a single UE. The use of OFDMA for the LTE uplink would have been attractive due to the possibility for full uplinkdownlink commonality. In principle, an OFDMA scheme similar to the LTE downlink could satisfy all the uplink design criteria listed above, except for low PAPR. SC-FDMA combines the desirable characteristics of OFDM with the low CM/PAPR of singlecarrier transmission schemes. Like OFDM, SC-FDMA divides the transmission bandwidth into multiple parallel subcarriers, with the orthogonality between the subcarriers being maintained in frequency-selective channels by the use of a Cyclic Prefix (CP) or guard period. The use of a CP prevents Inter- Symbol Interference (ISI) between SC- FDMA information blocks. It transforms the linear convolution of the multipath channel into a circular convolution, enabling the receiver to equalize the channel simply by scaling each subcarrier by a complex gain factor. However, unlike OFDM, where the data symbols directly modulate each subcarrier independently (such that the amplitude of each subcarrier at a given time instant is set by the constellation points of the digital modulation scheme), in SC-FDMA the signal modulated onto a given subcarrier is a linear combination of all the data symbols transmitted at the same time instant. Thus in each symbol period, all the transmitted subcarriers of an SC-FDMA signal carry a component of each modulated data symbol. This gives SC-FDMA its crucial single-carrier property, which results in the PAPR being significantly lower than pure multicarrier transmission schemes such as OFDM. (f) SC-FDMA Principles (i) SC-FDMA Transmission Structure An SC-FDMA signal can, in theory, be generated in either the timedomain or the frequency domain. Although the two techniques are duals and functionally equivalent, in practice, the time-domain generation is less bandwidth-efficient due to timedomain filtering and associated requirements for filter ramp-up and ramp-down times. (ii) T-Domain Signal Generation Time-domain generation of an SC-FDMA signal is shown in Fig. 4. It is similar to conventional singlecarrier transmission. Fig. 4. SC-FDMA time-domain transmit processing.

11 The input bit stream is mapped into a single-carrier stream of QPSK or QAM symbols, which are grouped into symbol-blocks of length M. This may be followed by an optional repetition stage, in which each block is repeated L times, and a user-specific frequency shift, by which each user s transmission may be translated to a particular part of the available bandwidth. A CP is then inserted. After filtering (e.g. with a root-raised cosine pulse-shaping filter), the resulting signal is transmitted. The repetition of the symbol blocks results in the spectrum of the transmitted signal only being non-zero at certain subcarrier frequencies (namely every L th subcarrier in this example) as shown in Fig. 5. Fig. 5. Distributed transmission with equal-spacing between occupied subcarriers. Thus, the transmitted signal spectrum in this case is similar to what would be obtained if data symbols were only modulated on every L th subcarrier of an OFDM signal. Since such a signal occupies only one in every L subcarriers, the transmission is said to be distributed and is one way of providing a frequency-diversity gain. By varying the block length M and the repetition factor L, under the constraint that the total number of possible occupied subcarriers in the bandwidth is constant (ML = constant), a wide range of data rates can be supported. When no symbol-block repetition is performed (L = 1), the signal occupies consecutive subcarriers and the transmission is said to be localized. Localized transmissions are beneficial for supporting frequency-selective scheduling, for example when the enodeb has knowledge of the uplink channel conditions (e.g. as a result of channel sounding), or for inter-cell interference coordination. Localized transmission may also provide frequency diversity if the set of consecutive subcarriers is hopped in the frequency domain, especially if the time interval between hops is shorter than the duration of a block of channel-coded data. Different users transmissions, using different repetition factors or bandwidths, remain orthogonal on the uplink when the following conditions are met: The users occupy different sets of subcarriers. This may in general be

12 accomplished either by introducing a user-specific frequency shift (typically for the case of localized transmissions) or alternatively by arranging for different users to occupy interleaved sets of subcarriers (typically for the case of distributed transmissions). The latter method as Interleaved Frequency Division Multiple Access (IFDMA). The received signals are properly synchronized in time and frequency. The CP is longer than the sum of the delay spread of the channel and any residual timing synchronization error between the users. The SC-FDMA time-domain generated signal has a similar level of PAPR as pulse-shaped single-carrier modulation. ISI in multipath channels is prevented by the CP, which enables efficient equalization at the receiver by means of a Frequency Domain Equalizer (FDE). (iii) F-Domain Signal Generation (DFT-S-OFDM) Generation of an SC-FDMA signal in the frequency domain uses a Discrete Fourier Transform-Spread OFDM (DFT-S-OFDM) structure as shown in Fig. 6. Fig. 6. SC-FDMA frequency-domain transmission processing (DFT-S-OFDM) showing localized and distributed subcarrier mappings. The first step of DFT-S-OFDM SC-FDMA signal generation is to perform an M-point DFT operation on each block of M QAM data symbols. Zeros are then inserted among the outputs of the DFT in order to match the DFT size to an N-subcarrier OFDM modulator (typically an Inverse Fast Fourier Transform (IFFT)). The zeropadded DFT output is mapped to the N subcarriers, with the positions of the

13 zeros determining to which subcarriers the DFT-precoded data is mapped. Usually N is larger than the maximum number of occupied subcarriers, thus providing for efficient oversampling and sinc (sin(x)/x) pulse-shaping. The equivalence of DFT-S-OFDM and a time-domaingenerated SC-FDMA transmission can readily be seen by considering the case of M = N, where the DFT operation cancels the IFFT of the OFDM modulator resulting in the data symbols being transmitted serially in the time domain. However, this simplistic construction would not provide any oversampling or pulseshape filtering. As with the time-domain approach, DFT-S-OFDM is capable of generating both localized and distributed transmissions: Localized transmission. The subcarrier mapping allocates a group of M adjacent subcarriers to a user. M<N results in zero being appended to the output of the DFT spreader resulting in an upsampled/interpolated version of the original M QAM data symbols at the IFFT output of the OFDM modulator. The transmitted signal is thus similar to a narrowband single carrier with a CP (equivalent to time-domain generation with repetition factor L = 1) and sinc pulse-shaping filtering (circular filtering). Distributed transmission. The subcarrier mapping allocates M equally-spaced subcarriers (e.g. every Lth subcarrier). (L 1) zeros are inserted between the M DFT outputs, and additional zeros are appended to either side of the DFT output prior to the IFFT (ML<N). As with the localized case, the zeros appended on either side of the DFT output provide upsampling or sinc interpolation, while the zeros inserted between the DFT outputs produce waveform repetition in the time domain. This results in a transmitted signal similar to timedomain IFDMA with repetition factor L and sinc pulse-shaping filtering. As for the time-domain SC- FDMA signal generation, orthogonality between different users with different data rate requirements can be achieved by assigning each user a unique set of subcarriers. The CP structure is the same as for the timedomain signal generation, and therefore the same efficient FDE techniques can be employed at the receiver. It is worth noting that, in principle, any unitary matrix can be used in the place of the DFT for the spreading operation with similar performance. However, the use of non- DFT spreading would result in increased PAPR since the transmitted signal would no longer have the single carrier characteristic. 3- Key technologies of Mobile WiMAX: (a) Spectrum, bandwidth options and duplexing arrangement The Release 1 WiMAX profiles cover operation in licensed spectrum allocations in the 2.3, 2.5, 3.3, and 3.5 GHz bands. The channel bandwidths supported are 5, 7, 8.75, and 10 MHz. While WiMAX supports TDD, FDD, and half-duplex FDD, the first release only supports TDD operation. TDD enables adjustment of the downlink/uplink ratio for asymmetric traffic, does not require paired spectrum, and has a less complex transceiver design. To counter interference issues, TDD does, however, require system-wide

14 synchronization and use of the same uplink/downlink ratio in neighboring cells. The reason is the potential for mobile-to-mobile and base station-tobase station interference if uplink and downlink allocations overlap, which becomes an issue in multi-cell deployments. Because of adjacent channel interference, system-wide synchronization may also be required for TDD operators deployed on adjacent or near-adjacent channels. While the initial profiles and deployment of WiMAX use TDD as the preferred mode, FDD may be introduced in the longer term. The reason may be local regulatory requirements or to address need for more extended multi-cell coverage where FDD may become more suitable. (b) Quality-of-service handling A connection-oriented Qualityof-Service (QoS) mechanism is implemented, enabling end-to end QoS control. The QoS parameters are set per service flow, with multiple service flows possible to/from a mobile station. The parameters define transmission ordering and scheduling on the air interface and can be negotiated statically or dynamically through MAC messages. Applications supported through the WiMAX QoS mechanism are: Unsolicited Grant Service (UGS): VoIP Real-Time Polling Service (rtps): Streaming Audio or Video Extended Real-Time Polling Service (ErtPS): Voice with Activity Non-Real-Time Polling Service (nrtps): File Transfer Protocol (FTP) Best-Effort Service (BE): Data Transfer, Web Browsing, etc. (c) Mobility Mobile WiMAX supports both sleep mode and idle mode for more efficient power management. In sleep mode, there is a pre-negotiated period of absence from the radio interface to the serving base station, where the mobile station may power down or scan other neighboring base stations. There are different power saving classes suitable for applications with different QoS types, each having different sleep mode parameters. There is also an idle mode, where the terminal is not registered to any base station and instead periodically scans the network at discrete intervals. There are three handover methods supported, with Hard Handover (HHO) being mandatory and Fast Base-Station Switching (FBSS) and Diversity handover (MDHO) being optional. (d) Fractional frequency reuse WiMAX can operate with a frequency reuse of one, but co-channel interference may in this case degrade the quality for users at the cell edge. However, a flexible subchannel reuse is made possible by dividing the frame into permutation zones as described above. In this way, it is possible to have a subchannel reuse by proper configuration of the subchannel usage for the users. For users at the cell edges, the Base Station operates on a zone with a fraction of the subchannels, while users close to the Base Station can operate on a zone with all subchannels. As shown in the example in Figure 7, there can be an effective reuse of not supported frequencies for users at the cell edge, while still maintaining a reuse of one for the OFDMA carrier as a whole.

15 Fig.7 Fractional frequency reuse III.PHY LAYER 1- PHY layer for WiMAX (IEEE802.16e based) (a) System block diagram Fig.8 The block diagram of the transmitter of the downlink PHY layer of mobile WiMAX with 2- antennas. (b) Overview of the PHY layer blocks (1) padding one: is used if the data size from the MAC layer is less than the frame size according to the selected modulation scheme and code rate, so this block pads ones to reach the frame size. (2) slot concatenation is used if the data size from the MAC layer is larger than the number of data to be transmitted in one slot, so it divides the data into blocks, each of them is with the suitable size that can be transmitted in one frame. (3) Randomizer: Aim: The randomization process ensures that there is no long runs of ones or zeros in the input bits. This will result in: Decrease the Peak to average power ratio (PAPR). Ensure the clock synchronization at the receiver as the transition

16 between bit values helps the receiver in synchronization. If we have long runs of ones the power of the signal will be decreases until the threshold and hence error happened due to Gibbs phenomena. This can be achieved by: The randomization process is carried out using pseudo random binary generator (PRBG), as the output of PRBG is used as the input to an XOR Gate and the second input is the block of data to be transmitted. Only source bits are randomized.elements that are not a part of the source data, such as framing elements and pilot symbols shall not be randomized.the LFSR shall be preset at the beginning of each frame to the value and shall be clocked once per processed bit. The Derandomizer It has the same construction of the Randomizer, as the data has a XOR operation with the output of PRPG that has a linear feedback shift register (LFSR) has the same seed value of the Randomizer used at the Transmitter (4) Channel Coding: The OFDMA PHY supports Mandatory tail-biting Convolutional Coding, The convolutional encoder uses a constituent encoder with a constraintlength 7 and a native code rate 1/2 The 6 bits from the end of the data block are appended to the beginning, to be used as flush bits. These appended bits flush out the bits left in the encoder by the previous FEC block. The first 12 parity bits that are generated by the convolutional encoder which depend on the 6 bits left in the encoder by the previous FEC block are discarded and four optional coding schemes: Zero Tailing Convolutional code, Convolutional Turbo code(ctc) along with H-ARQ, and Block Turbo code(btc) and low density parity check (LDPC) codes The most popular optional channel coding scheme is (CTC) WiMAX uses duobinary turbo codes with a constituent recursive encoder of constraint length 4. In duo binary turbo codes two consecutive bits from the uncoded bit sequence are sent to the encoder simultaneously, have been defined in WIMAX as optional channel coding schemes but are unlikely to be implemented in fixed or mobile WiMAX. (5) Puncturing: In order to achieve code rates higher than 1/2, the output of the encoder is punctured, using a specified puncturing pattern (6) Interleaving: The interleaver is defined by a two step permutation:

17 The first step ensures that the adjacent coded bits are mapped onto nonadjacent subcarriers, which provides frequency diversity and improves the performance of the decoder. The second step ensures that adjacent bits are alternately mapped to less and more significant bits of the modulation constellation, thus avoiding long runs of lowly reliable bits., The interleaver indices are determined using following equations (7) Symbol Mapping: Mobile WiMAX supports QPSK, 16QAM and 64QAM in DL, but In the UL, 64QAM is optional in gray coded scheme. Each modulation constellation is scaled such that the average transmitted power is unity, assuming that all symbols are equally likely. The symbols are further multiplied by a pseudorandom unitary number to provide additional layer 1 encryption. Preamble and midamble symbols are further scaled by 2 2 which allowsboost in the power and allows for more accurate synchronization and various parameter estimations, such as channel response and noise variance. (8) OFDMA: (i) OFDM Symbol Structure The flexibility of the WiMAX PHY layer allows one to make an optimum choice of various PHY layer parameters, such as cyclic prefix length, number of subcarriers, subcarrier separation, and preamble interval, such that the performance degradation owing to ICI and ISI (intersymbol interference) is minimal without compromising the performance. The four primitive parameters that describe an OFDM symbol, and their respective values in IEEE e-2005, are shown in Table 2. Table 1 Primitive Parameters for OFDM Symbol The OFDMA symbol structure consists of three types of sub-carriers: 1. Data subcarriers are used for carrying data symbols.

18 2. Pilot subcarriers are used for carrying pilot symbols. The pilot symbols are known a priori and can be used for channel estimation and channel tracking. 3. Null subcarriers have no power allocated to them, including the DC subcarrier and the guard subcarriers toward the edge. The DC subcarrier is not modulated, to prevent any saturation effects or excess power draw at the amplifier. No power is allocated to the guard subcarrier toward the edge of the spectrum in order to fit the spectrum, of the OFDM symbol within the allocated bandwidth and thus reduce the interference between adjacent channels. The power in the pilot subcarriers, as shown here, is boosted by 2.5 db, allowing reliable channel tracking even at low-snr conditions. (ii) Scalable OFDMA The IEEE e Wireless MAN OFDMA mode is based on the concept of scalable OFDMA (S- OFDMA). S-OFDMA supports a wide range of bandwidths to flexibly address the need for various spectrum allocation and usage model requirements. The scalability is supported by adjusting the FFT size while fixing the sub-carrier frequency spacing at khz. Since the resource unit sub-carrier bandwidth and symbol duration is fixed, the impact to higher layers is minimal when scaling the bandwidth. The system bandwidths for the initial planned profiles being developed by the WiMAX Forum Technical Working Group for Release-1 are 5 and 10 MHz. Table2 OFDMA Scalability Parameters

19 (9) Subchannelization & subcarrier permutation: In order to create the OFDM symbol in the frequency domain, the modulated symbols are mapped on to the subchannels that have been allocated for the transmission of the data block. A subchannel is a logical collection of subcarriers. The number and exact distribution of the subcarriers that constitute a subchannel depend on the subcarrier permutation mode. The number of subchannels allocated for transmitting a data block depends on various parameters, such as the size of the data block, the modulation format, and the coding rate. In the time and frequency domains, the contiguous set of subchannels allocated to a single user or a group of users, in case of multicast is referred to as the data region of the user(s) and is always transmitted using the same burst profile. A burst profile refers to the combination of the chosen modulation format, code rate, and type of FEC. Table3 Parameters of DL FUSC Permutation Four subcarrier permutation are applied: FUSC: Each slot is 48 subcarriers by one OFDM symbol. Downlink PUSC: Each slot is 24 subcarriers by two OFDM symbols. Uplink PUSC and TUSC: Each slot is 16 subcarriers by three OFDM symbols. Band AMC: Each slot is 8, 16, or 24 subcarriers by 6, 3, or 2 OFDM symbols. (i) DL Full Usage of Subcarriers All data subcarriers are used to create various subchannels. Each subchannel is made up of 48 data subcarriers. The pilot subcarriers are allocated first then the data subcarriers are mapped using permutation scheme. Set of pilot subcarriers is divided into 2 constant sets and 2 variable sets. Variable set allows receiver to estimate channel response across the entire frequency band. When transmit diversity of 2, for example, is used, each antenna uses half of number of pilots.

20 (ii) Downlink Partial Usage of Subcarriers All subcarriers are divided into 6 groups. All subcarriers (except null subcarriers) are arranged into clusters. Cluster = 14 adjacent subcarriers 2 OFDM symbols. Cluster = 24 data subcarriers + 4 pilot subcarriers. The clusters are then renumbered. The clusters are then divided into 6 groups. A subchannel is formed using 2 clusters from the same group. (iii) Uplink Partial Usage of Subcarriers Subcarriers are divided into tiles. Tile = 4 subcarriers 3 OFDM symbols. Subcarrier = 8 data subcarriers + 4 pilot subcarriers. Tiles are renumbered and divided into 6 groups. Subchannel = 6 tiles from a single group A special case from the UL PUSC is Uplink Optional Partial Usage of Subcarriers (OPUSC) where: Tile = 3 subcarriers 3 OFDM symbols. Subcarrier = 8 data subcarriers + 1 pilot subcarrier.

21 (iv) Band Adaptive Modulation and Coding Unique to the band AMC permutation mode, all subcarriers constituting a subchannel are adjacent to each other. Although frequency diversity is lost to a large extent with this subcarrier permutation scheme, exploitation of multiuser diversity is easier. Multiuser diversity provides significant improvement in overall system capacity and throughput, since a subchannel at any given time is allocated to the user with the highest SNR/capacity in that subchannel. Nine adjacent subcarriers with eight data subcarriers and one pilot subcarrier are used to form a bin. Four adjacent bins in the frequency domain constitute a band. An AMC subchannel consists of six contiguous bins from within the same band. An AMC subchannel can consist of 1 bin 6 consecutive symbols, 2 bins 3 symbols, or 3 bins 2 consecutive symbols.

22 (10) Channel estimation and equalization form an estimate of the amplitude and phase shift caused by the wireless channel from the available pilot information. The equalization removes the effect of the wireless channel and allows subsequent symbol demodulation. In WiMAX the reference design estimates the channel frequency response using linear interpolation in time and frequency on a tile-by-tile basis for each subchannel. The pilot structure is also outlined by Fig (6.31). In the first and third OFDMA symbol, the outer carriers of each tile are pilot subcarriers, and so it is possible to make an estimate of the channel response at these frequencies by comparison with the known reference pilot subcarrier. The frequency response of the two inner subcarriers may be estimated by linear interpolation in the frequency domain. r p t, k is the pth received pilot subcarrier s p t, k is the pth transmitted pilot subcarrier Subsequently, frequency domain linear interpolation is performed to calculate channel estimates using the following equations: h 12 = 1 3 h 14 h 11 + h 11,h 13 = 2 3 h 14 h 11 + h 11 h 32 = 1 3 h 34 h 31 + h 31, h 32 = 2 3 h 34 h 31 + h 31 Finally, time domain linear interpolation is achieved as follows: h 21 = 1 3 h 11 + h 31,h 22 = 1 3 h 12 + h 32 h 21 = 1 3 h 13 + h 33, h 22 = 1 3 h 14 + h 34 When the data and pilot information has been assembled as shown in Fig (6.31), it is possible to calculate h 11, h 14, h 31, h 34 using the equation h p t, k = r p(t, k) s p (t, k) for the tile t of OFDMA symbol k where: When all of the channel estimates have been formed,a single-tap zero forcing equalizer removes the channel distortion by dividing the received signal by the estimated channel frequency response. Only a single-tap equalizer is required, as the time dispersion of the channel has been removed by the use of OFDM and the addition of a cyclic prefix. (11) Advanced and Multiple antenna support: there several advanced multiple antenna techniques supported in the IEEE standard including adaptive antenna systems (AAS), space time coding(stc), multiple input multiple output (MIMO) to provide

23 significant improvement in the overall system capacity and spectral efficiency of the network There are 2 modes: (1) open-loop mode the transmitter does not know the CSI (2) closed-loop mode, the transmitter knows the CSI, either due to channel reciprocity, in case of TDD, or to explicit feedback from the receiver, in the case of FDD. (a) Open loop mode: (1) Space time coding(alamouti 2 2) For 2x2 Alamouti case, we have two transmit antennas, Ant0 and Ant1. At a given time instant, t, the transmitted sympols are S 0, S 1 respectively. At instant t+t, where T is the sympol duration, the transmitted signals are S 1 *, S 0 *. The received signals are as follow: r 0 = h 0 S 0 + h 1 S 1 + n 0 r 1 = h 0 S 1 + h 1 S 0 + n 1 r 2 = h 2 S 0 + h 3 S 1 + n 2 r 3 = h 2 S 1 + h 3 S 0 + n 3 where r 0 is the received signal at Ant0 at time t, r 1 is the received signal at Ant0 at time t+t, r 2 is the received signal at Ant1 at time t, r 3 is the received signal at Ant1 at time t+t. To decode, the combiner builds the signals for S 0, S 1 ŝ 0 = (α α α α 3 2 1)s 0 + h 0 n 0 + h 1 n 1 + h 2 n 2 + h 3 n 3 ŝ 1 = (α α α α 3 2 1)s 1 h 0 n 1 + h 1 n 0 h 2 n 3 + h 3 n 2 Then a Maximum Likelihood detector searches for S i that minimizes: (α α α α 3 2 1) s 0,1 2 + d 2 (ŝ 0,1, s i ) for both S 0, S 1.

24 (1) Spatial multiplexing (SM) Multiplex a data stream into several branches and transmit via several independent channels overlapping in time and frequency. SM Transmission (using V-BLAST algorithm) We consider a V-BLAST system with 2 transmit antennas and 2 receive antennas. At the transmitter, bit stream is modulated then demultiplexed into 2 substreams, and each substream is sent to its respective transmit antennas. At the receiver, after estimating the channel parametrs, the received signal and and channel parameters are sent to V-BLAST signal processing decoder, which performs ordered successive cancellation, taking the following steps: Ordering: selects the data stream with the highest signal to interference ratio Nulling: remove the effect of other streams by multipling the received signal by zeroing weights Slicing: quantize the output to get the received symbol. (2) Frequency-Hopping Diversity Code WiMAX also defines an optional transmit diversity mode, known as the frequency-hopping diversity code (FHDC), using two antennas in which the encoding is done in the space and frequency domain, as shown in Figure 8 rather than the space and time domain. In FHDC, the first antenna transmits the OFDM symbols without any encoding, much like a singleantenna transmission, and the second antenna transmits the OFDM symbol by encoding it over two consecutive subchannels, using the 2 2 Alamouti encoding matrix (b) Closed loop mode: The various transmit diversity and spatial-multiplexing schemes of IEEE described in the previous section do not require the transmitter to know the CSI for the receiver of interest. MIMO and diversity schemes can benefit significantly if the CSI is known at the transmitter. CSI information at the transmitter can be used to select the appropriate MIMO mode number of transmit antennas, number of simultaneous streams, and space/time encoding matrix as well as to calculate an optimum precoding matrix that maximizes system capacity. The CSI can be known at the

25 transmitter due to channel reciprocity, in the case of TDD, or by having a feedback channel, in the case of FDD. The uplink bandwidth required to provide the full CSI to the transmitter the MIMO channel matrix for each subcarrier in a multiuser FDD MIMO- OFDM system is too large and thus impractical for a closed-loop FDD MIMO system. For practical systems, it is possible only to send some form of quantized information in the uplink. The framework for closed-loop MIMO in IEEE , as shown in consists. of a space/time encoding stage identical to an open-loop system and a MIMO precoding stage. The MIMO precoding matrix in general is a complex matrix, with the number of rows equal to the number of transmit antennas and the number of columns equal to the output of the space/time encoding block. The linear precoding matrix spatially mixes the various parallel streams among the various antennas, with appropriate amplitude and phase adjustment Closed-loop MIMO framework in IEEE In order to determine the appropriate amplitude and phases of the various weights, the transmitter requires some feedback from the MS. In the case of closed-loop MIMO, the feedback falls broadly into two categories: long-term feedback and short-term feedback. The long-term feedback provides information related to the maximum number of parallel streams: the rank of the precoding matrix to be used for DL transmissions. The short-term feedback provides information about the precoding matrix weights to be used. The IEEE standard defines the following five mechanisms so that the BS can estimate the optimum precoding matrix for closed-loop MIMO operations: 1. Antenna selection. The MS indicates to the BS which transmit antenna(s) should be used for transmission in order to maximize the channel capacity and/or improve the link reliability. 2. Antenna grouping. The MS indicates to the BS the optimum permutation of the order of the various antennas to be used with the current space/time encoding matrix. 3. Codebook based feedback. The MS indicates to the BS the optimum precoding matrix to be used, based on the entries of a predefined codebook. 4. Quantized channel feedback. The MS quantizes the MIMO channel and sends this information to the BS, using the MIMO_FEEDBACK

26 message. The BS can use the quantized MIMO channel to calculate an optimum precoding matrix. 5. Channel sounding. The BS obtains exact information about the CSI of the MS by using a dedicated and predetermined signal intended for channel sounding. (c) AAS support in IEEE Std Through the AAS options, the IEEE standard supports the use of smart antennas to perform beam forming. Beam forming can effectively create a narrower signal beam, resulting in increased gain and, therefore, higher range. This in turn increases capacity by increasing the range at which a particular PHY burst profile can be received. AAS also allows for the suppression of noise sources, improving the SNR at the receiver, and discrimination on the AoD allows energy to be concentrated in the direction of the intended recipient, enabling large cell ranges. In addition, nulls can be steered in particular directions, enhancing the interference resistance of the system. Drawbacks of these approaches include the increased system complexity and the inability to broadcast messages, reducing the spectral efficiency due to repetition of broadcast MAC messages to the various recipients. (12) Mobile WiMAX TDD Frame Structure: The e PHY supports TDD and Full and Half-Duplex FDD operation, however the initial release of Mobile WiMAX certification profiles will only include TDD. With ongoing releases, FDD profiles will be considered by the WiMAX Forum to address specific market opportunities where local spectrum regulatory requirements either prohibit TDD or are more suitable for FDD deployments. To counter interference issues, TDD does require system-wide synchronization; and TDD is the preferred duplexing mode for the following reasons: TDD enables adjustment of the downlink/uplink ratio to efficiently support asymmetric downlink/uplink traffic, while with FDD, downlink and uplink always have fixed and generally, equal DL and UL bandwidths. TDD assures channel reciprocity for better support of link adaptation, MIMO and other closed loop advanced antenna technologies. Unlike FDD, which requires a pair of channels, TDD only requires a single channel or both downlink and uplink providing greater flexibility for adaptation to varied global spectrum allocations. Transceiver designs for TDD implementations are less complex and therefore less expensive. BASICS OF OFDMA FRAME STRUCTURE: There are three types of OFDMA subcarriers: 1. Data subcarriers for data transmission. 2. Pilot subcarriers for various estimation and synchronization purposes. 3. Null subcarriers for no transmission at all, used for guard bands and DC carriers.

27 Active subcarriers are divided into subsets of subcarriers called subchannels. The subcarriers forming one subchannel may be, but need not be, adjacent. The pilot allocation is performed differently in different subcarrier allocation modes. Fig illustrates the OFDMA frame structure for a Time Division Duplex (TDD) implementation. Each frame is divided into DL and UL sub-frames separated by Transmit/Receive and Receive/Transmit Transition Gaps (TTG and RTG, respectively) to prevent DL and UL transmission collisions. The downlink-to-uplinksubframe ratio may be varied from 3:1 to 1:1 to support different traffic profiles. The relevant information about the starting position and the duration of the various zones being used in a UL and DL subframe is provided by control messages in the beginning of each DL subframe. In a frame, the following control information is used to ensure optimal system operation: Preamble: The preamble is the first OFDM symbol of the frame. The preamble can be used for a variety of PHY layer procedures, such as time and frequency synchronization, initial channel estimation, and noise and interference estimation. To create the preamble in frequency domain, BPSK modulation is used. Frame Control Header (FCH): The FCH follows the preamble. It provides the frame configuration information such as MAP message length and the modulation and coding scheme and usable sub-channels. DL-MAP and UL-MAP: The DL-MAP and UL-MAP provide subchannel allocation and Multiple users data regions within the frame and other control

28 information for the DL and UL sub-frames respectively. Since MAP contains critical information that needs to reach all users, it is often sent over a very reliable link, such as BPSK with rate 1/2 coding and repetition coding. The BS also transmits the downlink channel descriptor (DCD) and the uplink channel descriptor (UCD) following the UL-MAP message, which contains additional control information pertaining to the description of channel structure and the various burst profiles that are allowed within the given BS. In order to conserve resources, the DCD and the UCD are not transmitted every DL frame. UL Ranging: The UL ranging sub-channel is allocated for mobile stations (MS) to perform closed-loop time, frequency, and power adjustment as well as bandwidth requests. UL CQICH: The UL CQICH channel is allocated for the MS to feedback channel state information. UL ACK: The UL ACK is allocated for the MS to feedback DL HARQ ( Hybrid Automatic Repeat Request ) acknowledge. Burst Regions is used as Data regions from different users each burst has the same modulation and code rate for all users that are included in this burst. TTG & RTG : Transmit/Receive and Receive/Transmit Transition Gaps. Frame duration is almost 5 ms (it is variable from 2 ms to 20 ms). Each frame has 47 OFDM symbols each symbol duration is µs Downlink Physical layer: 2-3GPP LTE PHY layer:

29 (1) CRC insertion: In the first step of the transportchannel processing, a 24-bit CRC is calculatedfor and appended to each transport block. The CRC allows for receiverside detection of errors in the decoded transport block. The corresponding error indication is then, for example, used by the downlink hybrid-arq protocol as a trigger for requesting retransmissions. (2) Code-block segmentation and per-code-block CRC insertion: The LTE Turbo-coder internal interleaver is only defined for a limited number of code-block sizes with a maximum block size of 6144 bits. In case the transport block, including the transport-block CRC, exceeds this maximum code-block size, code-block segmentation as illustrated in fig. is applied before Turbo coding. Codeblock segmentation implies that the transport block is segmented into smaller code blocks that match the set of code-block sizes defined for the Turbo coder. In order to ensure that the size of each code block is matched to the set of available code-block sizes, filler bits may have to be inserted at the head of the first code block. Note that filler bits may be needed also if there is no actual codeblock segmentation, that is if the transportblock size does not exceed the maximum code-block size. 15 As can be seen in Figure code-block segmentation also implies that an additional (24 bits) CRC is calculated for and appended to each code block. 16 Having a CRC per code block allows for early detection of correctly decoded code blocks and corresponding early termination of the iterative decoding of that code block. This can be used to reduce the terminal processing effort and power consumption. It should be noted that, in case of no code-block segmentation, that is in case of a single code block, no additional code-block CRC is applied (3) Turbo coding The overall structure of the LTE Turbo encoding is illustrated in Figure The Turbo encoding reuses the two WCDMA/HSPA rate-1/2, eight-state constituent encoders, implying an overall code rate of 1/3. However, the WCDMA/ HSPA Turbo encoder internal interleaver has, for LTE, been replaced by QPPbased17 interleaving, the QPP interleaver provides a mapping from the input (noninterleaved) bits to the output (interleaved) bits according to the function: where i is the index of the bit at the output of the interleaver, c(i) is the index of the same bit at the input of the interleaver, and K is the codeblock/interleaver size. The values of the parameters f1 and f2 depend on the

30 code-block size K. The LTE specification lists all supported codeblock sizes, ranging from a minimum of 40 bits to a maximum of 6144 bits, together with the associated values for the parameters f1 and f2. (4) Rate-matching and physicallayer hybrid-arq functionality The task of the rate-matching and physical-layer hybrid-arq functionality is to extract, from the blocks of code bits delivered by the channel encoder, the exact set of bits to be transmitted within a given TTI. As illustrated in Figure 16.30, the outputs of the Turbo encoder (systematic bits,first parity bits, and second parity bits) are first separately interleaved. The interleaved bits are then inserted into what can be described as a circular buffer with the systematic bits inserted first, followed by alternating insertion of the first and second parity bits. The bit selection then extracts consecutive bits from the circular buffer to the extent that fits into the assigned resource. The set of bits to extract depends on the redundancy version corresponding to different starting points for the extraction of coded bits from the circular buffer. As can be seen, there are four different alternatives for the redundancy version.

31 (5) Bit-level scrambling LTE downlink scrambling implies that the block of code bits delivered by the hybrid-arq functionality is multiplied ( exclusive-or operation) by a bit-level scrambling sequence. In general, scrambling of the coded data helps to ensure that the receiver-side decoding can fully utilize the processing gain provided by the channel code. Without downlink scrambling, the channel decoder at the mobile terminal could, at least in principle, be equally matched to an interfering signal as to the target signal, thus not being able to properly suppress the interference. By applying different scrambling sequences for neighbor cells, the interfering signal(s) after descrambling are randomized, ensuring full utilization of the processing gain provided by the channel code (6) modulation The downlink data modulation transforms the block of scrambled bits to a corresponding block of complex modulation symbols. The set of modulation schemes supported for the LTE downlink includes QPSK, 16QAM, and 64QAM, corresponding to two, four, and six bits per modulation symbol, respectively. All these modulation schemes are applicable to the DL-SCH, PCH, and MCH transport channels. As will be described in Chapter 18, only QPSK modulation can be applied to the BCH transport channel. (7) Antenna mapping The Antenna Mapping jointly processes the modulation symbols corresponding to, in the general case, two transport blocks, and maps the result to the different antenna ports (8) Resource-block mapping The resource-block mapping maps the symbols to be transmitted on each antenna port to the resource elements of the set of resource blocks assigned by the MAC scheduler for transmission of the transport block(s) to the terminal. Each resource block consists of 84 resource elements (12 subcarriers during 7 OFDM symbols) when deciding what set of resource blocks to use for transmission to a specific terminal, the network may take the downlink channel conditions in both the time and frequency domain into account. Such time/ frequency-domain channel-dependent scheduling, taking channel variations However, in some cases downlink channel-dependent scheduling is not suitable an alternative means to handle radio-channel frequency selectivity is to achieve frequency diversity by distributing a downlink transmission in the frequency domain. In order to provide the possibility for distributed resource-block allocation in case of resource allocation type 2, as well as to allow for distributing the transmission of a single resource-block pair in the frequency domain, the notion of a Virtual Resource Block (VRB) has been introduced for LTE. What is being provided in the resource allocation is the resource allocation in terms of VRB pairs. The key to distributed transmission then lies in the mapping from VRB pairs to Physical Resource Block (PRB) pairs, that is, to the actual physical resource used for transmission. The LTE specification defines two types of VRBs: localized VRBs and distributed VRBs. In case of localized VRBs, there is a direct mapping from VRB pairs to PRB pairs as illustrated in Figure. However, in case of distributed VRBs, the mapping from VRB pairs to PRB pairs is more elaborate in the sense that Consecutive VRBs are not mapped to PRBs that are consecutive in the frequency domain, even a single VRB pair is distributed in the frequency

32 domain.the basic principle of distributed transmission consists of two steps: o A mapping from VRB pairs to PRB pairs such that consecutive VRB pairs are not mapped to frequency-consecutive PRB pairs This provides frequency diversity between consecutive VRB pairs. The spreading in the frequency domain is done by means of a block-based interleaver operating on resource-block pairs. o A split of each resource-block pair such that the two resource blocks of the resource-block pair are transmitted with a certain frequency gap in between (second step of Figure ). This provides frequency diversity also for a single VRB pair. This step can be seen as the introduction of frequency hopping on a slot basis. (9) Multi-antenna transmission LTE supports the following multiantenna transmission schemes or transmission modes, in addition to single-antenna transmission: Transmit diversity Closed-loop spatial multiplexing including codebook-based beamforming Open-loop spatial multiplexing Transmit diversity In case of two antenna ports, LTE transmit diversity is based on Space Frequency Block Coding (SFBC). As can be seen from Figure 16.33, SFBC implies that consecutive modulation symbols 1 are mapped directly on adjacent subcarriers on the first antenna port. On the second antenna port, the swapped and transformed symbols are transmitted on the corresponding subcarriers. In case of four antenna ports, LTE transmit diversity is based on a combination of SFBC and Frequency Shift Transmit Diversity (FSTD). As can be seen in combined SFBD/FSTD implies that pairs of modulation symbols are transmitted by means of SFBC with transmission alternating between pairs of antenna ports (antenna ports 0 and 2 and antenna ports 1 and 3, respectively). Closed loop Spatial multiplexing As described in Chapter 6, spatial multiplexing implies that multiple streams or layers are transmitted in parallel, thereby allowing for higher data rates within a given bandwidth. LTE spatial multiplexing allows for the transmission of a variable number of layers, up to a maximum of NA layers, where NA is the number of antenna ports. The LTE spatial multiplexing may operate in two different modes: closed-loop spatial multiplexing and open-loop spatial multiplexing where

33 closed-loop spatial multiplexing relies on more extensive feedback from the mobile terminal. One or two codewords, corresponding to one or two transport blocks, are mapped to the NL layers. The number of NL layers may range from a minimum of one layer up to a maximum number of layers equal to the number of antenna ports After layer mapping, a set of NL symbols (one symbol from each layer) is linearly combined and mapped to the NA antenna ports. This combining/mapping can be described by means of a pre-coder matrix W of size NA *NL As LTE supports multi-antenna transmission using two or four antenna ports, pre-coding matrices are defined for: two antenna ports ( NA = 2) and one and two layers, corresponding to precoder matrices of size 2 1 and 2 2, respectively; four antenna ports ( NA = 4) and one, two, three, and four layers, corresponding to pre-coder matrices of size 4 1, 4 2, 4 3, and 4 4, respectively. Open loop SM LTE also supports open-loop spatial multiplexing, also sometimes referred to as large-delay CDD The structure of large-delay CDD is illustrated in Figure As can be seen, the overall pre-coding functionality can in this case be seen as a combination of two pre-coder matrices, a matrix P of size NL NL and a matrix W of size NA NL. The matrix P in Figure can be expressed as a product of two matrices P =U. D, where U is a constant matrix of size NL NL and D ( i ) is matrix of size NL NL that varies between subcarriers. As an example, the matrices U and D ( i ) for the case of two layers ( NL 2) are given by:

34 The basic idea with the matrix P, that is the large-delay CDD part of the open-loop spatial multiplexing, is to average out any differences in the channel conditions as seen by the different layers. General beam-forming As described above, closed-loop spatial multiplexing includes beamforming as a special case when the number of layers NL equals one. This kind of beamforming can be referred to as codebook-based beam-forming. SC-FDMA Design in LTE Transmit Processing for LTE Although the frequency-domain generation of SC-FDMA (DFT-S- OFDM) is functionally equivalent to the time-domain SC-FDMA signal generation, each technique requires a slightly different parameterization for efficient signal generation. The pulseshaping filter used in the time domain SC-FDMA generation approach in practice has a non-zero excess bandwidth, resulting in bandwidth efficiency which is smaller than that achievable with the frequency domain method with its inherent sinc (zero excess bandwidth) pulse-shaping filter which arises from the zero padding and IFFT operation. For example, for a 5 MHz operating bandwidth, physical layer parameters optimized for time-domain implementation might have a sampling rate of Mps (256 subcarriers with 16 khz subcarrier spacing) resulting in bandwidth efficiency of 82%. An equivalent set of parameters optimized for the frequency-domain generation can support a bandwidth efficiency of 90% (with 300 occupied subcarriers and 15 khz subcarrier spacing). Thus, with frequency-domain processing, a 10% increase in bandwidth efficiency can be achieved, allowing higher data rates. The non-zero excess bandwidth pulseshaping filter in the time-domain generation also requires ramp-up and ramp-down times of 3 4 samples duration, while for DFT-S-OFDM there is no explicit pulse-shaping filter, resulting in a much shorter ramp time similar to OFDM. However, the pulseshaping filter in the time-domain generation does provide the benefit of reduced CM by approximately db compared to DFT-S-OFDM, as shown in Fig.?. Thus there is a tradeoff between bandwidth efficiency and CM/PAPR reduction between the timeand frequency-domain SC-FDMA generation methods. Frequency-domain signal generation for the LTE uplink has a further benefit in that it allows a very similar parameterization to be adopted as for the OFDM downlink, including the same subcarrier spacing, number of occupied subcarriers in a given bandwidth, and CP lengths. This provides maximal commonality between uplink and downlink, including for example the same clock frequency. For these reasons, the SC-FDMA parameters chosen for the LTE uplink have been optimized under the assumption of frequency-domain DFT- S-OFDM signal generation. An important feature of the LTE SC- FDMA parameterization is that the numbers of subcarriers which can be allocated to a UE for transmission are restricted such that the DFT size in LTE can be constructed from multiples of 2, 3 and/or 5. This enables efficient, low-complexity mixed-radix FFT implementations. SC-FDMA Parameters for LTE The same basic transmission resource structure is used for the uplink as for the downlink: a 10 ms radio frame is divided into ten 1 ms subframes each consisting of two 0.5 ms slots. As LTE

35 SC-FDMA is based on the same fundamental processing as OFDM, it uses the same 15 khz subcarrier spacing as the downlink. The uplink transmission resources are also defined in the frequency domain (i.e. before the IFFT), with the smallest unit of resource being a Resource Element (RE), consisting of one SC-FDMA data block length on one subcarrier. As in the downlink, a Resource Block (RB) comprises 12 REs in the frequency domain for a duration of 1 slot, as detailed in Section 6.2. The LTE uplink SCFDMA physical layer parameters for Frequency Division Duplex (FDD) and Time Division Duplex (TDD) deployments are detailed in Table 6. Table 6. LTE uplink SC-FDMA physical layer parameters Two CP durations are supported a normal CP of duration 4.69 μs and an extended CP of μs, as in the downlink. The extended CP is beneficial for deployments with large channel delay-spread characteristics, and for large cells. The 1 ms subframe allows a 1 ms scheduling interval (or Transmission Time Interval (TTI)), as for the downlink, to enable low latency. However, one difference from the downlink is that the uplink coverage is more likely to be limited by the maximum transmission power of the UE. In some situations, this may mean that a single Voice-over-IP (VoIP) packet, for example, cannot be transmitted in a 1 ms subframe with an acceptable error rate. One solution to this is to segment the VoIP packet at higher layers to allow it to be transmitted over several subframes. However, such segmentation results in additional signaling overhead for each segment (including resource allocation signaling and Hybrid ARQ acknowledgement signaling). A more efficient technique for improving uplink VoIP coverage at the cell edge is to use so-called TTI bundling, where a single transport block from the MAC layer is transmitted repeatedly in multiple consecutive subframes, with only one set of signaling messages for the whole transmission. The LTE uplink allows groups of 4 TTIs to be bundled in this way, in addition to the normal 1 ms TTI. In practice in LTE, all the uplink data transmissions are localized, using contiguous blocks of subcarriers. This simplifies the transmission scheme, and enables the same RB structure to be used as in the downlink. Frequencydiversity can still be exploited by means of frequency hopping, which can occur both within one subframe (at the boundary between the two slots) and between subframes. In the case of

36 frequency hopping within a subframe, the channel coding spans the two transmission frequencies, and therefore the frequency diversity gain is maximized through the channel decoding process. The only instance of distributed transmission in the LTE uplink (using an IFDMA-like structure) is for the Sounding Reference Signals (SRSs) which are transmitted to enable the enodeb to perform uplink frequency-selective scheduling. Like the downlink, the LTE uplink supports scalable system bandwidths from approximately 1.4 MHz up to 20 MHz with the same subcarrier spacing and symbol duration for all bandwidths. The uplink scaling for the bandwidths supported in the first release of LTE is shown in Table 7. Note that the sampling rates resulting from the indicated FFT sizes are designed to be small rational multiples of the UMTS 3.84 MHz chip rate, for ease of implementation in a multimode UE. Note that in the OFDM downlink parameter specification, the d.c. subcarrier is unused. In contrast, no unused d.c. subcarrier is possible for SC-FDMA as it can affect the low CM/PAPR property of the transmit signal. Table 7. LTE Uplink SC-FDMA parametrization for selected carrier bandwidths. d.c. Subcarrier in SC-FDMA Direct conversion transmitters and receivers can introduce distortion at the carrier frequency (zero frequency or d.c. in baseband), for example arising from local oscillator leakage. In this section we explore three possible configurations of the d.c. subcarrier which were considered in the design of the LTE uplink in order to minimize d.c. distortion effects on the packet error rate and the CM/PAPR. Option 1. The d.c. subcarrier distortion region falls in the middle of a RB, such that one of the RBs includes information modulated at d.c. (e.g. 600 subcarriers for 10MHz operation bandwidth with the d.c. subcarrier being one of the subcarriers for RB 26). The performance of the RB containing the d.c. subcarrier would be reduced at the receiver; this effect would be most noticeable with a narrow bandwidth transmission consisting of a single RB. Option 2. One more subcarrier is configured than is required for the number of RBs (e.g. 601 subcarriers for the 10 MHz bandwidth case). This option would be beneficial for the case of a system bandwidth with an even number of RBs where the additional subcarrier would be unused and correspond to the d.c. subcarrier located between RBs allocated to different UEs. Option 3. The subcarriers are frequency-shifted by half a subcarrier spacing (±7.5 khz), resulting in an offset of 7.5 khz for subcarriers relative to d.c. Thus two subcarriers

37 straddle the d.c. location. This is the option used in LTE, and is illustrated in Fig. 5 for deployments with even and odd numbers of RBs across the system bandwidth. IV. System Architecture

38 Specifying the PHY and MAC of the radio link alone is not sufficient to build an interoperable broadband wireless network. Rather, the network architecture framework that deals with the end-to-end service aspects is needed. a- WiMAX Fig 9 shows the WiMAX network reference model (NRM), which is a logical representation of the network architecture. The NRM identifies the functional entities in the architecture and the reference points between the functional entities over which interoperability is achieved. The NRM divides the end-to-end system into three logical parts: (1) mobile stations used by the subscriber to access the work; (2) the access service network (ASN) which is owned by a NAP and comprises one or more base stations and one or more ASN gateways that form the radio access network; and (3) the connectivity service network (CSN), which is owned by an NSP, and provides IP connectivity and all the IP core network functions. The subscriber is served from the CSN belonging to the visited NSP; the home NSP is where the subscriber belongs. Fig.9 Network reference model 1- ASN Functions, Decompositions, and Profiles The ASN performs the following functions: IEEE e based layer 2 connectivity with the MS Network discovery and selection of the subscriber s preferred CSN/NSP AAA proxy: transfer of device, user, and service credentials to selected NSP AAA and temporary storage of user s profiles Relay functionality for establishing IP connectivity between the MS and the CSN Radio resource management (RRM) and allocation based on the QoS policy and/or request from the NSP or the ASP

39 Mobility-related functions, such as handover, location management, and paging within the ASN, including support for mobile IP with foreign-agent functionality The ASN may be decomposed into one or more base stations (BSs) and one or more ASN Gateways (ASN-GW) as shown in Fig.9. The WiMAX NRM defines multiple profiles for the ASN, each calling for a different decomposition of functions within the ASN. The BS is defined as representing one sector with one frequency assignment implementing the IEEE e interface to the MS. Additional functions handled by the BS in both profiles include scheduling for the uplink and the downlink, traffic classification, and service flow management (SFM) by acting as the QoS policy enforcement point (PEP) for traffic via the air interface, providing terminal activity (active, idle) status, providing DHCP proxy functionality, relaying authentication messages between the MS and the ASN- GW, reception and delivery of the traffic encryption key (TEK) and the key encryption key (KEK) to the MS, serving as RSVP proxy for session management, and managing multicast group association via Internet Group Management Protocol (IGMP) proxy. A BS may be connected to more than one ASN-GW for load balancing or redundancy purposes. The ASN-GW provides ASN location management and paging; acts as a server for network session and mobility management; acts as an authenticator and AAA; provides mobility tunnel establishment and management with BSs; acts as a client for session/mobility management; performs service flow authorization (SFA), based on the user profile and QoS policy; provides foreign agent functionality; and performs routing (IPv4 and IPv6) to selected CSNs. Table 2.1 lists the split of the various functional entities within an ASN between the BS and the ASN-GW, as per the ASN profiles defined by the WiMAX Forum. It should be noted that the ASN gateway may optionally be decomposed into two groups of functions: decision point (DP) functions and enforcement point (EP) functions. When decomposed in such a way, the DP functions may be shared across multiple ASN Gateways. Examples of DP functions include intra-asn location management and paging, regional radio resource control and admission control, network session/mobility management (server), radio load balancing for handover decisions, temporary caching of subscriber profile and encryption keys, and AAA client/proxy. Examples of EP functions include mobility tunneling establishment and management with BSs, session/mobility management (client), QoS and policy enforcement, foreign agent, and routing to selected CSN.

40 Table 2.1 Functional Decomposition of the ASN in Various Release 1 Profiles 2- CSN Functions The CSN provides the following functions: IP address allocation to the MS for user sessions. AAA proxy or server for user, device and services authentication, authorization, and accounting (AAA). Policy and QoS management based on the SLA/contract with the user. The CSN of the home NSP distributes the subscriber profile to the NAP directly or via the visited NSP. Subscriber billing and interoperator settlement. Inter-CSN tunneling to support roaming between NSPs. Inter-ASN mobility management and mobile IP home agent functionality. Connectivity infrastructure and policy control for such services as Internet access, access to other IP networks, ASPs, locationbased services, peer-to-peer, VPN, IP multimedia services, law enforcement, and messaging. 3- Reference Points The WiMAX NWG defines a reference point (RP) as a conceptual link that connects two groups of functions that reside in different functional entities of the ASN, CSN, or MS. Reference points are not necessarily a physical interface, except when

41 the functional entities on either side of it are implemented on different physical devices. Fig.9 shows a number of reference points defined by the WiMAX NWG. These reference points are listed in Table 2.2. Table 2.2 WiMAX Reference Points 4- Network Discovery and Selection WiMAX networks are required to support either manual or automatic selection of the appropriate network, based on user preference. It is assumed that an MS will operate in an environment in which multiple networks are available for it to connect to and multiple service providers are offering services over the available networks. To facilitate such operation, the WiMAX standard offers a solution for network discovery and selection. The solution consists of four procedures: