Vidyalankar B.E. Sem. VII [ETRX] Wireless Communication Prelim Question Paper Solution

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1 B.E. Sem. VII [ETRX] Wireless Communication Prelim Question Paper Solution 1. (a) Foliage Loss Foliage loss is the signal loss encountered in mobile communications due to propagation through forest or vegetation. To measure the foliage loss we have to take into account the following parameters : (i) The size, density and distribution of leaves, branches and trunks of trees in the propagation path (ii) Height of the trees relative to antenna heights. (iii) Spacing between adjacent trunks, branches and leaves. (iv) The texture and thickness of leaves. (v) Climatic conditions, depending on which trees shed their leaves. Close in foliage at the transmitter site always heavily attenuates signal reception. Therefore the cell site should be placed away from trees. If heavy foliage is close in at mobile unit, the additional foliage loss must be calculated using the diffraction loss formula. For a typical mobile communications environment, the total loss (including foliage) would be 60 db/ decade (20 db/decade of free space loss + additional 20 db due to foliage loss + additional 20 db due to mobile communication) 1. (b) Umbrella Cell Approach By using different antenna heights [often in the same building or tower] and different power levels, it is possible to provide large and small cells which are co located at a single location. This technique is called the Umbrella Cell approach. The figure above illustrates an umbrella cell which is co located with some smaller micro cells. Here a separate set of channels are reserved for the large umbrella cell. The Umbrella Cell approach is used to provide large area coverage to high speed users while providing small area coverage to users travelling at low speeds. This ensures that the number of handoffs is minimised for high speed users while at the same time additional micro cell channels are provided for pedestrian users. The speed of each user may be estimated by the base station or MSC by evaluating how rapidly the short term average signal strength on the RVC changes. Over time or more sophisticated algorithms may be used to evaluate and partition users. If a high speed user in the large umbrella cell is approaching the base station, and its velocity is rapidly decreasing, the base station may decide to hand the user to the co located micro cell, without MSC intervention. Fig.: The Umbrella Cell Approach 1113/Engg/BE/Pre Pap/2013/ETRX/Soln/WC 1

2 : B.E. WC 1. (c) Self Jamming in CDMA Self Jamming means that the receivers in the CDMA system have a tendency to jam or block their own signals. Self Jamming arises from the fact that spreading sequences of different users are not exactly orthogonal. Due to this, while de-spreading a particular PN code in the receiver, there are appreciable non-zero contributions due to spreading sequence transmissions from other users in the system i.e., there is some level of cross correlation. Self Jamming is a serious problem in CDMA systems, which is effectively tackled using power control. 1. (d) The features of TDMA include the following: TDMA shares a single carrier frequency with several users, where each user makes use of nonoverlapping time slots. The number of time slots per frame depends on several factors, such as modulation technique, available bandwidth, etc. Data transmission for users of a TDMA system is not continuous, but occurs in bursts. This results in low battery consumption, since the subscriber transmitter can be turned off when not in use (which is most of the time). Because of discontinuous transmissions in TDMA, the handoff process is much simpler for a subscriber unit, since it is able to listen for other base stations during idle time slots. An enhanced link control, such as that provided by mobile assisted handoff (MAHO) can be carried out by a subscriber by listening on an idle slot in the TDMA frame. TDMA uses different time slots for transmission and reception, thus duplexers are not required. Even if FDD is used, a switch rather than a duplexer inside the subscriber unit is all that is required to switch between transmitter and receiver using TDMA. Adaptive equalization is usually necessary in TDMA systems, since the transmission rates are generally very high as compared to FDMA channels. In TDMA, the guard time should be minimized. If the transmitted signal at the edges of a time slot are suppressed sharply in order to shorten the guard time, the transmitted spectrum will expand and cause interference to adjacent channels. High synchronization overhead is required in TDMA systems because of burst transmissions. TDMA transmissions are slotted, and this requires the receivers to be synchronized for each data burst. In addition, guard slots are necessary to separate users, and this results in the TDMA systems having larger overheads as compared to FDMA. TDMA has an advantage in that it is possible to allocate different numbers of time slots per frame to different users. Thus, bandwidth can be supplied on demand to different users by concatenating or reassigning time slots based on priority. 2. (a) Coherence Bandwidth [B c ] The coherence Bandwidth [B c ] is a statistical measure of the range of frequencies over which the channel passes all spectral components with approximately equal gain and linear phase. The coherence Bandwidth represents a frequency range for which either the amplitudes or phases of two received signals have a high degree of similarity [correlation]. A signal's spectral components in that frequency range are affected by the channel in a similar manner as for example, exhibiting fading or not. Consider two fading signal envelopes at frequencies f 1 and f 2 respectively, where f = f 1 f 2 If the two signals are further apart than the coherence bandwidth, B c the signals will fade independently. This concept is used for diversity reception. An exact relationship between Coherence Bandwidth and Delay Spread does not exist. Several approximations have been proposed two of which are listed below : If the coherence bandwidth is defined as the bandwidth over which the frequency correlation function is above 0.9, then coherence bandwidth is approximately /Engg/BE/Pre Pap/2013/ETRX/Soln/WC

3 Prelim Question Paper Solution 2. (b) 1 Bc 50 If the definition is relaxed so that the frequency correlation function is above 0.5, then the coherence bandwidth is approximately. l Bc 5 Coherence Time (T c ) Coherence time is a statistical measure of the time duration which the channel impulse response is essentially invariant, and quantifies the similarity of the channel response at different times. In other words, Coherence time is the time duration during which two received signals have a strong potential for amplitude correlation. The definition of coherence time implies that two signals arriving with a time separation greater than T c are affected differently by the channel. Coherence Time is the time domain dual of the Doppler Spread The Doppler spread and Coherence time are inversely proportional to one another. That is where f m = 1 Tc... [1] f m is the maximum Doppler shift If the Coherence Time is defined as the time over which the time correlation function is above 0.5, then the Coherence time is approximately. 9 Tc 16f m... [2] In practice equation [1] suggests a time duration during which a Rayleigh fading signal may fluctuate wildly and equation [2] is often too restrictive. A popular thumb rule for modern digital communications is to define the coherence time as the geometric mean of equations [1] and [2] Tc 2 16f f m m Security in GSM is achieved through authentication and encryption process. (i) Authentication protects against unauthorized Access Authentication involves two phases : First Phase This phase includes the Authentication of a Valid User for the SIM. A Personal Identification Number (PIN) code protects the SIM. The user needs to know the secret PIN to access the SIM. the PIN is checked by the SIM locally and not sent out by radio. Second Phase This phase includes Subscriber Authentication. Subscriber Authentication uses a challenge response method. The Access Control or Authentication Control AuC [network side] generates a Random number RAND as Challenge The SIM [within the MS] answers with SRES (Signed Response) as response. 1113/Engg/BE/Pre Pap/2013/ETRX/Soln/WC 3

4 : B.E. WC The figure below illustrates the Subscriber Authentication process in GSM MSC SIM with A3, A5, A8 and Ki 3 The various steps involved in the authentication process are as given below : i) The AuC performs the basic generation of random values RAND, signed responses SRES and cipher keys Kc for each IMSI and then forwards this information to the HLR. ii) At terminal location update, current VLR sends IMSI to the HLR iii) HLR returns security triplets Rand, SRES and K c to the VLR iv) For authentication and ciphering the VLR sends RAND to the SIM in the MS v) Using stored A3 algorithm and secret key K i stored in the SIM, along with RAND provided by the VLR the MS calculates the SRES and returns it to the VLR. [Using the A8 algorithm and K i, the MS also calculates the cipher key K c.] vi) If the SRES returned by the MS matches with the stored SRES in the VLR, the VLR accepts the subscriber, otherwise the subscriber is rejected. (ii) Encryption Process in GSM To ensure privacy, all messages containing user related information are encrypted in GSM over the air interface. Thus Encryption protects against unauthorized listening. After authentication, MS and BSS can start using encryption by applying the cipher key K c. The figure below illustrates the ciphering (Encryption) and Deciphering (Decryption) process. Operations at MS The MS uses the RAND received from the network and mixes K i through an algorithm called A8, which generates K c. The frame number and K c move to a ciphering algorithm A5 and generates S2 (114 bits) which performs an exclusive OR operation between 114 bits of plain text and ciphering sequence S2 producing encrypted data for transmission form MS to BTS. Operations at BTS The VLR sends the cipher key K c (generated during authentication) to the BTS. The frame number ad K c move to a ciphering algorithm A5 and generates S1 (114 bits) which performs an exclusive OR operation between 114 bits of plain text and ciphering sequence S1 producing encrypted data for transmission from BTS to MS. Frame Number (22 Bits) S 1 (114 bits) K s (64 bits) A5 K i A8 S 2 (114 bits) 4 VLR RAND 5 Frame Number (22 Bits) S 1 (114 bits) A5 K s 2 (64 bits) HLR/AU Plain Text Ciphering Deciphering S 2 (114 bits) Plain Text Plain Text Deciphering Ciphering SIM Network Ciphering & Deciphering Plain Text /Engg/BE/Pre Pap/2013/ETRX/Soln/WC

5 3. (a) Reverse Link Structure of IS-95 The fig. below shows the IS-95 CDMA Reverse Link Structure. Reverse CDMA channel (1.23 MHz) (channel received by base station) Prelim Question Paper Solution Access channel 1 Access channel n Structure of IS-95 CDMA Reverse Traffic Channel The Reverse Link is separated from the Forward Link by 45 MHz. It is used by all the mobiles in the cell coverage area to transmit to the base station. The reverse channel as it appears at the base station receiver is a composite of all the outputs from all the mobiles in the base station s coverage area. The reverse channel structure allows a maximum of 94 channels which include A maximum of 32 different access channels and 62 different Traffic channels Traffic channel 1 Traffic channel 2 User addressed by long code PNs Access Channels It is used by the mobile to transmit control information (non-traffic information) to the base station. When it needs to respond to messages received on a paging channel. Since multiple mobiles may attempt access at the same time, the access channel utilises some form of slotted random access protocol for contention management. It uses a data rate of 4800 bps. The number of access channels in a cell is configurable generally one or two access channels, for each paging channel. Each access channel is identified by a distinct access channel long code sequence having an access number, a paging channels number associated with the access channel, and other system data. The messages carried by the access channel include : i) Registration message The mobile station sends this message to inform the base station about its location, status, identification, and other parameters required to register with the system. This is necessary so that the base station can page the mobile whenever a call is to be delivered to the mobile. ii) Order message Typical order messages include base station challenge, SSD (Shared Secret Data) update confirmation, mobile station acknowledgement, local control response, mobile station reject. iii) Data Burst Message This is a user generated data message sent by the mobile station to base station. iv) Origination message This message allows the mobile station to place a call sending dialled digits. Traffic channel 1R v) Page Response message This message is used by the mobile to respond to a page in continuation of the process of receiving a call. vi) Authentication Challenge Response Message This message contains necessary information to validate the mobile stations identity. 1113/Engg/BE/Pre Pap/2013/ETRX/Soln/WC 5

6 : B.E. WC Reverse Traffic Channels Information on the traffic channel includes the primary traffic (voice), secondary traffic (data), and signalling in frames 20 ms in length. Signalling information during a call is transmitted using blank and burst or dim and burst mode. Each Reverse Traffic channel is paired with a corresponding forward traffic channel. When the reverse traffic channel is used for signalling the following are some of the typical messages that can be sent : i) Order messages : They typically include base station challenge, SSD update confirmation, SSD update rejection, parameter update confirmation, mobile station acknowledgement, service option request, service option response, release, long code transition request, connect, continuous DTMF tone (start and stop), service option control, mobile station reject and local control. ii) Authentication challenge response message It contains information to validate the mobiles identity. iii) Data Burst message It is a user generated message sent by the mobile to the base station. iv) Pilot Strength Measurement message It sends information about other pilot signals that are not associated with the serving base station. v) Power Measurement Report message It sends FER [Frame Error Rate] statistics to the base station. vi) Send Burst DTMF message It uses two tone DTMF one low and one high frequency to represent a dialed digit and transmits digits to the base station. vii) Handoff Completion message It is the mobile response to the Handoff Direction message. 3. (b) Consider a mobile moving at a constant velocity v, along a path segment having length d between points X and Y, while it receives signals from a remote source S as illustrated in the figure alongside. The difference in path lengths travelled by the wave from source S to the mobile at points X and Y is given as : = d cos = v t cos where, t = Time required for the mobile to travel from X to Y = Angle between direction of radiation towards mobile and direction of motion of mobile is assumed to be same at points X and Y since the source is assumed to be very far away. The phase change in the received signal due to difference in the received path lengths is therefore given as : 2 2dcos 2 vt cos Also, = 2f d = t Illustration of Doppler Effect /Engg/BE/Pre Pap/2013/ETRX/Soln/WC

7 Hence the apparent charge in frequency, or Doppler shift f d is given as : Prelim Question Paper Solution f d = 1 v cos 2t 4. (a) From the above equation the following conclusions can be made ; i) If the mobile is moving towards the direction of arrival of the wave, ii) If the mobile is moving away from the direction of arrival of the wave, = 0 = 180 cos = 1, is positive The Doppler shift f d is positive. Hence the apparent frequency is increased cos = 1, is negative The Doppler shift f d negative. Hence the apparent frequency is decreased. In a practical mobile environment multi path components from a CW signal that arrive from different directions contribute to Doppler spreading of the received signal, thus increasing the signal bandwidth. Space Division Multiple Access (SDMA) Space division multiple access (SDMA) controls the radiated energy for each user in space. It can be seen from figure that SDMA serves different users by using spot beam antennas. These different areas covered by the antenna beam may be served by the same frequency (in a TDMA or CDMA system) or different frequencies (in an FDMA systems). Sectorized antennas may be thought of as a primitive application of SDMA. In the future, adaptive antennas will likely be used to simultaneously steer energy in the direction of many users at once and appear to be best suited for TDMA and CDMA base station architectures. The reverse link presents the most difficulty in cellular systems for several reasons. Fig. : A spatially filtered base station antenna serving different users by using spot beams First, the base station has complete control over the power of all the transmitted signals on the forward link. However, because of different radio propagation paths between each user and the base station, the transmitted power from each subscriber unit must be dynamically controlled to prevent any single user from driving up the interference level for all other users. Second, transmit power is limited by battery consumption at the subscriber unit, therefore there are limits on the degree to which power may be controlled on the reverse link. If the base station antenna is made to spatially filter each desired user so that more energy is detected from each subscriber, then the reverse link for each user is improved and less power is required. 1113/Engg/BE/Pre Pap/2013/ETRX/Soln/WC 7

8 : B.E. WC 4. (b) Adaptive antennas used at the base station (and eventually at the subscriber units) promise to mitigate some of the problems on the reverse link. In the limiting case of infinitesimal beam-width and infinitely fast tracking ability, adaptive antennas implement optimal SDMA, thereby providing a unique channel that is free from the interference of all other users in the cell. With SDMA, all users within the system would be able to communicate at the same time using the same channel. In addition, a perfect adaptive antenna system would be able to track individual multipath components for each user and combine them in an optimal manner to collect all of the available signal energy from each user. The perfect adaptive antenna system is not feasible since it requires infinitely large antennas. RAKE Receiver In CDMA spread spectrum systems, the chip rate is typically much greater than the flatfading bandwidth of the channel. Whereas conventional modulation techniques require an equalizer to undo the intersymbol interference between adjacent symbols. CDMA spreading codes are designed to provide very low correlation between successive chips. Thus, propagation delay spread in the radio channel merely provides multiple versions of the transmitted signal at the receiver. If these multipath components are delayed in time by more than a chip duration, they appear like uncorrelated noise at a CDMA receiver, and equalization is not required. The spread spectrum processing gain makes uncorrelated noise negligible after despreading. However, since there is useful information in the multipath components, CDMA receivers may combine the time delayed versions of the original signal transmission in order to improve the signal-to-noise ratio at the receiver. A RAKE receiver does just this it attempts to collect the time-shifted versions of the original signal by providing a separate correlation receiver for each of the multipath signals. Each correlation receiver may be adjusted in time delay, so that a microprocessor controller can cause different correlation receivers to search in different time windows for significant multipath. The range of time delays that a particular correlator can search is called a search window. The RAKE receiver, shown in Figure below, is essentially a diversity receiver designed specifically for CDMA, where the diversity is provided by the fact that the multipath components are practically uncorrelated from one another when their relative propagation delays exceed a chip period. r(t) IF or baseband CDMA signal with multipath Correlator 1 Correlator 2 Correlator M Z 1 Z 2 Z 3 m (t) An M-branch (M-finger) RAKE receiver implementation. Each correlator detects a time shifted version of the original CDMA transmission, and each finger of the RAKE correlates to a portion of the signal which is delayed by at least one chip in time from the other fingers A RAKE receiver utilizes multiple correlators to separately detect the M strongest multipath path components. 1 2 M Z T ()dt 0 Z /Engg/BE/Pre Pap/2013/ETRX/Soln/WC

9 Prelim Question Paper Solution 5. (a) The outputs of each correlator are then weighted to provide a better estimate of the transmitted signal than is provided by a single component. Demodulation and bit decisions are then based on the weighted outputs of the M correlators. In outdoor environments, the delay between multipath components is usually large and, if the chip rate is properly selected, the low autocorrelation properties of a CDMA spreading sequence can assure that multipath components will appear nearly uncorrelated with each other. (i) Channel Assignment Strategies Channel Assignment strategies can be classified into two categories : 1) Fixed Channel Assignment and 2) Dynamic Channel Assignment. 1) Fixed Channel Assignment : In Fixed Channel Assignment strategy, each cell is allocated a pre determined set of voice channels. Any call attempt within the cell can only be served by the unused channels in that particular cell. If all the channels in that cell are occupied, the call is blocked and the subscriber does not receive service. A variation of the fixed assignment strategy is the Borrowing strategy. In this approach a cell is allowed to borrow channels from a neighbouring cell if all of its own channels are already occupied. The MTSO supervises such borrowing procedure and ensures that the borrowing of a channel does not disrupt or interfere with any of the calls in the donor cell. 2) Dynamic Channel Assignment : In Dynamic Channel Assignment strategy, each time a call request is made, the serving base station requests a channel from the MTSO. The MTSO then allocates a channel to the requesting base station following an algorithm that takes into account the likelihood of future blocking within the cell, the frequency of use of the candidate channel, the reuse distance of the channel and other cost functions. Accordingly, the MTSO only allocates a given frequency if that frequency is not presently in use in the cell or any other cell which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference. Advantages : It reduces the likelihood of blocking, which increases the trunking capacity of the system. This is because all channels in a market are accessible to all the cells. Increased channel utilisation. Disadvantages : It increases the storage and computational load on the system. This is because the MTSO is required to collect real time data on channel occupancy, traffic distribution and Radio Signal Strength Indication (RSSI) of all channels on a continuous basis. 1113/Engg/BE/Pre Pap/2013/ETRX/Soln/WC 9

10 : B.E. WC 5. (a) 5. (b) (ii) Comparison between Small Scale Fading and Large Scale Fading Small Scale Fading It is caused by interference between two or more versions of the transmitted signal which arrive at the receiver at slightly different times. These waves called multipath waves combine at the receiver antenna to give a resultant signal which can vary widely in amplitude and phase. It is characterized by deep and rapid amplitude fluctuations which occur as the mobile moves over distances of just a few wavelengths. For narrow band signals small scale fading typically results in a Rayleigh fading distribution of signal strength over small Large Scale Fading It is caused by shadowing due to variations in both the terrain profile and the nature of the surroundings. In deeply shadowed conditions, the received signal strength at the mobile can drop well below that of free space. It is characterized by a gradual change in average signal strength as the mobile moves over large distances In urban environments, Large scale Fading is log Normally Distributed with a standard deviation of 10 db. distances. The signal fluctuates in a range of about 40 db (10 db above and 40 db below the average signal). Microscopic diversity techniques can be used Macroscopic diversity techniques are used to to prevent deep fades from occurring. prevent large scale fading. For e.g. For e.g. If two antennas separated by a fraction of a The mobile can greatly improve the average meter, one may receive a null while the other signal to noise ratio on the forward link by receives a strong signal. By selecting the best selecting a base station which is not shadowed signal at all times, a receiver can reduce small when others are. scale fading effects. Also, by using base station antennas that are sufficiently separated in space, the base station is able to improve the reverse link by selecting the antenna with the strongest signal from the mobile. UPPER LAYERS Figure 1 shows the layer structure of cdma2000. The upper layers contain three basic services: 1) Voice services Voice telephony services, including public switched telephone network (PSTN) access, mobile-to-mobile voice services, and Internet telephony. 2) End user data-bearing services Services that deliver any form of data on behalf of the mobile end user, including packet data (e.g., Internet protocol (IP) service), circuit data services (e.g., B-ISDN emulation services), and SMS. Packet data services conform to industry standard connection-oriented and connectionless packet data including IP-based protocols (e.g., transmission control protocol [TCP] and user datagram protocol [UDP]) and ISO/OSI connectionless interworking protocol (CLIP). Circuit data services emulate international standards-defined, connection-oriented services such as asynchronous (async) dial-up access, fax, V.120 rate-adapted ISDN, and B-ISDN services /Engg/BE/Pre Pap/2013/ETRX/Soln/WC

11 Prelim Question Paper Solution 3) Signaling Services that control all aspects of operation of the mobile. New for cdma2000 Fig. 1 : cdma2000 Layering Structure LOWER LAYERS The link layer provides varying levels of reliability and quality of service (QoS) characteristics according to the needs of the specific upper layer service. It gives protocol support and control mechanisms for data transport services and performs all functions necessary to map the data transport needs of the upper layers into specific capabilities and characteristics of the physical layer. The link layer is subdivided into sublayers: Link access control (LAC) (see Figure 2) Media access control (MAC) (see Figure 3). The LAC sublayer manages point-to-point communication channels between peer upper layer entities and provides a framework to support a wide range of different end-to-end reliable link layer protocols. Fig. 2 : cdma2000 Link Layer Fig. 3 : cdma2000 Media Access Control Layer 1113/Engg/BE/Pre Pap/2013/ETRX/Soln/WC 11

12 : B.E. WC 6. (a) The cdma2000 system includes a flexible and efficient MAC sublayer that supports multiple instances of an advanced-state machine, one for each active packet or circuit data instance. Together with a QoS control entity, the media access control sublayer realizes the complex multimedia, multiservice capabilities of 3G wireless systems with QoS management capabilities for each active service. The media access control sublayer provides three important functions: Media access control state Procedures for controlling the access of data services (packet and circuit) to the physical layer (including contention control between multiple services from a single user as well as between competing users). Best-effort delivery Reasonably reliable transmission over the radio link with radio link protocol (RLP) providing a best-effort level of reliability. Multiplexing and QoS control Enforcement of negotiated QoS levels by mediating conflicting requests from competing services and appropriately prioritizing access requests. The MAC sublayer provides differing QoS to the LAC sublayer (e.g., different modes of operation). It may be constrained by backward compatibility (e.g., for IS-95B signaling layer 2) and it may have to be compatible with other link layer protocols (e.g., for compatibility with non- IS-95 air interfaces or for compatibility with future ITU-defined protocol stacks) The MAC sublayer is subdivided into Physical layer independent convergence function (PLICF) Physical layer dependent convergence function (PLDCF) is further subdivided into * Instance-specific PLDCF * PLDCF MUX and QoS sublayer PLICF provides service to the LAC sublayer and includes all MAC operational procedures and functions that are not unique to the physical layer. Each instance of PLICF maintains service status for the corresponding service. PLICF uses services provided by PLDCF to implement actual communications activities in support of media access control sublayer service. (i) Power Control Sub Channel in IS95 CDMA To minimise the average BER (Bit Error Rate) for each user, IS95 strives to force each user to provide the same power level at the base station receiver. The base station reverse traffic channel receiver estimates and responds to the signal strength (actually, the signal strength and the interference) for a particular mobile. Since both the signal and interference are continuously varying, power control updates are sent by the base station every 1.25 ms. Power control commands are sent to each subscriber unit on the forward control sub channel now referred to as the Power Control sub channel Instructions are issued to the mobile on the Power Control sub channel to raise or lower its power in 1 db steps as follows : If the received signal is low a 0 is transmitted over the power control sub channel, thereby instructing the mobile station to increase its mean output power level. If the mobiles power is high, a l is transmitted to indicate that the mobile station should decrease its power level. The power control bit corresponds to two modulation symbols on the forward traffic channel. Power control bits are inserted after data scrambling /Engg/BE/Pre Pap/2013/ETRX/Soln/WC

13 Transmission Technique Power control bits are transmitted using puncturing techniques. Illustration Prelim Question Paper Solution (a) 1 power control bit = 2 data bits ms 1.25 ms = 24 scrambled traffic data bits 1/64 Long Code Used for Scrambling These 4 bits of previous long code specified the starting point of power control bits value 1011 = 11; the power control bits starts at position 11. Position of Power Control Bits The power control bits puncture the modulated data symbols at a rate of 800 bps a single power control bit replaces 2 symbols. During a 1.25 ms period, twenty four data symbols are transmitted, and IS95 specifies 16 possible power control group positions for the power control bit. Each position corresponds to one of the first 10 modulation symbols. 24 bits from the 1/64 long code decimator are used for data scrambling in a period of 1.25 ms The location of a power control bit in a power control group is determined by bits 2023 of the 1/64 long code in the previous 1.25 ms control group. (ii) Advantages and disadvantages of CDMA over GSM Advantages of CDMA over GSM: GSM uses high-speed wireless data technology known as GPRS (General Packet Radio Service), this offers a slower data bandwidth for wireless data connection than CDMA s 1xRTT(short for single carrier radio transmission technology)high-speed technology, which has the capability of providing ISDN speeds of as much as 144Kbps. Increased cellular communications security. Simultaneous conversations. Increased efficiency, meaning that the carrier can serve more subscribers. Smaller phones. Low power requirements and little cell-to-cell coordination needed by operators. Extended reach - beneficial to rural users situated far from cells. 1113/Engg/BE/Pre Pap/2013/ETRX/Soln/WC 13

14 : B.E. WC 6. (b) Disadvantages of CDMA over GSM: The CDMA s 1xRTT requires a dedicated connection to the network, means that data calls made on a GSM handset don t block out voice calls like they do on CDMA phones. GSM uses "SIM cards" to store your account information and contact list, so you can switch phones. CDMA devices are all proprietary handsets and cannot be reprogrammed for another carrier. This means you cannot purchase a CDMAS print phone and use it with your local carrier, even if they also use CDMA. CDMA cannot offer international roaming, a large GSM advantage. GSM System Architecture Network and Switching Subsystem (NSS) The NSS includes : i) MSC as the central unit, ii) Three different databases called Home Location Register (HLR), Visitor Location Register (VLR) - and Authentication Centre (AuC). It handles the switching of GSM calls, between external networks (PSTN, ISDN) and BSCs in the radio subsystem. It is also responsible for managing and providing external access to several customer databases. MSC The MSC performs the following major functions: i) Call set up, supervision and release. ii) Digit collection and translation. iii) Call routing. iv) Billing information collection. v) Mobility management. vi) Paging and alerting. vii) Management of radio resources during a call. viii)echo cancellation. ix) Manage connections to BSS, other MSCs and PSTN/ISDN. x) Interrogation of appropriate registers (VLRs/HLRs). Databases Home Location Register (HLR) : The HLR represents a centralised data base that has a permanent data fill about the mobile subscribers in a large service area (generally one per GSM network operator). The HLR is kept updated with the current locations of all its mobile subscribers, including those who may have roamed to another network operator within or outside the country /Engg/BE/Pre Pap/2013/ETRX/Soln/WC

15 Prelim Question Paper Solution 7. (a) Besides the up to date location information for each subscriber, which is dynamic, the HLR maintains the following subscriber data on a permanent basis * International Mobile Subscriber Identity (IMSI), a unique number used to identify each home user. * Service subscription information * Service restrictions * Supplementary services subscribed to * Mobile terminal characteristics * Billing/Accounting information Visitor Location Register (VLR) : The VLR is a database which temporarily stores the IMSI and customer information for each roaming subscriber who is visiting the coverage area of a particular MSC. Generally there is only one VLR per MSC. The VLR maintains information about subscribers that are currently physically operational in the region covered by the MSC/VLR. The temporary subscriber information which resides in the VLR includes : * Features currently activated * Temporary Mobile Station Identity [TMSI] * Current location information about the MS (e.g. locations areas and cell identities). Once a roaming mobile is logged in the VLR, the MSC sends necessary information to the visiting subscriber s HLR so that calls to the roaming mobile can be appropriately routed over the PSTN by the roaming user s HLR. Even if the subscriber is in the area covered by its home MSC it is also represented in the switching centre s VLR for consistency. Authentication Centre [AuC] As the radio interface and mobile stations are very vulnerable to malicious misuse, a separate Authentication Centre [AuC] has been defined to protect user identity and data transmission. The Authentication Centre is a strongly protected data base which is generally associated with the HLR. It contains the algorithms for authentication as well as the keys for encryption and generates the values needed for user authentication in the HLR. Equipment Identity Register (EIR) The Authentication Centre contains a register called the Equipment Identity Register (EIR). The EIR maintains information to authenticate terminal equipment so that fraudulent, stolen or non type approved terminals can be identified and denied service. The information is in the form of White [Valid Terminals], Gray [Malfunctioning Terminals] and Black [Stolen or Locked Terminals] lists that may be consulted by the network, when it wishes to confirm the authenticity of the terminal requesting service. Discontinuous Transmission (DTx) used in GSM Discontinuous Transmission (DTx) technique is based on detecting voice activity and switching ON the transmitter only during periods when there is active speech to transmit and switching OFF the transmitter during silent periods. Advantages Switching off the transmitter during silent periods reduces interference in the air. This allows the use of smaller frequency reuse clusters leading to a gain in spectrum efficiency of up to 50%. Battery power consumption is reduced in the mobile terminal since transmitter is OFF during silent. In adaptive threshold Voice Activity Detector (VAD) algorithm is used in the GSM radio equipment to detect silent periods and interrupt transmission. At the receiving end, the DTx is 1113/Engg/BE/Pre Pap/2013/ETRX/Soln/WC 15

16 : B.E. WC detected and empty frames are filled with comfort noise. The VAD algorithm and its implementation need to be carefully designed to minimise speech clipping and the resulting degradation in speech quality. 7. (b) 7. (c) 7. (d) Near Far Effect in CDMA A mobile station close to the base station has much lower path loss than mobiles that are far away from the base station. Hence, if all mobile stations were to use the same transmit power then users near the base station are received with high power while those far away are received with low power. This would cause the mobiles close to the base station to effectively jam the signals from the mobiles far away from the base station. This is the Near Far effect in CDMA. The Near Far effect is more predominant when many mobile users share the same channel as is the case in CDMA. To overcome the Near Far problem, Power control is used in most CDMA implementations. Power control is provided by each base station in a cellular system and ensures that each mobile within the base station coverage area provides the same signal level to the base station receiver. This solves the problem of a nearby subscriber overpowering the base station receiver and drowning out the signals of far away subscribers. Walsh Code Administration IS-95 A/B uses fixed-length 64-chip Walsh codes. The new rate sets in cdma2000 require variable-length Walsh codes for traffic channels. In 3G1X, the Walsh codes used are from 128 chips to 2 chips in length. The F-FCH Walsh code is fixed (128 chips for RS3 and RS5, and 64 chips for RS4 and RS6), whereas, the length of the Walsh codes for F-SCH decreases as the information rate increases to maintain a constant bandwidth of the modulated signal. In addition to different Walsh code lengths, the coordination of allocation of Walsh codes across the 2G and the 3G system is necessary for overlay systems. The algorithm must ensure that Walsh codes assigned for different rate supplemental channels are always orthogonal to each other as well as to the fundamental traffic channels, paging channels, sync channel, and pilot channel. For example, if an all 0s 4-chip Walsh code ( ) is assigned, then there are two 8-chip Walsh codes that are not to be assigned at the same time ( , ); the remaining six 8-chip codes can be used since they are all orthogonal to it. By induction, four 16-chip, eight 32-chip, chip and chip codes must also be set aside to maintain orthogonality. The 2G and 3G Walsh code assignments must be coordinated to ensure that assigning the longer-lengths codes does not block out all of the shorter codes. GSM Channel Coding Channel coding improves transmission quality when interference, multi path fading and Doppler shift are encountered. As a result, the bit error rate and frame error rate are reduced, but throughput is also reduced. Four kinds of codings are used in GSM : i) Convolutional codes (L, k) are used to correct random errors, k is the input block bits and L is the output block bits. Conventional codes have three different rates in GSM: a. The one half late (L/k = 2) b. The one third rate (L/k = 3 and c. The one sixth rate (L/k = 6) ii) Fire codes (L, k) are used as a block code to detect and correct a single burst of errors, where k is the information bits and L is the coded bits /Engg/BE/Pre Pap/2013/ETRX/Soln/WC

17 Prelim Question Paper Solution iii) Parity check codes (L, k) are used for error detection. L is the bits of a block, k is the information bit, L-k is the parity check bits. iv) Concatenation codes use conventional code as an inner code and fire code as an outer code. Both the inner code and outer code reduce probability of error and correct most of the channel code. GSMs speech code is sent at a rate of 13 Kbps, which represent 260 bits in each 20 ms speech block. After channel coding each block contains 456 bits and the transmission rate is 22.8 Kbps or 114 bits per time slot. Adding the overhead bits such as tail bits (6), training bits (26), flag bits (2) and guard time bits (8.25), the total bits of a traffic channel is bits in one time slot of ms, as shown in the fig below. Normal Burst TDMA Frame (4.615 ms) T B SF = 3 Encrypted bits Training sequence Encrypted bits 3 58 (Bits) TDMA frame and normal burst T B 8.25 Guard 1113/Engg/BE/Pre Pap/2013/ETRX/Soln/WC 17