A COMPARISON STUDY OF 3G SYSTEM PROPOSALS: CDMA2000 VS. WCDMA

Transcription

1 A COMPARISON STUDY OF 3G SYSTEM PROPOSALS: CDMA2000 VS. WCDMA Emre A. Yavuz and Dr. Victor Leung University of British Colombia, 2356 Main Mall, Vancouver, BC, CANADA, V6T 1Z4 ABSTRACT With the introduction of the third generation systems (IMT-2000), second generation capabilities (voice and low/medium rate data) are extended adding multimedia capabilities to second-generation platforms such as support for high bit rates and introduction of packet data/ip access. In this paper the technical features of the two IMT-2000 radio interface proposals, ARIB/ETSI s WCDMA and TIA s cdma2000, are discussed and a comparison is being made regarding their performances. After the similarities are given briefly, the study is more focused on the differences that are affecting the performance. Main issues of the differences are examined in detail to find out the benefits or the drawbacks that those issues bring to each proposal. KEY WORDS WCDMA, cdma2000, 3G Proposals, Standardization, radio access networks, IMT 2000, TIA TR45.5. I. Introduction Wireless communication is taking the lead role in the communication business, as Internet is growing explosively and the users of the services like data and multimedia are demanding that those services are also available on the move. Research has been going on for more than a decade to find enabling techniques to increase capacity, data rates and thus to introduce multimedia and new supported services into mobile communications. These research efforts have been aligned with efforts in the International Telecommunication Union (ITU) and other bodies to find standards and recommendations to develop a mobile communication network which has an access to multimedia capabilities and service quality similar to the fixed network. With the introduction of the third generation systems (IMT-2000), second generation capabilities (voice and low/medium rate data) are extended adding multimedia capabilities to second generation platforms such as support for high bit rates and introduction of packet data/ip access. European Telecommunications Standards Institute (ETSI) / Special Mobile Group (SMG), has been responsible for UMTS standardization since the early 1990s. The wideband CDMA (WCDMA) scheme has been developed as a joint effort between ETSI and ARIB during the second half of 1997 and was chosen in January 1998 as the basic technology for the UMTS terrestrial radio access (UTRA) system. On the other hand, in the United States in March 1998, the TIA (Telecommunications Industry Association) TR45.5 committee, adopted a framework for wideband CDMA, backward compatible with IS-95, which is called cdma2000. Originally under TIA TR45.5, the responsibility of the cdma2000 work has been moved to a newly formed organization called The Third Generation Partnership Project 2, or 3GPP2, thereby benefiting from the expertise of specialists from Chinese (CWTS), Japanese (ARIB and TTC), Korean (TTA), and North American (TIA) standards bodies. The ITU approved the cdma2000 radio access system as the CDMA Multi- Carrier (MC) member of the IMT-2000 family of standards. This paper will discuss the similarities and the differences between the two radio interface proposals, ARIB/ETSI s WCDMA and TIA s cdma2000, for future mobile multimedia communications (IMT-2000) by comparing the technical features of each proposal. After the similarities between the proposals are given briefly in the following section, the study will be focused on the differences that are affecting the performance and examining the main issues in detail to find out the benefits or the drawbacks that those issues bring to each proposal. Only the frequency division duplexing (FDD) mode will be considered in this paper. II. Similarities between cdma2000 and WCDMA Although they are developed by different organizations, there are many similarities between the two systems, since both employ key design concepts of IS-95. Below is a brief summary of the important similarities both systems have [3]: Coherent downlink and reverse link. Fast power control on forward (downlink) link as well as reverse (uplink) link. Variable length orthogonal Walsh sequences used for forward (downlink) and uplink channelization in order to separate users..

2 Complex QPSK spreading on downlink. Convolutional codes used as baseline (identical polynomials). Variable spreading factor to achieve higher data rates and to support blind rate estimation for simple services (e.g. voice). In current WCDMA systems this is done by TFIC (transport format Combination Indicator) Soft handover and mobile assisted inter-frequency hard handoff procedures. Turbo codes for higher data rates. It should be noted that while the concepts are similar, the details of the physical layers, which are going to be mentioned later, are different because of the separate standardization efforts. Nevertheless, although it is mentioned that both proposals employ key design concepts of IS-95, there are also differences in parameters like chip rate selection, pilot structure, base station synchronization, and frame length as well as the details of physical layers, procedures and etc. This is also due to another fact that WCDMA has to be compatible with GSM while cdma2000 has to be compatible with IS-95. III. Differences between WCDMA and cdma2000 Differences between the two systems include parameters like: a) Forward (downlink) link RF channel structure - beside the direct spread technique used for both channel structures, multicarrier cdma technique is also used for cdma2000. In this technique, multiple narrowband DS CDMA waveforms, each at a distinct carrier frequency, are combined to yield a composite wideband CDMA signal. Among the advantages of such an approach is the ability to achieve the same type of system performance that a conventional signal carrier, wideband CDMA signal, would provide, such as diversity enhancement over a multipath channel; however this is achieved without the need for a contiguous spectral band over which to spread. It also allows backward compatibility with IS-95 systems. On the other hand as it s indicated in the cdma2000 proposal [2] and shown in Table 1, DS is more spectrally efficient in the vehicular environment than the multicarrier method, whereas it s only slightly better or comparable in the pedestrian and indoor environments. b) Chip rates - the most discussed and debated parameter, which will be mentioned later in detail. c) Frame lengths - 10 ms/20 ms (optional) for WCDMA, 20 ms for data and control / 5 ms for control information on the fundamental and dedicated control channel of cdma2000. Coding operations are performed based on Tranmision Time Interval (TTI), 10 to 80 ms. This parameter will also be mentioned later in detail. TABLE 1. Spectrum efficiency comparison of MC and DS modes in cdma2000. Service Voice 9.6 kbps 1% FER Spectrum Efficiency (RL/FL) Environment (users/mhz/cell) for voice (Mbps/MHz/cell) for data Multicarrier Direct (MC) Spread (DS) Vehicular 29 / / 45.1 Pedestrian 42.1 / / 45.3 Indoor 38.9 / / 33.6 Mixed 34.1 / / 46.1 Vehicular / / Pedestrian / / Packet Data 76.8 kpbs 10% FER Indoor / / d) Coherent detection - while there is only common pilot for forward (downlink) link in WCDMA, there is a common continuous and common and dedicated auxiliary pilot for forward (downlink) link in cdma2000. This parameter will be discussed later in detail regarding the differences in the physical layer parameters of both systems. e) multiplexing in forward (downlink) link - A combined IQ and code multiplexing solution (dual channel QPSK) is used in WCDMA uplink to avoid electromagnetic compatibility (EMC) problems with discontinuous transmission (DTX)., pilot, fundamental and supplemental channels are also code multiplexed in cdma2000 uplink. However, in the downlink of WCDMA, the dedicated channels (DPDCH and DPCCH) are time multiplexed as the EMC problem caused by discontinuous transmission is not considered difficult because of the parallel transmitted signals to several users and having the base stations not so close to other electrical equipment, like hearing aids. Nevertheless, this last issue may also become a more important concern in the future when closer deployment of new base stations is considered. f) Power control - both systems have similar open and fast closed loops other than the rates that they are

3 FORWARD CDMA CHANNEL for Spreading Rates 1 and 3 (SR1 and SR3) Assignment Power Pilot s s Sync s Broadcast Paging s (SR1) Quick Paging Forward Pilot Transmit Diversity Pilot Aux. Pilot Auxiliary Transmit Diversity Pilot Fundamental Auxiliary Pilot Supplemental Code s (Radio Configurations 1-2) Figure 1. Forward channel structure of cdma2000. using: 1.5 khz for WCDMA vs. 800 Hz in cdma2000. g) Spreading for both forward (downlink) and reverse (uplink) link - variable length orthogonal sequences for channel separation and Gold sequences 2 18 for cell and user separation are used in downlink for WCDMA while variable length Walsh sequences for channel separation and M-sequence 2 15 (same sequence with time shift utilized in different cells, different sequence in I&Q channel) are used in downlink for cdma2000. In uplink variable length orthogonal sequences for channel separation and Gold sequences 2 24 for user separation (different time shifts in I and Q channel) are used for WCDMA while variable length orthogonal sequences for channel separation, M- sequence 2 15 (same sequence for all users, different sequences in I&Q channels) and M-sequence 2 41 for user separation (different time shifts for different users) are used for cdma2000. h) Base station synchronization - WCDMA utilizes asynchronous base station operation which is composed of 3 step parallel code search for base station detection and slot/ frame, while cdma2000 uses synchronous base station operation that is through time shifted PN correlation. IV. Selection of Chip Rates At first, a chip rate of Mbps was assigned for WCDMA, while a rate of Mbps was proposed for cdma2000. But then, despite the fact that was claimed by the WCDMA proponents that the lower cdma2000 rate degrades the performance, this rate has been changed to 3.84 Mbps after considering the inefficiencies that deteriorated the system capacity. The case of the deployment of scenarios, is mentioned in [4]. On the other hand, it has to be highlighted that, chip rate, alone does not determine the overall system capacity. V. Frame Length The WCDMA RTT uses a 10 ms frame length, however 20 ms is also considered as an optional frame length. While 5 and 10 ms frame lengths can be appropriate for certain type of control messages and low-delay data applications, 20 ms based frame length is considered as the basis for voice and data applications. Since one of the common objectives is to reduce the endto-end delay, it s the 10 ms frame length to be chosen to reduce it regarding the target set by ITU. Both frame lengths couldn t meet the original end-to-end delay of 40 ms set by ITU.

4 REVERSE CDMA CHANNEL for Spreading Rates 1 and 3 (SR1 and SR3) Phy. Data s (RC 1 or 2) Enhanced Pilot Fundamental Power Subchannel However the ITU requirement has been relaxed, taking the constraint on using the 20 ms frame length away, while the end-to-end delay with 10 ms frame length will still be less. In spite of this advantage, it has been demonstrated that 10 ms frame reduces time diversity and increases E b /N o requirements. At 30 km/h, up to 2 db additional E b /N o may be required, based on simulation results on RL, 800 bps PC, and two antennas per one path per antenna [3]. Another advantage of using 20 ms frames is the overhead percentage (convolutional tail bits, CRC etc.). While the overhead is 11% for 8.6 kbps voice services (per cdma2000 numerology) with a 20 ms frame length, it s 20% with a 10 ms frame length for 8kbps voice services (per WCDMA/NA numerology) [3]. Existing vocoders are widely deployed based on 20 ms frames, which seem to offer the best tradeoff between vocoder delay and interleaver depth. On the other hand, 5 ms frame length is also used for control information on the fundamental and dedicated control channel in cdma2000. VI. Physical Layers The forward and reverse channel structures of cdma2000 and WCDMA are given in figures 1,2,3 and 4. i. Pilot Structures Supplemental Code s Figure 2. channel structure of cdma2000. In the current WCDMA description, time multiplexed pilot symbols are used in the downlink as opposed to code multiplexed pilot symbols in the uplink. Time multiplexed common pilot channel (CPICH) of WCDMA has two types of channels named Primary and Secondary which are similar to forward pilot channel (F-PICH) and forward common auxiliary pilot channel (F- CAPICH) respectively in cdma2000. In addition to these, cdma2000 has forward dedicated auxiliary pilot channel (F-DAPICH) to increase the coverage or data rate towards a particular mobile station, forward transmit diversity pilot channel (F-TDPICH) to be used for transmit diversity coherent detection and forward auxiliary transmit diversity pilot channel (F- ATDPICH) to be used for transmit coherent detection for a group of users. In WCDMA, the pilot symbols are transmitted by the code multiplexed dedicated physical control channel (DPCCH) in the uplink. However, cdma2000 has a pilot channel (R- PICH), which consists of a fixed reference value (pilot symbol) and multiplexed forward power control (PC) information. Although this is time multiplexed, these pilot symbols and the muxed power symbols are all sent with the same power level in order not to arise EMC problems. ii. Synchronization channels WCDMA has a synchronization channel (SCH), which consists of two sub channels named primary and secondary corresponding to forward synchronization channel (F- SYNC) in cdma2000. The advantage of two subchannels is the limitation of the search of long codes to a subset of all the codes. The unmodulated primary SCH is used to acquire the timing for secondary SCH, while the modulated secondary SCH code carries information about the long code group, which the long code of the BS belongs to. iii. Broadcasting channels In WCDMA, broadcast channel (BCCH) has a fixed rate of 32 kbps whereas the broadcast channel (F- BCCH) of cdma2000 has a frame containing 744 bits, transmitted at 19.2, 9.6 or 4.8 kbps. The transmission of 744 bits for once, twice or four times has a benefit of reducing battery consumption since a terminal that is able to successfully receive the first copy of the broadcast frame can stop monitoring the channel after.

5 FORWARD WCDMA CHANNEL Sync Primary Downlink Shared s Page Indicator Secondary Acquisition Indicator Primary Pilot Broadcast Paging Secondary Pilot Forward Physical Data Physical (Pilot + Power control + Transport format) Figure 3. Forward channel structure of WCDMA. iv. Paging channels Paging channel (PCH) of WCDMA is carried by secondary common control physical channel (S-CCPCH) whose rate may be different for different cells and can be set to provide the required capacity for PCH and FACH in each specific environment. The correspondent paging channel in cdma2000 is F-PCH, which is backward compatible with IS- 95. In addition to that, cdma2000 has a quick paging channel (F-QPCH), which is used for informing mobiles to listen to the paging, or forward common control channels when it s necessary to save power and maximize battery life. Usage of quick paging channel is more advantageous when it s used with synchronized base stations. v. Other downlink channel structures With an exception of slight differences, dedicated physical control and data channels (DPCCH and DPDCH) in WCDMA correspond to forward dedicated control channel (F- DCCH) and fundamental channel (F-FCH) in cdma2000 respectively. Other than those above, there are some other proposed channels for both systems, which can t be directly corresponded to each other but have some functions in common. In WCDMA, there is physical downlink shared channel (PDSCH), which is shared by users based on code multiplexing and acquisition indication channel (AICH). In cdma2000, there are forward common power control channel (F-PCCH), which supports the transmission of multiple power control bits associated with various reverse common control channels (R-CCCH) and forward common control channel (F-CCCH) which is used for communication of layer 3 and MAC messages from BS to MS. In addition to those in cdma2000, there is another channel called forward common assignment channel (F-CACH), which can be related to forward access channel (FACH) in WCDMA. vi. Uplink channel structures In the uplink, dedicated physical data channel (DPDCH) in WCDMA correspond to both reverse fundamental and supplemental channels (R-FCH and R-SCH) of cdma2000. physical control channel (DPCCH) of WCDMA corresponds to reverse pilot and dedicated control channels (R- PICH and R-DCCH) of cdma2000. Physical random access channel (PRACH) and physical common packet channel (PCPCH) of WCDMA are correspondents for reverse access channel (R-ACH) and reverse common control channel (R-CCCH) of cdma2000, respectively. cdma2000 has an enhanced access channel (R-EACH) for random access control information. Although we ve used the term correspondence for the channels that are proposed for similar functions in both systems, it should also be noted that there are slight differences in their implementations and functioning. VII. Synchronization Base stations are synchronized in cdma2000 while WCDMA utilizes asynchronous BS operation. A comparison of asynchronous and synchronous base stations is given in [3] in detail and will be shortly

6 REVERSE WCDMA CHANNEL Random Physical Data Packet Physical discussed here: synchronized base stations with time -shifted common pilots permit fast one-step correlation resulting in quick acquisition and neighbor detection. Complexity in the mobile station is kept to a minimum since there is no need to adjust the timing of individual mobile in soft hand-off. experience with current commercial CDMA networks has shown that synchronized networks are reliable and efficient. Utilizing asynchronous BS operation, WCDMA requires highly stable timing references in the infrastructure for timing alignment. synchronized base stations permit the operation of common overhead and signaling channels (such as common control and paging channels) into soft handoff. Those techniques cannot be used in asynchronous networks without the expense of additional complexity and reduced performance that outweigh any benefits. MS terminal battery saving techniques such as Quick Paging channel work best with synchronized base stations whereas with asynchronous base stations, different cells are not time aligned and the MS must wake up multiple times and for larger periods to monitor the different base stations. performance issues like hard, inter-frequency handoff are still to be resolved regarding asynchronous operation. With asynchronous cells in WCDMA, it turns out that prior to handling off to the new frequency, the mobile has to shift to the new frequency channel for a delay of 40 ms, which is long and performance degrading, before decoding. although it s often claimed that an asynchronous system can be operated in synchronous mode and benefit from synchronous operation while it can operate asynchronously when timing reference is not available, this will lead to a higher mobile complexity, reduced capacity (due to non-orthogonal acquisition channels and large signaling overhead) and increased designed cycle. VIII. Conclusion This paper introduced a summary of comparison between the two dominant CDMA pro posals, cdma2000 and WCDMA for third generation systems. REFERENCES [1] 3 rd Generation Partnership Project; Technical Specification Group Radio Network; Physical channels and mapping of transport channels onto physical channels (FDD) (3G TS version 3.0.0). Pilot Subchannel Power Subchannel Transport Format Indicator Figure 4. channel structure of WCDMA [2] TIA/TR45.5, The cdma2000 ITU-R RTT Candidate Submission ( 1998). [3] Rao, Y.S. and Kripalani, A., cdma2000 Mobile Radio for IMT 2000, ICPWC, Jaipur, India, 1999, [4] CDG: 3G:Detailed Info: 3rd Generation Sy stems White Paper, asp [5] ETSI Tech. Rep , UMTS Terrestrial radio access: Concept evaluation (UMTS 30.06) (Version 3.0.0, Dec. 1997). [6] Willeneger, S., cdma2000 Physical Layer: An Overview, Journal of Communications and Networks, Vol.2, No: 1, March 2000, [7] Prasad Ramjee, Ojanpera Tero, An Overview of CDMA Evolution toward Wideband CDMA, IEEE Communication Surveys, [8] Dahlman E., Beming P., Knutsson J, Ovesjo, F., Persson, M., and Roobol, C., WCDMA The Radio Interface for Future Mobile Multimedia Communications, IEEE Transactions on Vehicular Technology, Vol. 47, No 4, Nov. 1998, [9] Adachi, F., Sawahashi, M., Suda, H., Wideband DS-CDMA for Next Generation Mobile Communication Systems, IEEE Communications Magazine, Sep. 1998, [10] Dahlman E., Gudmundson B., Nilsson M, and Skold J. UMTS/IMT-2000 Based on Wideband CDMA, IEEE Communication Magazine, September 1998, [11] Knisely, D., N., Kumar, S., Laha, S., and Nanda, S., Evolution of Wireless Data Services : IS-95 to cdma2000, IEEE Communication Magazine, October 1998,