WIRELESS COMMUNICATIONS AND MOBILE COMPUTING Wirel. Commun. Mob. Comput. 2003; 3:987 992 (DOI: 10.1002/wcm.134) Third generation WCDMA radio evolution Harri Holma*,y and Antti Toskala Nokia Networks, IP Mobility Networks, PO Box 301, FIN-00045 Nokia Group, Finland Summary Third Generation Partnership Project (3GPP) produced the first full version of the WCDMA standard at the end of 1999. This release, called Release 99, contains all the necessary elements to meet the requirements for IMT-2000 technologies, including 2 Mbps data rate with variable bit-rate capability, support of multi-service, QoS differentiation and efficient packet data. The Release 5 specifications were created in March 2002 and they contain downlink packet data operation enhancement, under the title high speed downlink packet access (HSDPA). HSDPA utilizes Hybrid ARQ and higher order modulation for improving data-spectral efficiency and for pushing bit rates beyond 10 Mbps. The further 3GPP releases will study the enhancements of packet-data performance in uplink. Other important features in future 3GPP releases include advanced antenna technologies and WCDMA standard for new spectrum allocations. The paper describes the main solutions of 3GPP WCDMA standard in more detail. Copyright # 2003 John Wiley & Sons, Ltd. KEY WORDS: third generation partnership project (3GPP); high speed downlink packet-access (HSDPA); WCDMA Release 99; beamforming 1. 3GPP Release 99 Commercial WCDMA Deployment The WCDMA system deployment has reached the level where first operators in Asia and Europe have launched their networks for commercial operation during 2003. The WCDMA bit-rate capabilities reach beyond 2 Mbps in the first version of the specifications, with later evolution reaching up to 10 Mbps downlink capability. In the first phase terminals, up to 384 kbps data-transmission capability is available with the support for the new data services. A single physical connection (channel) can support more than one service, even if the services have different quality requirements. This avoids multi-code transmission in case there are two services running simultaneously, as they can be dynamically multiplexed on a single physical resource. This also allows easy terminal support for service multiplexing such as simultaneous speech and multimedia messaging. WCDMA Release 99 supports variable user data rates, Bandwidth on Demand (BoD). Each user is allocated frames of 10 ms duration, during which the user data rate is kept constant. However, the data capacity among the users can change from frame to frame. This radio-capacity allocation is controlled and co-ordinated by the radio resource management functions in the network to achieve optimum throughput for packet-data services and to ensure sufficient QoS. WCDMA Release 99 supports QoS mechanisms to provide services from low-delay conversational class to flexible background class. These QoS mechanisms improve the efficiency of the air interface as the radio network can optimize the resource allocations *Correspondence to: Harri Holma, Nokia Networks, IP Mobility Networks, PO Box 301, FIN-00045 Nokia Group, Finland. y E-mail: harri.holma@nokia.com Copyright # 2003 John Wiley & Sons, Ltd.
988 H. HOLMA AND A. TOSKALA Fig. 1. Variable bit rate and service multiplexing in WCDMA Relelease 99. according to the QoS requirements of each service, thus avoiding over-dimensioning of the network. The variable bit-rate capability is illustrated in Figure 1 and QoS mechanism in Figure 2. WCDMA Release 99 supports several transport channels for flexible packet data operation. The following alternatives exist in WCDMA downlink for packet data: dedicated channel (DCH) downlink shared channel (DSCH) forward access channel (FACH) The DCH can be used for any type of the service up to 2 Mbps and has a fixed spreading factor in the downlink. The DCH is fast power controlled and may be operated in macrodiversity as well. The DSCH has been developed to always operate together with an associated DCH. This allows defining the DSCH channel properties to be best suited for packet data while leaving the data with tight delay budget to be carried by DCH. The DSCH, in contrast to DCH, has dynamically varying spreading factor informed to the terminal on a 10 ms frame-by-frame basis with physical layer signalling carried on the DCH. This allows dynamic multiplexing of several users to share the DSCH code resource and thus optimizing the orthogonal code resource and base station hardware usage in the downlink. DSCH can utilize power control based on the associated DCH. DSCH is not operated in soft handover. Fig. 2. QoS mechanisms provides more efficient utilization of radio resources. The FACH can be used for downlink packet data as well. The FACH is operated on its own, and it is sent with a fixed spreading factor and typically with rather high power level since it does not support fast power control as power control feedback in the uplink is not available. FACH does not use soft handover. The uplink counterpart for FACH is RACH, which is intended for short duration, 10/20 ms packet-data transmission in the uplink. There exists another uplink option also, named Common Packet Channel (CPCH), to enable longer packet bursts up to 640 ms with power control, but that is not foreseen to be part of the first phase WCDMA network deployment nor the terminal implementations. In WCDMA downlink, there exist advanced performance improvement methods utilizing transmit diversity, where base station transmission utilizes two transmit antennas while terminal reception is done with a single antenna to keep the terminal complexity reasonable. Release 99 contains two modes of operation: open loop transmit diversity, intended to be used e.g. on common channels; and closed loop transmit diversity, which relies on the uplink feedback to adjust phase or phase and amplitude between the two transmit antennas. This is applicable only when there is feedback existing in the uplink, which means in Release 99 DCH/DSCH. Typical WCDMA Release 99 spectral efficiency is approximately 0.2 bits/s/hz/cell in macrocells and higher in microcells. The gain of transmit diversity in spectral efficiency is typically 25% 40%[1]. WCDMA Release 99 provides smooth interworking with the GSM/EDGE networks including common core network, multimode terminals, inter-system handovers and harmonized QoS parameters. The interworking is illustrated in Figure 3. GSM and WCDMA are two complementary radio access systems, one optimized for existing GSM spectrum and the other for new UMTS spectrum. The standardization of both WCDMA and GSM/EDGE takes place in
THIRD GENERATION WCDMA RADIO EVOLUTION 989 Fig. 4. WCDMA HSDPA operation. 2 ms frame length, while with DSCH the frame length is 10, 20, 40 or 80 ms; fixed spreading factor 16 with maximum of 15 codes, while with DSCH the spreading factor may vary between 256 and 4; and HS-DSCH supports also 16QAM modulation, in addition to QPSK of DSCH. Fig. 3. Harmonized GSM/EDGE and WCDMA standards. 3GPP. Further alignment of the WCDMA and GSM/ EDGE radio access network architecture has taken place in Release 5 as the Iu-mode was introduced to GSM/EDGE radio access network. 2. 3GPP Release 5 High Speed Downlink Packet Access for Higher Bit Rates For Release 5 specifications, release 03/2002, the High Speed Downlink Packet Access (HSDPA) was completed, which is the most significant radio-related update since the release of the first version of 3GPP WCDMA specifications. HSDPA is based on distributed architecture where the processing is closer to the air interface at the base station (Node B) for low delay link adaptation. The key technologies used with HSDPA are: Node B based scheduling for the downlink packet data operation; higher order modulations; and hybrid ARQ. The HSDPA principle with Node B based scheduling is illustrated in Figure 4. The HSDPA operation is carried out on the HS- DSCH (High Speed Downlink Shared Channel), which has fundamental differences when compared to DSCH in Release 99. The key differences to the DSCH are: The HS-DSCH is accompanied by the HS-SCCH (High Speed Shared Control Channel) which carries the necessary information for demodulation of the HS-DSCH such as what codes to despread and what is the modulation and necessary information for HARQ, whether the transmission is a new one or a retransmission. As HSDPA operation is intended for high datarate services, only Turbo coding is supported on HS- DSCH. HSDPA extends the WCDMA bit rates up to 10 Mbps. The higher peak bit rates are obtained with higher order modulation, 16-QAM, and with adaptive coding and modulation schemes. The theoretical bit rates bit rates are shown in Table I. The maximum bit rate with QPSK modulation is 5.3 Mbps and with 16-QAM 10.7 Mbps. With even lower rate, coding up to 14.4 Mbps can be achieved. The terminal capability classes start from 900 kbps and 1.8 Mbps with QPSK being the only modulation to support low-cost implementation without 16-QAM requirements. The highest capability class supports the maximum bit rate of 14.4 Mbps. The HSDPA concept offers over 100% higher peak user bit rates than Release 99 in practical deployments. HS-DSCH bit rates are comparable to Digital Subscriber Line, (DSL) modem bit rates. The mean Table I. Theoretical bit rates with HSDPA. Modulation Coding rate Max. bit rate 1/4 1.8 Mbps QPSK 2/4 3.6 Mbps 3/4 5.3 Mbps 16-QAM 2/4 7.2 Mbps 3/4 10.7 Mbps
990 H. HOLMA AND A. TOSKALA Table II. Cell throughput with Release 99 DSCH and R5 HSDPA in macrocells [2]. DSCH HSDPA Cell throughput 1.4 Mbps 2.7 Mbps user bit rates in large macrocell environment can exceed 1 Mbps and in small microcells 5 Mbps. The HSDPA concept is able to support not only non-real time UMTS QoS classes but also real time UMTS QoS classes with guaranteed bit rates. The cell throughput refers here to the total number bits transmitted to the users through one cell. The cell throughput increases with HSDPA compared to the Release 99 because Hybrid ARQ combines packet retransmission with the earlier transmission, and no transmissions are wasted. 16-QAM modulation provides higher bit rates than QPSK of Release 99 with the same usage of orthogonal codes. Typical throughput values are shown in Table II. HSDPA increases the cell throughput typically 100% compared to the Release 99. The cell throughput with HSDPA depends on the interference environment: the inter-path interference and the inter-cell interference. For microcell, the HS-DSCH can support up to 5 Mbps per sector per carrier, i.e. 1 bits/hz/cell. Short 2 ms frame length in HSDPA also allows to minimize the round trip time which enables shorter network latency and better response times. 3. WCDMA Performance Enhancement Smart Antenna Beamforming for Higher Capacity and Coverage The WCDMA air interface spectral efficiency and user bit rates are interference limited, in particular with HSDPA. If the interference levels could be reduced, the bit rates could be further improved. Base station beamforming allows to reduce the interference levels by transmitting the signal via a narrow beam to the desired user, thus causing less interference to the other users. The beamformining is supported in WCDMA Release 99 air interface. The support of radio resource management functionalities for beamforming in Iub interface between base station and radio network controller (RNC) is part of Release 6. Beamforming is illustrated in Figure 5 where one scrambling code is shared between several beams. That approach brings the advantage that the spreading Fig. 5. Beamforming smart antennas in WCDMA. codes under one scrambling code are orthogonal, reducing the interference levels further. The capacity of the WCDMA sector can be typically improved 100 150% with 4 beams compared to single antenna transmission [3]. 4. WCDMA Evolution Beyond Release 5 In 3GPP standardization, further work will be carried on for Release 6 and beyond. The radio-related work areas include: Enhanced uplink DCH, which studies similar methods as with HSDPA for the uplink operation. The focus is on the improved coverage and capacity of the uplink services, especially with packet-based traffic. Also, the delay related to initiating uplink packet transmission is an area that is being studied for improvements. The proposed concept under study is illustrated in Figure 6. New frequency variants of WCDMA such as the utilization of 2.5 GHz spectrum and 1.7/2.1 GHz. The latter is relevant in U.S.A. Advanced antenna technologies, including enhancements for the beamforming capabilities in the network side as well as transmit diversity technologies with single terminal receiver antenna or with two receiver antennas in the terminal. The latter work is carried on under the multiple input multiple output (MIMO) term. Further topics include UTRAN architecture evolution to get the most of IP transport, part of Release 5 standard, by using distributed architecture that can benefit further from the properties of IP as the transport technology.
THIRD GENERATION WCDMA RADIO EVOLUTION 991 Fig. 6. Release 6 study item, Enhanced uplink DCH operation with HARQ in the uplink. Fig. 7. MIMO receiver. The support of multicast services is also being worked on to provide efficient means for sharing a common content with several users. In addition to the beamforming antenna solutions, the HSDPA bit rates could be increased by using several transmitter antennas in the base station and several receiver antennas in the mobile. This approach is called MIMO. The higher data rates are obtained since the same spreading code can be reused in different antennas. To distinguish the several substreams sharing the same code, the mobile uses multiple antennas and spatial signal processing. An example MIMO receiver with two antennas is shown in Figure 7. The space time RAKE combiner is the multiple antenna generalization of the conventional RAKE combiner. The performance of MIMO techniques is evaluated in Reference [2]. The results show that using two antennas in the base station and in the mobile, the bit rate can be increased from 10.8 Mbps to 14.4 Mbps. With four antennas, the bit rate can be increased to 21.6 Mbps with the same C/I requirement. During Release 6, further work on the performance and complexity aspects of MIMO technologies will be carried out in 3GPP standardization. 5. Conclusions WCDMA commercial deployment starts with 3GPP Release 99 specifications which support flexible data services together with voice service. The practical data rates with first terminal extend beyond 300 kbps. Release 99 is optimized for varying QoS requirements from real time video, interactive web browsing and gaming to delay tolerant data transmission. WCDMA Release 99 allows the introduction of a large variety of new attractive wireless data services. WCDMA Release 5 makes the data services more attractive and improves the efficiency of the service delivery with the introduction of HSDPA which boosts the practical user bit rates beyond 1 Mbps and the theoretical peak bit rates beyond 10 Mbps. WCDMA standardization work beyond Release 5 is going on with targets to improve the uplink data performance and to increase the practical bit rates of HSDPA with advanced antenna technologies. Further, on the radio access network side, the aim is for more efficient architecture that can meet the need for increased data services. All this is aiming to ensure that WCDMA can provide low enough cost per bit for emerging market of mobile data applications with increasing end user performance requirements. References 1. Holma H, Toskala A. WCDMA for UMTS, 2nd edn. John Wiley & Sons: Chichester, 2002. 2. 3GPP Technical Report 25.848. Physical layer aspects of UTRA High Speed Downlink Packet Access. version 4.0.0, March 2001. 3. Ramiro-Moreno J, Pedersen KI, Mogensen PE. Capacity gain of beamforming techniques in a WCDMA system under channelization code constraints. Accepted to IEEE Trans. on Wireless Communications.
992 H. HOLMA AND A. TOSKALA Authors Biographies Antti Toskala (M.Sc.) joined the Nokia Research Center in 1994 where he was involved with WCDMA system studies. In September 1995, he joined the ACTS FRAMES project. In later phase of the FRAMES project, he worked as the team leader for the work package which defined the FMA2 WCDMA concept. During 1997, he worked as a senior research engineer and CDMA Specialist participating in the ETSI SMG2 UMTS standardization work. Currently, he is with IP Mobility Networks, Nokia Networks, in Espoo, Finland. He is working as standardisation manager with WCDMA radio access network standardisation and product development. He was also working in 3GPP as chairman for the TSG RAN WG1 until 2003, the group being responsible for the physical layer of the WCDMA standard. He has a large number of publications in the field and is co-editor of WCDMA for UMTS published by Wiley. Harri Holma joined Nokia Research Center in 1994. He received his M.Sc. from Helsinki University of Technology in 1995. Since 1994, he has been working on 3rd generation WCDMA air interface with special interest on radio network performance. In January 1998, he joined Nokia Network and is currently working as principal engineer with EDGE/WCDMA radio network performance area. Mr Holma has co-edited the book WCDMA for UMTS and has a large number of publications in this field.