Performance of Hybrid ARQ Techniques for WCDMA High Data Rates

Performance of Hybrid ARQ Techniques for WCDMA High Data Rates Esa Malkamalu, Deepak Mathew, Seppo Hamalainen Nokia Research Center P.O. Box 47, FN-45 Nokia Group, Finland esa.malkamaki @nokia.com Abstract Hybrid ARQ is being considered as a potential method for improving the performance of WCDMA in UMTS. n this paper, we compare different hybrid ARQ techniques, namely the incremental redundancy type 1 hybrid ARQ schemes and type hybrid ARQ schemes with and without SOB combining. The emphasis is at high data rates and downlink shared channel (DSCH). Both link and system simulations are used for comparison. The hybrid ARQ schemes are shown to achieve higher throughput and capacity than the conventional ARQ schemes currently specified in the 3GPP WCDMA standard. Also, the combining of packets can cut the tail of the delay distribution which reduces the required maximum number of transmissions. However, the complexity increase due to physical layer buffering in the mobile station is too high if the round trip delay is large. 1. ntroduction Hybrid ARQ (Automatic Repeat request) is being considered as a potential method for improving the performance of WCDMA in UMTS. Currently, the retransmission (ARQ) function in the WCDMA uses type hybrid ARQ with either convolutional or turbo codes [l]. The packet data capacity in wideband CDMA system has been considered in [2] and [3]. n [2], the conventional type hybrid ARQ is considered and in [3], type hybrid ARQ with soft combining was studied with a data rate of 32 kbps. n this paper, we consider also other hybrid ARQ techniques, namely the incremental redundancy type 1 hybrid ARQ schemes and compare the performance with type hybrid ARQ schemes with and without soft combining. The emphasis is at high data rates and downlink shared channel (DSCH). Both link and system simulations are used for comparison. SF Channelbit rate (kbps) 256 128 64 32 16 8 4 3 6 12 24 48 96 192 2. WCDMA packet data services The WCDMA (wideband CDMA) air interface standardized in 3GPP (3rd Generation Partnership Project) provides currently packet data services up to 2 Mbps, see e.g., [l]. The standard provides two different channels for high data rate packet services in the downlink: dedicated channel (DCH) and downlink shared channel (DSCH). Both can provide variable bit rate. n the dedicated channel the spreading factor and spreading code is reserved from the OVSF (orthogonal variable spreading factor) code tree based on the highest data rate. This easily leads to shortage of downlink codes. Therefore, the DSCH is more appropriate for high data rate packet services. The DSCH is a common channel shared by several users. The users can be time or code multiplexed, i.e., the whole capacity of the DSCH can be allocated to a high bit rate user for some time or several lower bit rate users can be code multiplexed at the same time on the DSCH. Each user is also allocated an associated DCH which carries user specific signaling, keeps fast power control running and indicates when there is data on DSCH to receive. The associated DCH also tells the spreading factor, channelization codes as well as the transport format being used on the DSCH. The UE (user equipment) only needs to demodulate the DSCH when indicated on the associated DCH. The spreading factor of the physical downlink shared channel (PDSCH) can vary from 256 to 4 [9]. On PDSCH there are only data bits, all associated Layer 1 control information is transmitted on the DPCCH (dedicated physical control channel) part of the associated DCH. The channel bit rates of PDSCH are given in Table. Packet data services require use of some ARQ mechanism. Currently, the retransmission functionality in the WCDMA is implemented at the radio link control (RLC) layer. t uses type hybrid ARQ techniques with either convolutional or turbo codes. The basic code rate is R=1/3 [ 11. Other code rates are achieved by puncturing or repetition. n the basic type hybrid ARQ scheme, when a received packet is detected to be erroneous, it is discarded and a negative acknowledgement is sent to the transmitter -783-6728-611/$1 21 EEE 272 VTC'O

requesting retransmission. The retransmitted packet is identical with the first transmission. The same ARQ mechanism is used both with DCH and DSCH. 3. Hybrid ARQ schemes A transmission scheme based on Hybrid ARQ combines an error detectiodcorrection plus a retransmission of the erroneous packet [4]. The following types of Hybrid ARQ are defined: Type : n the basic type hybrid ARQ, the data is always encoded with a FEC (forward error correction) code besides the error detection code. n the receiver, the FEC code is first decoded and if there are still errors in the packet, a retransmission of the packet is requested. The erroneous packet is discarded. The retransmission uses the same FEC code. Type 11: The type 1 hybrid ARQ schemes are also called incremental redundancy (R) ARQ schemes. n R ARQ schemes, a packet that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. For type 1 hybrid ARQ schemes the retransmissions are typically not identical with the first transmission but instead carry additional redundancy for error correction. This additional redundancy is combined with the previously received packets and the resulting more powerful FEC code is decoded. Type 111: Type 11 hybrid ARQ schemes [5] are a special case of type 1 hybrid ARQ schemes: each packet is self decodable, i.e., the retransmitted packed may be combined with the previous versions if available, but each version contains all the information necessary for a correct reception of the data. Type hybrid ARQ with soft combining: With type hybrid ARQ, it is also possible to store the erroneous packet in the receiver and combine it with the retransmitted packet. This can be considered as incremental redundancy in the form of repetition code. Since all the packets are self decodable this can also be considered as a special case of type 11 hybrid ARQ scheme. This type of combining is also called metric combining [6] or Chase combining [7]. 4. Performance of hybrid ARQ with high data rates n this section, we present some performance results for different hybrid ARQ techniques using WCDMA shared channel parameters. The performance of the ARQ techniques is analyzed both with link and system simulations. ARQ scheme Code rate R, Packet size [bits] ## of packets per frame Type wlo 112 4796 1 combining -, 213 3196 2 Type with 1 2 4796 1 combining 213 3 196 2 Type 1 213,113 3 196 2 1,112 4796 2 Type1 wlo 112 4796 1 4.2 Link simulations The average single user throughput is defined as Rb U, =- [kbpsl (1) T, where Tr is the average number of transmissions and Rb is the data rate [kbps]. The delay is estimated here simply by the number of transmissions assuming that the delay between each transmission and retransmission is constant. The average number of transmissions T, can be calculated as -783-6728-6/1/$1. 21 EEE 272 1 VTC'O 1

T =+< +<* +<?, +... (2) where 6 is the probability that the first transmissions is in error, 42 is the joint probability that the first as well as the combined packet are in error, etc. (2) can be upper bounded by assuming an inferior system where only two latest packets are combined: +< T, =+< +<2 +P,P2, +t2p34 +...=- (3) 1-42 where we have assumed that on average t2 = P2, = e4 =... n downlink simulations the performance is typically measured as a function of,,,, where, is the average energy per chip for a particular Walsh channel, measured at the base station.,, is the total power density of the base station., /,, tells the proportion of the total power that a single user requires. Therefore, assuming that all users require the same, /,,, the number of users K, in the cell is inversely proportional to the required, /,, 1 K,=-. 1, 1 'or The cell throughput can then be defined as (4)._ Figure 1 shows the single user throughput vs., / for different hybrid ARQ schemes. The hybrid ARQ schemes with higher starting code rate achieve higher throughput at high,,,. 7 1 * Type 1 Rate 2/3 Type 2 Rate (1,112) + Type 1 Rate 1/2 SC E L 2 1-14 -12-1 -8-6 -4 dlor [db] Fig. 1 Average single user throughput 1 Y q, = KJ, = L. (5) 1, 1 'or The link simulations are so called geometry simulations where the own cell interference is modeled explicitly (common pilot channel and other users) and other cell interference and thermal noise with Gaussian noise. The geometry G is defined as the ratio of the received own cell power,, to other cell interference:.(i ) G = J (6) 'or + No Power control is modeled only for the desired user, for interfering users no power control is used. The common pilot channel is also transmitted with constant power. Table 11 shows assumptions used for simulations. Table 111 Simulation assumotions Parameter Explanation/ Assumption Chip Rate 3.84 Mcps Closed 1~ Pwr Control ON Channel Estimation From common pilot symbols PC error rate 14% -1 db,, /,, values (G) Delay between trans. 6 db looms -783-6728-6/1/$1. 21 EEE 2722 VTC'O 1

tail of the distribution. Type 1 hybrid ARQ scheme with high starting coding rate requires practically always at least two transmissions. 1. v.9.5 U B.4.3.2.1 -.1 2.3.4.5.6.7.6.9 1 FER Fig. 3 Cell throughput vs. FER ll;ll,,, 1 Type 1 Rate 112 Type 1 Rate 112 SC Type 2 Rate (1, 112).],, 3 2 1 123456 123456 123456 No of Transmissions No of Transmissions No of Transmissions Fig. 4 Number of transmissions making calls and transmitting data according to a packet data traffic model which simulates a typical WWW browsing session [ 121. Also, the Radio Resource Management (RRM) algorithms, such as power control (inner and outer loop, open loop), packet scheduling and handover, are implemented in a detailed way. The radio link performance is modeled using look-up tables [ 1 ]. Type- hybrid ARQ is modeled using the same look-up tables as for the case without soft combining. n the system simulator, for each slot, the SRS of different transmissions are summed together. Summing of SR values models maximal ratio combining. After that the block SR is calculated and mapped to FER by using the look-up tables. Finally, erroneous frames are randomly generated. With type-1 hybrid ARQ, separate look-up tables are generated for l", 2"d, 3rd and 41h transmission. n these simulations no re-transmission protocol is assumed. This means that a packet is transmitted so long that it has been correctly received by the UE. Feedback channel is assumed to be error free and with no delay. Some system simulation parameters are given in Table V. For further details about the basic simulator, see, e.g., [ ]. 4.3 System simulations n order to have a more realistic view of the cell throughput some system simulations were carried out, too. The simulated environment is a system with hexagonal macro cells with 3 sectors. For each sector real 3-D antenna patterns are assumed. The used propagation model is a modified Hata model specified in [12]. The shadowing modeling is also adopted from [ 121. The shadowing model is a time correlated process. There exists also correlation between cells and base stations. The multipath propagation is also modeled in the simulator. Each multipath component has its own fast fading process. n these simulations, however, only one multipath component is used, i.e., single tap Rayleigh channel has been assumed. The simulator is a dynamic simulator: the users are moving in the simulation area according to the pedestrian mobility model [21 at a speed of 3 km/h. The users are n Fig. 5, comparison of mean user throughputs (number of correctly transmitted bits for user i divided by the active time of user i including queuing, averaged over active users) is shown for type hybrid ARQ (conv. ARQ), type hybrid ARQ with soft combining (R,=1/2) and type- 1 hybrid ARQ (&=, 1/2). For type- hybrid ARQ with soft combining, gains in order of 1-6 % can be seen over type hybrid ARQ without soft combining. The corresponding gains for type 1 hybrid ARQ are up to 35 %. n Fig. 6, downlink capacity (total number of correctly transmitted bits in the system divided by the number of base stations, total simulation time and bandwidth) is shown for the three ARQ methods. For type hybrid ARQ -783-6728-6/1/$1. 2 EEE 2723 VTC'O

with soft combining gains ranging from to 5 % are seen, while for type 1 hybrid ARQ gains are up to 11 9%. Mean throughput 14 12 ; 1 : 8 2 6 9 4 2 2m 5m 1OOOm cell range Fig. 5 Mean throughput per user Max. DL average bitrate 16-14 a? 12 2 1 = 8 5 6, 4 2 2m 5m lm Cell range Fig. 6 Downlink capacity 5. Discussion 6type-l Both link and system simulation results show that some gain in throughput and capacity is achieved with hybrid ARQ schemes, i.e., with soft combining and incremental redundancy schemes. Hybrid ARQ schemes naturally add some complexity, especially the buffering requirements in the UE physical layer are increased since all the erroneous packets are buffered and combined with the retransmissions. The memory increase is proportional to the round trip delay of the ARQ scheme, i.e., the time for waiting the retransmission, since more erroneous packets are received during that time. The round trip delay for the RLC level hybrid ARQ is at least 1-2 ms which means significant increase in the physical layer memory, especially for high data rates. This is mainly because RLC layer is typically located in the radio network controller (RNC). Due to the huge memory requirements of the RLC level hybrid ARQ, possible ways to decrease the round trip delay are currently being studied. n 3GPP, hybrid ARQ is currently studied as part of high speed downlink packet access (HSDPA) scheme, where a fast hybrid ARQ scheme is being specified between the base station and the UE. 6. Conclusions The hybrid ARQ schemes studied in this paper were shown to achieve higher throughput and capacity than the conventional ARQ schemes currently specified in the 3GPP WCDMA standard. Also, the combining of packets can cut the tail of the delay distribution which reduces the required maximum number of transmissions. However, the complexity increase due to physical layer buffering in the UE is too high if the round trip delay is large (as is the case with the RLC level ARQ). References [] H. Holma and A. Toskala, WCDMA for UMTS. Chichester: John Wiley & Sons, Ltd, 2. [2] B. C. V. Johansson, Packet data capacity in a wideband CDMA system, in Proc. EEE Vehicular Technology Conference, 1998, pp. 1878-1883. [3] M. Raitola and H. Holma, Wideband CDMA packet data with hybrid ARQ, in Proc. EEE nternational Symposium on Spread Spectrum Techniques and Applications, 1998, pp. 318-322. [4] S. Lin and D. J. Costello, Jr., Error Control Coding: Fundamentals and Applications. Englewood Cliffs: Prentice-Hall, 1983. [5] S. Kallel, Complementary punctured convolutional (CPC) codes and their applications, EEE Trans. Commun., vol. 43, pp. 25-29, June 1995. [6] T. Ji and W. E. Stark, Turbo-coded ARQ schemes for DS- CDMA data networks over fading and shadowing channels:throughput, delay, and energy efficiency, EEE J. Select. Areas Commun., vol. 18, pp. 1355-1364, Aug. 2. [7] D. Chase, Code combining - a maximum-likelihood decoding approach for combining an arbitrary number of noisy packets, EEE Trans. Commun., vol. COM-33, pp. 385-393, May 1985. [8] P. Robertson, P. Hoeher, and E. Villebrun, Optimal and sub-optimal maximum a posteriori algorithms suitable for turbo decoding, En, vol. 8, pp. 119-125, March-April 1997. [9] Physical channels and mapping of transport channels onto physical channels (FDD), 3GPP technical specification, TS 25.211, ~3.3. (2-6). [ 11 Multiplexing and channel coding (FDD), 3GPP technical specification, TS 25.212, v3.3. (2-6). [l]hiimidainen, S., Holma, H. and Sipila, K., Advanced WCDMA Radio Network Simulator, in Proc. PMRC99 conference, 1999 [ 121 Universal Mobile Telecommunications System (UMTS); Selection procedures for the choice of radio transmission technologies of the UMTS, TR 11 112 v3.1. (1997-11). UMTS3.3 O-7SOX728-6 Ol /$. 21 EEE 2724 VTC O