Network Performance of Mixed Traffic on High Speed Downlink Packet Access and Dedicated Channels in WCDMA

Network Performance of Mixed Traffic on High Speed Downlink Packet Access and Dedicated Channels in WCDMA Klaus I. Pedersen, Tako F. Lootsma, Michael Støttrup, Frank Frederiksen, Troels E. Kolding, Preben E. Mogensen Nokia Networks, Niels Jernes Vej 10, DK-9220 Aalborg East, Denmark Email: Klaus.I.Pedersen@nokia.com Abstract Downlink throughput results are presented for cases where the available cell transmission resources are shared between HSDPA and DCH users. It is shown that the total cell throughput can be increased by 69% by allocating only 5 HS-PDSCH codes and 7 W for HSDPA transmission, compared to a scenario without HSDPA enabled. These results are obtained for a macro cellular scenario with best effort packet traffic. It is demonstrated that part of this throughput increase originates from a better utilization of available cell transmit power when HSDPA is introduced, since reservation of less power control headroom is required. I. INTRODUCTION High speed downlink packet access (HSDPA) is introduced in Release 5 of 3GPP UTRAN [1]. HSDPA includes a number of performance enhancing features such as adaptive modulation (QPSK/16QAM) and coding, a new physical layer retransmission mechanism (Hybrid ARQ), location of the medium access control layer (called MAC-hs) in the base station (Node- B), and a short transmission time interval (TTI) of 2 ms. The shared transport channel for HSDPA is the high speed downlink shared channel (HS-DSCH), which is mapped to the high speed physical downlink shared channel (HS-PDSCH). HSDPA is designed so that it can co-exist in the same 5 MHz bandwidth as the dedicated channels (DCH) defined in Rel 99 of 3GPP. This implies that part of the Node-B transmit power and the channelization codes in one cell may be used for HSDPA, while another part is allocated to DCH transmission. Not many studies have been published where the network performance of WCDMA is studied under mixed traffic conditions on DCH and HSDPA on the same carrier frequency. We will therefore focus on the joint performance of DCH and HS- DPA for scenarios where the available Node-B transmit power and the channelization code resources are shared between those two channel types. Such scenarios are important to study as the penetration of HSDPA capable users might be moderate during the initial roll-out of HSDPA. The network performance as a function of the power and the channelization code split between DCH and HSDPA is studied for a macro cellular scenarion with best effort packet traffic. The resource split that results in the highest average throughput per cell is quantified, and the potential gain of introducing HSDPA is discussed. As it is almost impossible to evaluate the performance of a system with mixed traffic on HSDPA and DCH under realistic conditions by simple theoretical methods, we have chosen to primarily study the performance by means of extensive dynamic network simulations. The paper is organized as follows: Section II presents the most important HSDPA specific algorithms, while Section III outlines the simulation methodology. Section IV includes the simulation results. Finally, Section V contains concluding remarks. II. ALGORITHM CONSIDERATIONS A. Resource split between HSDPA and DCH The total available transmit power in each cell is shared between HSDPA and DCH users. The total transmit power from one cell can be expressed as P tot = P common + P DCH + P HSDPA, (1) where P common is the transmit power of common channels such as the primary common pilot channel (P-CPICH), P DCH is the total transmit power of all the DCHs that are transmitted in the cell, and P HSDPA is the HSDPA transmit power. Notice that the HSDPA transmit power includes the power for both the high speed shared control channel (HS-SCCH) and the HS-PDSCHs. Conventional power based radio resource management (RRM) algorithms typically try to load the cell to the point where [1] E{P tot } P target, (2) where E{ } denotes time averaging over 100 ms and P target is a radio network planning parameter. Assuming a maximum transmit power in the cell of P max, the power (P max P target ) can be regarded as the reserved power control headroom that is required for the power fluctuations from using fast closed loop power control (PC) on the DCHs. In cells with no HSDPA traffic it is often chosen to have P target = KP max, where a typical value for the constant is K = 0.5 [1]. However, as more power is being allocated to HSDPA and less power to the DCHs, P target can be increased, as there is a need for less power control headroom for the DCHs. Hence, P target is adjusted as a function of the power that is allocated to HSDPA transmission, so P target =(P max P HSDPA )K + P HSDPA. (3) Notice that when using (3), we always reserve the same relative power control headroom for the dedicated channels. The optimal value of P HSDPA that maximizes the total cell throughput 4496

is presented in Section IV under the assumption that there is sufficient offered traffic to fully utilize the allocated HSDPA power. Another common transmission resource at the Node-B is the orthogonal channelization codes. In this study we will assume that 5 HS-PDSCH codes with spreading factor 16 are allocated, plus one channelization code with spreading factor 128 for HS- SCCH transmission. The remaing channelization codes can be used for other common and dedicated channels. Using 5 HS-PDSCH codes and 16QAM modulation with an effective code rate of 3/4 limits the peak bit rate to 3.6 Mbps on the HS- DSCH. In this study it is assumed that the radio network controller (RNC) allocates power and channelization codes to each cell for HSDPA transmission. B. Dynamic radio link adaptation for HSDPA Constant transmit power is assumed for the HS-DSCH. The bit rate for the HS-DSCH is adjusted every TTI depending on the channel quality indicator (). The from the user expresses the recommended transmission format. This information is signaled from the user to the Node-B via a index n [0 31], where the integer index number n can be regarded as a pointer to a vector, i.e, f (n) =[A n,m n,x n, n ] n [0 31], (4) where A n is the recommended transport block size (number of bits in a TTI), M n is the recommended number of HS-PDSCH codes, X n [QP SK, 16QAM] denotes whether the user recommends the Node-B to use QPSK or 16QAM modulation, and n is a recommended power offset for the n-th index, respectively. 3GPP has specifed mapping tables for the function f (n) for different user terminal categories in [11]. Notice that the mapping tables are organized so that A n A n+1 n [0 30]. The index estimated by the user fulfils n = arg max n {A n BLEP < 0.1} (5) in the 2 ms interval that ends one slot before the is sent, where BLEP is the per TTI block error probability [11]. However, using the recommended transmission format by the report for the HS-DSCH transmission does not necessarily guarantee that the BLEP for the transmission does not exceed 0.1. This is the case because the signal quality at the user might have changed from the time where the was estimated until the actual transmision on the HS-DSCH. Hence, in addition to using the, we also monitor Ack/Nack s from past transmissions on the HS-DSCH, so that the selected transmission format on the HS-DSCH corresponds to a case where the residual BLEP on the first Layer-1 retransmission does not exceed 0.01 (1%). This is achieved by using an outer loop link adaptation algorithm that is based on the same principle as was discussed in [9]-[10]. The HS-SCCH transmit power is adjusted every TTI based on feedback information from the user that is being scheduled. The HS-SCCH transmit power is controlled so that the residual BLEP equals 1%. Hence, it happens with a 1% probability that the transport block transmitted on the HS-DSCH is not being decoded by the user as it has failed to decode the Layer-1 control information on the HS-SCCH. For the latter case, the following retransmission on the HS-DSCH does not benefit from Layer-1 Hybrid ARQ soft combining. C. MAC-hs packet scheduling The simplest packet scheduler is round robin (RR), where the users are served in sequential order, independent of the radio channel conditions for the users. The RR scheduler is therefore often characterized as a blind scheduler. A more advanced scheduler is the so-called proportional fair (PF) algorithm, where the user with the highest priority metric is served at every TTI. The priority metric for user number n is expressed as R n /T n, where R n is the instantaneous data rate user number n can support during the next TTI, while T n is the average throughput delivered to user number n in the past [3], [4]. The PF algorithm uses a priori knowledge from the radio channel (R n ) and therefore provides a potential multi-user diversity gain. The PF is used as the default MAC-hs packet scheduler in this study. III. SIMULATION METHODOLOGY A. Overall approach and network topology Dynamic Monte-Carlo network simulations are conducted with a time-resolution of one slot (0.66 ms). A standard three sector network topology is considered with a site-to-site distance of 2.8 km, where 70 degree wide antennas are used at the Node-Bs. Users in the network are uniformly distributed (with respect to area) and assumed to move with a constant speed of 3 kmph in a random direction that is selected at the begining of each call. The deterministic distance dependent path loss between each Node-B and user is modeled according to the COST231 Hata model, while the stochastic shadow fading component is modeled with a lognormal distributed random variable with 8 db standard deviation. The fast fading and the temporal dispersion is modeled according to the ITU Vehicular-A radio channel profile. Hence, the considered scenario corresponds to a macro cellular environment. The default simulation parameters are summarized in Table I. B. Simulated channels and radio link modeling Each of the HSDPA-users are assumed to receive the HS- SCCH, the HS-PDSCH, and one associated DPCH. The associated DPCH is assumed to only carry Layer-3 signaling information at a bit rate of 3.4 kbps, i.e. requiring a channelization code with the spreading factor 256. As illustrated in Fig. 1, transmission on the HS-SCCH starts two slots before the actual data transmission on the HS-PDSCH. The user only decodes the data transmission on the HS-PDSCH if the HS-SCCH is correctly decoded. The radio link performance 4497

of each channel is depending on the experienced energy-persymbol-to-interference-ratio (E s /N 0 ). The E s /N 0 is therefore computed for all channels every slot, assuming that a standard single antenna Rake receiver is applied. The E s /N 0 for user number n is calculated according to, L SF α l P n ρ[n] = P other + P no + P own l=1 m =l α, (6) m where SF is the spreading factor for the channel, P n is the transmitted power to UE number n on the channel, P other is the received wideband other cell interference, P no is the received noise power, P own is the total transmitted wideband own cell power, while α l is the power attenuation of the radio channels path number l including fast fading, shadow fading, and the deterministic path loss. Finally, L is the number of resolvable multipath component in the radio channel. Notice that the effect of using orthogonal channelization codes is included in (6), since the time-synchronized orthogonal own cell interference is excluded from the interference term in the denominator. After transmission of a number of slots corresponding to one TTI the geometrically averaged E s /N 0 is computed, ρ db [n] =E{10 log 10 (ρ[n])}, (7) where E{ } denotes averaging over one TTI. The BLEP for the transmission on the HS-DSCH is obtained as BLEP = f(ρ db [n],q,m), (8) where Q is the used modulation and coding scheme, and M is the number of used HS-PDSCH codes for the transmission. The function f( ) is obtained from extensive link level simulations and is often called the actual value interface [7]. The Hybrid ARQ soft combining gain of retransmissions from using chase combining on the HS-DSCH is modeled according to [2]. The same AVI principle as in (8) is used for HS-SCCH and DCH, except that these channels are transmitted with a fixed modulation and coding scheme. Transmission of Ack/Nack and on the uplink high speed dedicated physical control channel (HS-DPCCH) is modeled with a fixed signaling delay and error probabilities. The conditional probabilities for erroneously receiving an Ack or Nack at the Node-B, depending on the transmitted information are: Pr(Ack Nack) = 0.01%, Pr(Nack Ack) = 1%, and Pr(Ack DTX) =1%. The is estimated by the user by first measuring the ideal narrowband SINR on the P-CPICH during the 2 ms interval that ends one slot before the is transmitted. The SINR is afterwards mapped to an estimate of the E s /N 0 on the HS- DSCH since the user knows the Node-B transmit power offset between the HS-DSCH and P-CPICH. Finally, the estimated E s /N 0 is mapped to the index via a mapping function n = g(ρ db + ɛ) [0 31], (9) where ρ db is the ideally estimated E s /N 0 on the HS-DSCH in decibel while ɛ is a zero mean Gaussian distributed random variable with 1.0 db standard deviation that is added to model estimation errors and other imperfections. The function g( ) is generated from extensive link level simulations so it fulfils (5). Uplink Downlink DPCH HS-SCCH HS-PDSCH HS-DPCCH DPCH Slot 2ms TTI ~7.5 slots Ack Fig. 1. Overview of the donwlink and uplink channel structure. Notice that for this example, the is only transmitted in every second TTI from the UE. C. Offered traffic New packet calls are generated according to a homogeneous Poisson process. A standard closed loop TCP web-browsing packet call model is used, assuming a lognormal distributed packet call length with an average of 100 kbytes. The TCP packet size is fixed at 1500 bytes. RLC acknowledged mode is used, with an RLC PDU size of 336 bits for the HSDPA-users. The data are transmitted with a constant bit rate of 64 kbps to DCH-users, while the bit rate for the HSDPA-users are controlled by the MAC-hs packet scheduler as discussed in Section II-C. The offered traffic is adjusted so that there is full utilization of the HS-DSCH in every cell, and so that all the available power for DCH transmission is used, i.e. all cells are fully loaded so E{P tot } P target. D. Radio resource management algorithms In addition to the radio resource management (RRM) algorithms discussed in Section II, a standard closed loop power control (PC) algorithm and an outer loop PC algorithm is also simulated for the dedicated channels [1]. A simple power based admission control (AC) scheme is used for DCH users, so that users are only admitted provided that the estimated average Node-B transmit power cell is below P target if the user is admitted. Similarly, a simple AC scheme for HSDPA users is implemented to limit the maximum number of users per cell that are sharing the HS-DSCH. A conventional soft handover algorithm is used for the DCH [1], while change of the serving HS-DSCH cell is handled according to [8]. IV. RESULTS A. Transmitted cell power distribution As argumented in Section II-A the parameter P target can be increased as a function of the allocated HSDPA power according to (3), so a higher average cell transmit power is allowed as more power is used for HSDPA. In order to further motivate the latter claim the cumulative distribution function (cdf) of the cell total transmit power is plotted in Fig. 2 for two cases. It is observed that for low HSDPA power allocations there are rather large power fluctuations, which calls for reservation of a large power control headroom. On the contrary, for high HSDPA power allocations, the variations of the cell transmit 4498

TABLE I SUMMARY OF THE MAIN DEFAULT SIMULATION PARAMETERS. Parameter Setting Max cell Tx power (P max ) 20 W P-CPICH Tx power 2W K (used in eq. 3) 0.5 MAC-hs scheduler PF Number of HS-PDSCH codes 5 Number of HS-SCCH codes 1 Hybrid ARQ combining Chase HSDPA terminal category 6 User receiver type 1-Rx Rake Path loss model COST231 Hata Shadow fading std. 8dB Power delay profile Veh-A Site-to-site distance 2.8 km User speed 3kmph reporting interval 2ms error (std{ɛ}) 1.0 db power are much less, which makes it possible to better utilize the available cell transmit power by increasing P target without increasing the probability of saturating the power amplifier. Cumulative distribution 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 3W HSDPA Power 9W HSDPA Power 0.0 5 10 15 20 Cell Transmit Power [W] Fig. 2. Cumulative distribution function of the total transmit power per cell depending on the allocated HSDPA power. B. Cell throughput results Fig. 3 shows the achievable average cell throughput on DCH, HSDPA, and the total cell throughput (sum of throughput on DCH and HSDPA) versus the amount of power allocated for HSDPA. Notice that the number of HS-PDSCH codes is fixed to five for all the cases. As expected, the HSDPA cell throughput increases as more power is allocated to HS- DPA, while the DCH throughput decreases as there will be less power for transmission of these channels. It is observed that the HSDPA power allocation that maximizes the total cell throughput is 7 W, which results in a total cell throughput of 1320 kbps, with 900 kbps carried on HSDPA. However, the total cell throughput does not change significantly for HSDPA power allocations in the range from 4-9 W, as the number of HS-PDSCH codes is fixed to five codes. For comparison, the achievable cell throughput on DCH in cells with no HSDPA traffic equals 780 kbps, so by introducing HSDPA with 7 W and 5 HS-PDSCH codes the total cell throughput is increased by 69% ([1320/780 1] 100%). Cell Throughput [kbps] 1400 1200 1000 800 600 400 Total throughput HSDPA throughput DCH throughput 200 3 4 5 6 7 8 9 Allocated HSDPA Power [W] Fig. 3. Average cell throughput versus the allocated HSDPA power. PF scheduling is assumed and 5 HS-PDSCH codes. Six HSDPA-users per cell. The gain in cell throughput from introducing HSDPA mainly comes from; (i) A higher spectral efficiency for the HS-DSCH over the DCH by using Layer-1 Hybrid ARQ and adaptive modulation and coding, (ii) The multi-user diversity gain from using fast PF scheduling, (iii) A better utilization of the available cell transmission power as discussed in Section IV-A. Furthermore, part of the capacity gain also comes from the PF MAC-hs scheduler, which tends to giver higher bit rates to users close to the Node-B, while all the DCH-users are given the same bit rate. In order to quantify the multi-user diversity gain from using PF scheduling, the HSDPA cell throughput has been compared for a 7 W HSDPA power allocation against a scenario where blind RR scheduling is used. These results showed an HSDPA capacity gain of 36% from using PF instead of RR. However, the multi-user diversity gain is only available provided that the experienced E s /N 0 at the different HSDPAusers is time-varying at a rate that the MAC-hs packet scheduler can track. Fig. 4 shows the multi-user diversity on HSDPA from using PF instead of RR scheduling. It is observed that the multi-user diversity gain is marginal for user speeds larger than 15 kmph as the MAC-hs packet can no longer accurately track the E s /N 0 variations for the users. This is due to the delay from the time where the user terminal estimates the until the time where the HS-DSCH is transmitted. However, even though the multi-user diversity gain becomes marginal at high speeds, HSDPA still provides a gain over DCH. The latter is mainly caused by the Layer-1 Hybrid ARQ mechanism 4499

that adds robustness to the system by using soft combining of retransmissions. Multi-user diversity gain [%] 40 35 30 25 20 15 10 5 0 0 5 10 15 20 25 30 35 40 45 50 UE Speed [kmph] Fig. 4. The multi-user diversity gain of PF over RR scheduling for HSDPA with 7 W and 5 HS-PDSCH codes. The number of HSDPA-users equal 6. C. Per HSDPA-user throughput results Fig. 5 shows the cdf of the experienced per HSDPA-user throughput depending on the offered HSDPA load. For the case with only one HSDPA-user per cell (and several DCH-users), the median throughput is on the order of 410 kbps, while 10% of the users are experiencing bit rates higher than 800 kbps (typically those that are close to the Node-B). Notice that these bit rates are achieved with only 5 HS-PDSCH codes and 7 W for HSDPA, while the rest of the available transmit power up to P target is used by DCH-users and common channels. It is furthermore observed that when increasing the number of HSDPA-users per cell from one to three, the median throughput decreases with a factor less than three. This is observed because of the added multi-user diversity gain. Cumulative distribution 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 5 HSDPA-users 3 HSDPA-users 1 HSDPA-user 0.0 0 100 200 300 400 500 600 700 800 900 1000 Per HSDPA-user throughput [kbps] Fig. 5. Cumulative distribution function of the experienced per HSDPA-user throughput for different number of users sharing the HS-DSCH. 7 W HS- DPA power and 5 HS-PDSCH codes are allocated. D. Environment sensitivity The presented results are obtained for a macro cellular scenario, assuming an ITU Vehicular-A power delay profile. If the ITU Pedestrian-A power delay profile is used instead, then it is found that the HSDPA cell throughput increases from 900 kbps (for Vehicular-A) to 1240 kbps for 7 W HSDPA power and 5 HS-PDSCH codes. This increase in capacity mainly comes from an improved downlink orthogonality, as the ITU Pedestrian-A channel has less temporal dispersion. Furthermore, if the simulations are repeated for a micro cell scenario with higher interference isolation between adjacent cells, then the cell capacity increases further [12]. Hence, the achievable throughput results on DCH and HSDPA depend strongly on the environment settings. V. CONCLUDING REMARKS Network performance results from dynamic WCDMA simulations have been presented for cases with mixed traffic on DCH and HSDPA. It has been shown that the available cell transmit power can be better utilized when introducing HSDPA as no power control headroom is required for HS-DSCH transmission. The sensitivity on the cell throughput depending on the HSDPA power allocation has been presented, as well as the per HSDPA-user experienced throughput depending on the number of active HSDPA-users. It is found that the total cell capacity can be increased by 69% in a macro cell environment by allocating only 5 HS-PDSCH codes and 6-7 W for HSDPA transmission, while the remaing resources are used for Rel 99 channels. REFERENCES [1] H. Holma, A. Toskala (Editors), WCDMA for UMTS - Radio Access for Third Generation Mobile Communications, John Wiley and Sons, Third Edition, 2004. [2] F. Frederiksen, T.E. Kolding, Performance and modeling of WCDMA/HSDPA transmission/h-arq schemes, IEEE Proc. VTC, pp. 472-476, September 2002. [3] J.M. Holtzman, CDMA Forward Link Water Filling Power Control, IEEE Proc. VTC, pp. 1663-1667, May 2000. [4] A. Jalali, R. Padovani, R. Pankaj, Data Throughput of CDMA-HDR a High EfficiencyHigh Data Rate Personal Communication Wireless System, IEEE Proc. VTC, pp. 1854-1858, May 2000. [5] R.C. Elliott, W.A. Krzymieh, Scheduling Algorithms for the cdma2000 Packet Data Evolution, IEEE Proc. VTC, September 2002. [6] H. Zheng, On UMTS-HSDPA TCP Throughput with Scheduling and Hybrid ARQ, Proc. IEEE VTC, Otober 2003. [7] S. Hamalainen, et.al., A novel interface between link and system level simulations, Procs. of ACTS Summit 1997, pp. 509-604, Aalborg, October 1997. [8] K.I. Pedersen, A. Toskala, P.E. Mogensen, Mobility Management and Capacity Analysis for High Speed Downlink Packet Access in WCDMA, IEEE Proc. VTC-2004-Fall, September 2004. [9] M. Nakamura, Y. Awad, S. Vadgama, Adaptive Control of Link Adaptation for High Speed Downlink Packet Access (HSDPA) in W-CDMA, in Proc. WMPC, pp. 382-386, 2002. [10] D.W. Paranchych, M. Yavuz, A Method for Outer Loop Rate Control in High Data Rate Wireless Networks, IEEE Proc. VTC 2002, pp. 1701-1705, 2002. [11] 3GPP TS 25.214, Physical Layer Procedures, Version 6.1.0., March 2004. Available at: www.3gpp.org. [12] T. E. Kolding, K. I. Pedersen, J. Wigard, F. Frederiksen, P. E. Mogensen, High Speed Downlink Packet Access: WCDMA Evolution, IEEE Vehicular Technology Soceity (VTS) News, Vol. 50, No. 1, pp. 4-10, February 2003. 4500