W-CDMA RANDOM ACCESS CHANNEL TRANSMISSION ENHANCEMENT FOR SATELLITE-UMTS V. Y. H. Kueh1, A. Capellacci, R. Tafazolli, B. G. Evans Centre for Communication Systems Research (CCSR), University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom 1.
[email protected]
Abstract – The random access channel (RACH) is an uplink shared channel for initial channel access to the network as well as for short data bursts. In this paper, we propose modifications to the RACH preamble for satellite-UMTS using higher order of Hadamard codes, and it is shown that these enhancements have a better performance than that of the current Third Generation Partnership Project (3GPP) specifications and the European Telecommunications Standards Institute (ETSI) proposal. Keywords – Random access, Satellite-UMTS, ETSI, 3GPP
I. INTRODUCTION Third generation wireless systems, like Universal Mobile Telecommunication Systems (UMTS), are paving the way towards the much anticipated integration of the most successful technologies of the last decade – the Internet and cellular mobile telephony. These wireless Internet systems promise to provide a more exciting range of applications, some not even imagined up to a few years ago, such as wireless web browsing, video telephony, e-commerce and streaming multimedia with a higher data rate of up to 2 Mbps. For the efficient use of the scarce wireless spectrum, packet transmission over the uplink shared channels is used. The random access channel (RACH) plays a crucial part in UMTS as one of the uplink shared channels, as it is used not only for initial channel access to the network (for e.g. for call origination, paging response and registration messages) but also for sending short data bursts (for e.g. Short Messaging Service (SMS) packets). Since satellite has been identified to play an integral role in UMTS, it is desirable that the RACH transmissions over satellite-UMTS (SUMTS) follow closely the 3GPP W-CDMA RACH specifications [1], so as to achieve maximum commonality with the terrestrial part. Nevertheless, due to the different characteristics of the satellite channel, some modifications to the 3GPP RACH structure and procedure are necessary. The adaptation for the satellite environment proposed by ETSI in [2] degrades in performance when the random access attempt rate is high. Hence we propose modifications to the RACH preamble for S-UMTS using higher order of Hadamard codes and show that their performance is better than that in the current 3GPP and ETSI proposals.
0-7803-7589-0/02/$17.00 ©2002 IEEE
The paper is structured as follows. In Section II, the RACH procedure and structure optimised for S-UMTS are described. The simulators used at the link-level and system level are presented in Section III. In Section IV, simulation results comparing the 3GPP and the ETSI proposals with our modifications for the RACH in different satellite environments are shown and discussed. Finally, some conclusions are drawn in Section V.
II. RACH TRANSMISSION PROCEDURE AND STRUCTURE In the 3GPP specifications [1], the random-access transmission is based on a Slotted ALOHA approach with preamble power ramping and fast acquisition indication, sent in the downlink Acquisition Indication Channel (AICH). Only when the preamble is successfully detected, does the mobile terminal (UE) transmit the message part with a power related to the detected preamble and with a channelization code corresponding to the selected signature. There are 15 access slots per two frames and 16 signatures to choose from. Each preamble is of length 4 096 chips, consisting of 256 repetitions of a signature of length 16 chips. C sig,16 (i ) = P16 (i modulo 16), i = 0,...,4 095
(1)
where P16(k) is chosen from the set of 16 Hadamard codes (k = 0 corresponds to the first bit of the signature). The evaluation of the method of preamble power ramping with fast acquisition in the terrestrial-UMTS can be found in [3]. However the larger propagation delay experienced over the satellite channel renders this method of preamble power ramping with fast acquisition ineffective. This is because the UE has to wait for a duration of at least the round trip time of the satellite transmission before it can decide whether to send the message part (when the preamble transmission is successful) or to retransmit the preamble (if the preamble transmission is not successful). Also power is more likely to be wasted since the UE may have to send a few preambles before detection is successful; furthermore successful message transmission is not guaranteed as channel conditions may change from the transmission of the last preamble to the actual message transmission [4], and this is more apparent for the satellite case compared to the
PIMRC 2002
terrestrial due to the longer propagation delay. As a result, it is better to send the preamble together with the message part in the satellite case. Only when the preamble is successfully detected can the message part be decoded. If the attempt is not successful, then the whole block is retransmitted with a higher power. Using this technique, the preamble power ramping method is not employed and no AICH channel is needed. Instead, acquisition at the first attempt is preferred and in order to achieve one-shot acquisition, a longer preamble structure than the one defined in 3GPP is required. In [2], it is proposed by ETSI to have a preamble length of 36 864 chips, consisting of 8 x 256 repetitions of a 16-chips long signature and a unique word (UW) of 4 096 chips.
A link-level simulator was used to evaluate the preamble detection probability (Pd) with varying Ec/No, fixing the probability of false alarm (Pfa), defined as the probability that the preamble is detected when the preamble is not sent, equal to 10-3 or less according to [5]. We have performed simulation of a passive matched filter acquisition system adopting both a coherent and differential algorithm. The structure of the detector consists of a matched filter tuned to the beam’s scrambling code and a number of accumulators that perform correlation between each available signature and the received sequence. Assuming
C sig,16 (i ) = P16 (i modulo 16 ), i = 0,...,32 767
where r(k) is the received preamble, Cn the nth scrambling code and hm,X the mth Hadamard code (user’s signature) of length X = 16, 32, 64, 128, the output of each accumulator, when the coherent algorithm is considered, can be represented as
(2)
However, with this proposal, there is now only one access slot per frame. This will certainly degrade the performance of the RACH when the access attempt rate increases since the collision probability is higher. To reduce the risk of collision, we propose a modification to the preamble structure using higher order of Hadamard codes so as to extend the signature set. By extending the signature set to 32, 64 and 128, the preamble sequence now becomes C sig,X (i ) = PX (i modulo X ), i = 0,...,32 767
y m, X (s ) =
X −1
∑ C (k + X ⋅ s)r(k + X ⋅ s)h n
k =0
= ∑ s y m, X (s )
m, X
m, X
(k ), s = 0..... 32768 − 1
X
2
(4)
(5)
As for the differential accumulation, this is performed by breaking up the sequence into segments of length 1024 chips. The output of each accumulator is given by accumulating within a segment and then taking the conjugate product of consecutive sum as described below.
(3)
* 1024 1024 −1 X −1 X 1024 1024 (i − 1) = ∑ ∑ y m, X s + ⋅ i ∑ y m, X k + X X i =1 s = 0 k = 0 31
where X is equal to 32, 64 and 128 and PX is taken from the set of Hadamard codes of length X. The sequence is still of length 32 768 chips, but now the signature is repeated 1024, 512 and 256 times, respectively. This sequence together with a unique word of 4 096 chips still gives us the same length of 36 864 chips for the preamble, but now there are more signatures to choose from. By following the same scheme proposed for the terrestrial case, as found in [1], the available spreading factors for the message part are now limited since there is a one-to-one correspondence between the signatures used with the channelization codes used for the message part. The spreading factors that are available for the message part for the different sets of signatures are outlined in Table 1. Table 1 Available Spreading Factor for the Message Part Available Signatures 16 32 64 128
Message Part Spreading Factors 256, 128, 64, 32 256, 128, 64 256, 128 256
III. SIMULATION MODEL A. Link level simulations
m, X
(6) The maximum of these values is then selected and compared to a threshold. If the maximum value exceeds the threshold, the signature corresponding to that value is assumed as the detected signature. The threshold has been evaluated so that when only additive noise is present, the probability of the statistic being greater than the threshold is equal to 10-3. In evaluating Pd, we refer to the best case, assuming that the received sequence is already aligned at the receiver. Neither unique word nor any Doppler compensation techniques are adopted. Simulations using the 3GPP preamble structure are also carried out in order to show the improvements in performance when a longer preamble sequence is adopted. Thus for the 3GPP preamble structure, (6) can be rewritten as
m,16
* 63 3 63 = ∑ ∑ y m,16 (s + 64 ⋅ i ) ∑ y m,16 (k + 64 ⋅ (i − 1)) k=0 i =1 s = 0
(7)
B. System level simulations A system-level simulator was developed in OPNET (OPNET is a trademark of Opnet Technologies Inc), using the results from the link level simulations as input. Here it is
assumed the UEs access the gateway via a transparent GEO satellite. The performance of our proposed schemes is compared with the ETSI proposal in terms of throughput, determined as the average number of successful RACH attempts per frame (10 ms), and delay, defined here as the time from when a RACH request is generated to the time an acknowledgement (ACK) is successfully received from the gateway.
work. Capture effect is considered here, in that if there is more than one request selecting the same slot and signature (and are above the detection power threshold), the request with the higher power is accepted if the difference in power level compared to the rest is greater than the capture ratio. The main simulation parameters are summarized in Table 2. Table 2 Simulation Parameters
The random access transmissions are generated according to a Poisson process with a specified arrival rate λ. Each user can have at most one packet in its buffer. After a packet generation, the packet has to be transmitted and correctly acknowledged before the next one can be generated. If no ACK is received when the retransmission timer has elapsed, the user performs a random backoff for a random number of transmission time intervals (TTIs) uniformly distributed in [NBomin, NBomax]. It will then retransmit the request with a higher power according to the ramping step size, and this is repeated until Mmax attempts are reached. The mobile satellite channel model used follows the Lutz statistical model [6], where the fading process is switched between a Rician fading, representing unshadowed areas (good channel state) and a Rayleigh/lognormal fading, representing shadowing areas (bad channel state). The corresponding Doppler shift is applied to both the direct and multipath components in the good state condition. The transition between the good and bad states is characterized by a two-state Markov chain. The statistical parameters of the channel depend on the propagation environment, elevation angles of the satellite as well as the mobile speed. Here we assume the UE to be moving at a speed of 90 km/hr in an urban environment with satellite elevation of 24o and percentage of shadowed time of 66%, and in a highway environment with satellite elevation of 13o and percentage of shadowed time of 24%.
Parameter UE arrival rate λ Mobile speed Channel model
Spreading factor Retransmission timer Message length Mmax NBomin NBomax Power increase for message ramping Capture ratio Persistence TTI
Value 1- 16 attempts per frame 90 km/hr Lutz’s 2 state channel model (urban and highway) 256 600 ms 10 ms 5 0 8 3 dB 6 dB 1 10 ms
Note that in 95% of all cases, confidence measurements confirmed that the confidence interval of the simulation results is within 3% of the expected value.
IV. PERFORMANCE EVALUATION: RESULTS A. Link level results
For the interference, we consider both the uplink interference level caused by voice users and other random access bursts. The interference caused by the voice users is assumed to be fixed during the random access burst, and thus a 3 dB increase at the receiver, as in [7]. The interference caused by the multiple random access burst (due to the fact that the preamble signatures are not perfectly orthogonal to each other) is evaluated slot by slot and is thus varying during the duration of the desired burst.
Herein we have named our proposals with signatures of 32, 64 and 128 to be ETSI32, ETSI64 and ETSI128, respectively. Figures 1 and 2 show the probability of misdetection in a highway and urban environment, respectively. As expected, a longer preamble performs better in terms of probability of detection than the preamble defined in the 3GPP specifications. For example, using a differential detection, the required SIR for Pd = 0.9 of a longer preamble is about 10 dB below that of the 3GPP preamble in a highway environment.
An access request is considered as successful if no other request selects the same slot and signature, the received signal-to-interference ratio (SIR) at the satellite exceeds a required threshold, SIRt (set to the SIR when Pd is 0.9), and the message part is decoded correctly. In our simulation model, if the received SIR value exceeds the desired SIRt, we assume that the access attempt can be detected and decoded successfully. The effect of frame error rate (FER) is not considered herein and remains to be added in future
Comparing coherent and differential accumulation, it is observed that no much difference in performance gain is obtained when a differential detection is used for the 3GPP preamble. Nevertheless, when a longer preamble is used, the differential algorithm gives clear improvements. For instance, to achieve a Pd = 0.9, the required SIR is almost 7 dB below that of the coherent detection in both the highway and urban environments.
0
0
10
10
−1
−1
10
1−Pd
1−Pd
10
−2
10
−3
10
−4
10 −25
3GPP coherent ETSI coherent ETSI32 coherent ETSI64 coherent ETSI128 coherent 3GPP differential ETSI differential ETSI32 differential ETSI64 differential ETSI128 differential
−20
−2
10
−3
10
−15
−10
−5
Ec/No
−25
3GPP coherent ETSI coherent ETSI32 coherent ETSI64 coherent ETSI128 coherent 3GPP differential ETSI differential ETSI32 differential ETSI64 differential ETSI128 differential
−20
−15
−10
−5
Ec/No
Figure 1 Evaluation of probability of misdetection (1-Pd) in highway environment
Figure 2 Evaluation of probability of misdetection (1-Pd) in urban environment
Comparing the curves obtained in figures 1 and 2, a degradation of about 1 to 3 dB can be seen in an urban environment compared to the highway, since the bad state in an urban environment is more frequent and the shadowing is stronger than in a highway environment.
proposals over ETSI is much more evident in a highway environment than in an urban environment, while in an urban environment, the gain obtained from our proposals over ETSI is more pronounced when differential detection is used, compared to coherent method.
The significant observation in all cases is that when the set of signatures is extended to 32, 64 and 128, the detection probability performances in terms of the required SIR are very similar to those of the ETSI case. Hence we can expect improvements in the RACH throughput and delay since there are more signatures to choose from.
From our proposals, it is seen that ETSI128 has the best performance. This is not surprising since ETSI128 has the least risk of collisions since it has the largest number of signatures available to choose from. Nevertheless the spreading factor that can be used for the message part is now limited to 256. This spreading factor, corresponding to a channel bit rate of 15 kbps, can be supported by all terminals without limiting the system coverage and should be sufficient for signalling purposes and short data bursts. If a higher bit rate is needed (which means a lower spreading factor has to be used), then either ETSI64 or ETSI32 can be used, albeit a lower performance gain is obtained compared to ETSI128, but is still much better than ETSI.
B. System level results Figures 3-6 show throughput and delay results in highway and urban environments, comparing the proposed schemes with the ETSI proposal when both coherent and differential detection schemes were used. It is observed that when the number of arrivals is small, our proposals, ETSI32, ETSI64 and ETSI128, show performance close to those of the ETSI scheme. However, as the access rate increases, the performance of the ETSI proposal degrades, whereas the performance of our proposals do not degrade significantly even at high access rates. Hence improvements in our proposals over the ETSI proposal increase as the access request rate increases. For instance, for a differential detection in a highway environment, when the access rate is 4 per frame, the average throughput of ETSI128, ETSI64 and ETSI32 is 9%, 7.5% and 6% more than ETSI, respectively, while the delay is reduced by 23%, 20% and 16%, respectively. When the access rate is 16 per frame, the gain in throughput of ETSI128, ETSI64 and ETSI32 over ETSI is increased to 164%, 131% and 77%, respectively, while the delay has decreased to 58%, 42% and 19%, respectively. It is also noted that the improvement of our
V. CONCLUSIONS In this paper, modifications to the RACH preamble for SUMTS are proposed and their performances are compared to both the 3GPP and ETSI proposals in terms of misdetection probability, throughput and delay. It is shown that by modifying the ETSI preamble so that the signature set is extended does not degrade its performance in terms of probability of detection compared to the ETSI proposal. Hence it can be concluded that the detection probability of the preamble does not depend on the length of the signature but instead on the length of the preamble. With more signatures available, there is a reduced risk of collision and hence a higher throughput and lower access delay can be achieved. The only downsides of having a longer signature are increased complexity in the receivers as more matched
12
2600 ETSI coherent ETSI32 coherent ETSI64 coherent ETSI128 coherent ETSI differential ETSI32 differential ETSI64 differential ETSI128 differential
2200 Average delay for successful attempts (ms)
Average number of successful attempts per frame
10
ETSI coherent ETSI32 coherent ETSI64 coherent ETSI128 coherent ETSI differential ETSI32 differential ETSI64 differential ETSI128 differential
2400
8
6
4
2000
1800
1600
1400
1200
1000 2 800
0
0
2
4
6
8
10
12
14
600
16
0
2
Random access arrival rate [attempts/frame (10ms)]
8
10
12
14
16
Figure 5 Average random access delay vs arrival rate in highway environment
8
2600 ETSI coherent ETSI32 coherent ETSI64 coherent ETSI128 coherent ETSI differential ETSI32 differential ETSI64 differential ETSI128 differential
6
ETSI coherent ETSI32 coherent ETSI64 coherent ETSI128 coherent ETSI differential ETSI32 differential ETSI64 differential ETSI128 differential
2400
2200 Average delay for successful attempts (ms)
7
Average number of successful attempts per frame
6
Random access arrival rate [attempts/frame (10ms)]
Figure 3 Throughput performance vs arrival rate in highway environment
5
4
3
2
1
0
4
2000
1800
1600
1400
1200
1000
0
2
4 6 8 10 12 Random access arrival rate [attempts/frame (10ms)]
14
16
800
0
2
4
6
8
10
12
14
16
Random access arrival rate [attempts/frame (10ms)]
Figure 4 Throughput performance vs arrival rate in urban environment
Figure 6 Average random access delay vs arrival rate in urban environment
filters are needed, and also a higher value of spreading factor needs to be used.
[4] J. Moberg, M. Lofgren and R. S. Karlsson, “Throughput of the WCDMA Random Access Channel”, IST Mobile Communication Summit, Galway, Ireland, 1-4 October 2000 [5] 3GPP RAN WG#1, Technical Specification Group (TSG), 25.104 V4.1.0 [6] E. Lutz, D. Cygan, M. Dippold, F. Dolainsky, and W. Papke “The land mobile satellite communication channel-Recording, statistics and channel model”, IEEE Trans. Veh. Technol. Vol. 40, no. 2, pp. 375-386, 1991 [7] P. Choundhary, A. Ghosh and L. Jalloul, “Simulated Performance of W-CDMA Random Access Channel”, in Proc. VTC 2001 Spring, Rhodes, Greece, 6-9 May 2001
REFERENCES [1] 3GPP RAN WG#1, Technical Specification Group (TSG), 25.211-25.214 V4.1.0 [2] ETSI TS 101 851-3: “Satellite Component of UMTS/IMT2000; A-family; Part 3: Spreading and modulation”, V.1.1.1 [3] H. Olofsson, M. Edvardsson and G. Frank, “Performance Evaluation of Different Random Access Power Ramping Proposals for the WCDMA System”, Personal, Indoor and Mobile Radio Communications (PIMRC ’99), Osaka, Japan, 12-14 Sept. 1999
