An Enhanced DQRUMA/MC-CDMA Protocol for Packet-Based Multi-Service Traffic Jae Yoon Park , Dan Keun Sung , and Won Jin Park ;
Dept. of EECS, Korea Advanced Institute of Science and Technology Taejon, 305-701, Korea Korea Telecom R&D Group, Woomyun-dong 17, Seocho-gu, Seoul, 137-792, Korea
[email protected],
[email protected], and
[email protected] Abstract , In this paper we introduce an enhanced DQRUMA/ MC-CDMA protocol for wireless packet networks. The original DQRUMA/MC-CDMA protocol may suffer severe code collisions due to a limited number of request codes. An enhanced DQRUMA/MC-CDMA protocol using minislots for request accesses is proposed to reduce request collisions. In the proposed protocol, request attempts can be transmitted concurrently with data packets of other calls. The performance of the proposed protocol is evaluated by simulation and its result shows that the proposed protocol outperforms the original DQRUMA/MCCDMA protocol in terms of average total delay for data traffic and packet loss rate for voice/video traffic. I. Introduction It is important to develop appropriate multiple access techniques in the emerging third generation wireless communications such as International Mobile Telecommunications (IMT)-2000 [1]. A slotted Code Division Multiple Access (CDMA) system or a hybrid time division multiple access (TDMA)/CDMA system which combines TDMA and CDMA technologies can be a good candidate [2–5]. Liu et al.[5] proposed a DQRUMA protocol based on multi-code (MC) CDMA for wireless packet (ATM) networks. It has good features such as a piggybacking mechanism to reduce request accesses and a bandwidth-ondemand fair-sharing round-robin (BoD-FSRR) transmit scheduling policy with maximum capacity power allocation (MCPA) approach by slot-by-slot to fully utilize radio resources. Code collisions and packet corruptions due to multiple access interference (MAI) are important problems in a receiver-oriented slotted CDMA system. In a TDMA slot structure, minislots are generally used to reduce collisions [6], [7]. We define a lattice pool for request access (LPRA) which consists of codes on the ordinate and several minislots on the abscissa in hybrid CDMA/TDMA protocols. The LPRA scheme can be applicable to hybrid CDMA/TDMA protocols including [2], [5]. In the original DQRUMA/MC-CDMA protocol, the impact of code collisions on request access delay is severer than for packet corruptions due to MAI because request access packets are protected with powerful FEC codes. We apply this LPRA scheme to the DQRUMA/MC-CDMA protocol for performance enhancement in this paper. In order to apply an LPRA scheme to the DQRUMA/MC-CDMA protocol, we adopt a policy in which request packets are transmitted concurrently with data packets of other calls
during a time slot. Collision reduction yields a decrease in both average access delay and average total delay. The rest of the paper is organized as follows. In Section II a slot structure and access schemes of the proposed protocol are described. Its performance is evaluated and is compared with the original DQRUMA/MC-CDMA protocol in Section III. Finally, conclusions are drawn in Section IV. II. Enhanced DQRUMA/MC-CDMA Protocol A new slot structure based on an LPRA scheme is proposed for uplink where each slot is divided into a number of small minislots and several codes are assigned for requests and request packets are transmitted concurrently with data packets of other calls during a time slot. Since the length of request packets is generally much shorter than that of data packets, a time slot can be divided into a number of small request minislots. The LPRA size is defined as the product of the number of request minislots per slot and the number of codes assigned for request accesses. Thus, as the LPRA size becomes large the number of request collisions can be greatly reduced. This size is easily adjustable by only changing the number of codes for request accesses between a base station and mobile stations. In the slot structure of the original DQRUMA/MCCDMA protocol request packets are transmitted first and then data packets are transmitted on the uplink. In the proposed protocol, however, in order to reduce the overhead caused by LPRA, request packets in minislots are transmitted concurrently with data packets of other calls during a time slot. Hence, it is necessary to divide a set of codes for concurrent transmission of request and data packets. The whole set of pseudo-noise (PN) codes, C is partitioned into two non-overlapping subsets CTX and CREQ , which are used for data transmission and request accesses, respectively. Thus, unlike [5], it is necessary to reserve additional capacity margin for concurrent request accesses besides background noise. Thus, in the proposed protocol, capacity for data transmission is smaller than for the original DQRUMA/MC-CDMA protocol by a capacity margin for concurrent request accesses. Since request attempts are dispersed on the LPRA, actual interference on data transmission due to request attempts is small in the proposed protocol. In the proposed protocol the interference level of the LPRA on data packets is considered to be the same as that of a data packet of the basic rate mobile calls on the
Code no. for request accesses 12 11
1
Piggybacking
Voice/Video Data
0 0
1
2
3
6
7
8
Power
9
Minislot no. for request accesses
UPLINK
Allocated for request (LPRA) Allocated for receiving Req. Ack Allocated for receiving Xmt. Permission
Minislot (k)-th time slot on the uplink
(k)-th time slot on the downlink
t
Power
DOWNLINK
Req.Xmt. Ack Permission
Data Voice/Video
Fig. 1. The k-th time slot structure of the proposed protocol.
data transmission permission calculation. In order to determine the transmission power of request access packets it is necessary to calculate the relative power level of a request access packet to a basic rate data packet. Let d and r represent the required Eb =Io of a data packet and a request packet, respectively. Then, the relative channel capacity ( RCC ) is defined as the ratio of the channel capacity of a data packet to that of a request packet as follows:
RCC
1+ 3 G = 1 + 23 Gd ; 2 r
where 32 is a coefficient that arises in the Gaussian model because rectangular pulses are assumed. Thus, we can obtain the required Eb =Io , r , of request access packet if G, d and target RCC are given. For example, for G = 18 dB, d = 2.6 dB, and RCC = 0.5, r is given by -0.46 dB. Since request access packets of the original protocol are protected with a powerful FEC scheme, reduction of transmission power for request accesses is an efficient method to provide more data transmission capacity in the proposed protocol. Thus, the transmission power of request access packets is assumed to be reduced by half like the above example. In this case compensation for the increased bit error probabilities of request packets is achieved by a powerful FEC scheme. Figure 1 shows a slot structure of the proposed protocol when propagation delay is neglected and a slot is divided into 12 minislots considering guard time [8], [9], [10]. First ten minislots are available for requests and the last two minislots are used for reception of a request acknowledgement and a transmission permission from the BS. Requests attempted in the k -th uplink slot before the start of the k -th downlink slot can be acknowledged or permitted in the k -th downlink slot. The ending point of transmit
permission in the k -th downlink slot is aligned with the ending point of the k -th uplink slot to maximally receive request packets. Of course, requests may be attempted on the uplink during request acknowledgement and transmit permission minislots of the downlink and then, they are acknowledged or permitted in the next slot. However, this case is not considered in this paper. There are two access schemes, i.e., Schemes I and II, in the proposed protocol. In Scheme I request packets can be sent in one of many minislots with a randomly chosen code concurrently with data packets of other calls during a time slot. Request packets are assumed to collide only when both the same code and the same minislot are selected. Hence, the number of code collisions of request packets can be greatly reduced by LPRA. When an attempted call is admitted in a BS a unique, short temporary ID (TID) is assigned to the call. Scheme II is the same as Scheme I except that access requests are attempted in the pre-allocated minislot position with a request access code assigned during the call admission control (CAC) procedure. Thus, this scheme is available when the LPRA size is equal to or larger than the number of admitted calls. In Scheme II the request minislot number and the request code of each call are assigned in such a way that:
= bT ID=M c; Nm = T ID , Nc M; where Nc , Nm , M and bX c denote the request code numNc
ber, the minislot number, the number of request minislots per slot and the largest integer that is smaller than or equal to X , respectively. Thus, there are no request collisions in Scheme II. Schemes I and II may be used alternately from tome slot to time slot according to the number of admitted calls. If the LPRA size is large enough to allocate unique request access positions on the LPRA to all admitted calls, Scheme II is better than Scheme I in terms of average total delay because there is no request collision in Scheme II. III. Performance Evaluation All simulation environments and terminologies of the proposed protocol, except some of parameters listed in Table I, are the same as those in the original DQRUMA/MCCDMA protocol [5]. The ideal DQRUMA/MC-CDMA protocol is defined as an ideal access system in which the base station has complete, perfect knowledge about the request accesses of all mobiles in the system (with no overhead). In other words, the base station always knows which mobiles have packets to transmit and the request table is always up-to-date; no request access is needed and therefore, the request access delay is zero [5]. And the BoD-FSRR scheduling policy with an MCPA criterion of the original DQRUMA/MC-CDMA protocol is adopted for a packet transmission permission policy in the proposed protocol. The original protocol was designed for data traffic only, but we augment it to accommodate real-time traffic additionally and compare it with the proposed protocol. The
slot structure of the augmented protocol is the same as that of the original DQRUMA/MC-CDMA protocol and we call it Modified protocol in this paper. In other words, Modified protocol is the same as the original protocol in data traffic handling while it is different from the original protocol in aspect of packet dropping in real-time traffic handling. And the ideal DQRUMA/MC-CDMA protocol is also augmented to Modified ideal DQRUMA/MCCDMA protocol in the same manner. Herein, video traffic means video telephony traffic. The following assumptions are used for simulation: Assumptions – Each mobile supports only a single type of traffic among voice, data and video. – All the mobiles are assumed to have the same maximum transmission rate supporting up to 10 code channels per time slot. – All types of packet are generated at the starting point of each time slot. – When a voice/video packet can not obtain transmit permission in a time slot the packet is to be dropped out of its mobile buffer. – Processing and propagation delays are neglected. – Each packet is differentiated by using different BCH code schemes instead of different Eb =Io values according to traffic types. – Packets are optimally power controlled. – A traffic model is considered at the burst (packet) level. Thus, there are no calls generated or completed [11]. – The duration that voice sources stay in the on and of f states is geometrically distributed with a mean of 1=pon,off and 1=poff ,on, respectively. – Since voice/video services are delay-sensitive, the excessively delayed portion of speech/video traffic is clipped off. – Voice/video packets are clipped off if they experience request code collisions or if they do not receive transmit permission from a BS within a time slot. – Voice/video packets corrupted due to MAI during the transmission are lost and are not retransmitted. – Voice mobile terminals use a voice-activity monitoring technique. – In the on state a voice source generates a voice packet per time slot which can be accommodated by a single CDMA code on its transmission, whereas a voice source does not generate packets when it is in the of f state. – Burst arrivals of each data mobile follow a Bernoulli process. – Each data burst has two packets. – The buffer size of each mobile terminal is unlimited for data traffic. – A harmonic backoff algorithm with attempt probabilities of 1, 1/2, 1/3, 1/4, for data traffic is considered as a retransmission scheme in request access. – Corrupted data packets are retransmitted until the
packes are completely received. Thus, there are no data packet losses. – Bursts from each video mobile arrive according to an auto-regressive (AR) process [12]. – Each video burst has up to 2 packets. A traffic source at the burst level can be described using an on-off model. Each mobile terminal generates a new burst whenever a talk spurt or a video frame begins, and the burst is packetized into several packets for slotted transmission. Let pon,off and poff ,on be the transition probability of a voice source from the on to of f state and the transition probability of a voice source from the of f to on state, respectively. Voice, video, and data traffic have different characteristics and service requirements. Voice and video traffic are delivered in real-time with a negligible delay, while data traffic can be enqueued but requires lower bit error rates. Real-time packets have higher priorities than non-realtime packets in the procedure of transmission permission assignment in a base station and data packets corrupted due to MAI are retransmitted until they are received completely. We investigate three performance measures - average total delay, voice and video packet loss rates. The average total delay is defined as the duration taken from the time when a packet is created to the time when a BS receives the packet successfully. The packet loss rate is described as the sum of the rate of corrupted packets due to MAI on the air interface and the rate of clipped packets due to request code collisions, channel capacity limit, and piggybacking information loss. Thus, if exactly two requests are attempted in every request minislot, it takes the same interference level as a data (voice/video) packet (at a basic rate) transmission does during the span except guard time intervals between mini-slots. Thus, when the number of request accesses is less than two in a slot for the proposed protocol, the interference of packets transmitted during the span of the minislot is less than for the original DQRUMA/MC-CDMA protocol. Since most of voice and video packets are transmitted with the piggybacking scheme, the number of access requests of these calls are small. Figures 2 and 3 show the video and voice packet loss rate of the proposed protocol, respectively. The performance of the proposed schemes is better than for Ideal DQRUMA/MC-CDMA protocol as well as the original protocol because the interference amount caused by actual request accesses are less than the reserved interference margin for request accesses, and the proposed schemes yield lower packet corruption due to MAI than for the original or Ideal DQRUMA/MC-CDMA protocol. Figures 4 and 5 illustrate the average total delay versus the number of voice mobile calls for two different numbers of data calls. The average total delay of the proposed protocol is shorter than for the original protocol. The average total delay of data traffic of the original protocol in Fig.5 is longer than that in Fig.4 due to increased request collisions. But the average total delay of the
TABLE I S IMULATION PARAMETERS (VOICE /DATA /V IDEO ) Parameters Max. number of calls in the system Required Eb =Io of voice, data and video packets Processing gain Mobile’s max. transmit rate Request packet size Data (voice, video) packet size Time slot duration Voice source rate Voice activity p p
on,off off ,on
Data source rate Number of data calls in the system FEC for request packets FEC for Voice TX packets FEC for Data TX packets FEC for Video TX packets Data burst generation (burst/slot/call) Packets per data burst in a slot Video source peak rate Number of video calls in the system Video burst generation Voice and Video packet delay limit Time slot assignment policy (priority) Number of codes for LPRA, CREQ Number of minislots for LPRA, M System capacity reservation factor of the proposed protocol
value 128 2.6 dB 64 10 codes/slot 31 bits 1023 bits 11.75 ms 32 kbps 0.35 1/30 1/55 64 kbps 0, 20, 40 BCH(31,11,5) BCH(1023,648,41) BCH(1023,628,43) BCH(1023,588,47) 0.1 (Bernoulli) 2 (Deterministic) 64 kbps 5, 10 VBR (AR-model) 1 time slot Voice>Video>Data 13 10
traffic over UMTS,” IEEE Trans. Veh. Technol., vol. 47, pp. 11481161, Nov. 1998. [5] Z. Liu, M. J. Karol, M. E. Zarki, and K. Y. Eng, “Channel access and interference issues in multi-code DS-CDMA wireless packet (ATM) networks,” Wireless Networks, pp. 173-193, 1996, J.C. Baltzer AG, Science Publishers. [6] Y. Li and S. Andresen, “An extended packet reservation multiple access protocol for wireless multimedia communication,” in Proc. PIMRC‘94, pp. 1254-1259, 1994. [7] X. Qiu and V. O. K. Li, “Dynamic reservation multiple access (DRMA) scheme for personal communication system (PCS),” Wireless Networks, pp. 117-128, 1996, J.C. Baltzer AG, Science Publishers. [8] M. J. Karol, Z. Liu, and K. Y. Eng, “Distributed-queueing request update multiple access for wireless packet (ATM) networks,” in Proc. ICC, pp. 1224-1231, 1995. [9] M. J. Karol, Z. Liu, and K. Y. Eng, “An efficient demandassignment multiple access protocol for wireless (ATM) networks,” Wireless Networks, pp. 269-279, 1995, J.C. Baltzer AG, Science Publishers. [10] J. Sanchez, R. Martinez, and M. W. Marcellin, “A survey of MAC protocols proposed for wireless ATM,” IEEE Network, pp. 52-62, Nov/Dec. 1997. [11] T. K. Liu and J. A. Silvester, “Joint admission/congestion control for wireless CDMA systems supporting integrated services,” IEEE J. Select. Areas Commun., vol. 16, no. 6, pp. 845-857, Aug. 1998. [12] B. Maglaris, D. Anastassiou, P. Sen, G. Karlsson, and J. D. Robbins, “Performance models of statistical multiplexing in packet video communications,” IEEE Trans. Commun., vol. 36, pp. 834844, Nov. 1988.
2 0.1
Video packet loss rate
proposed protocol in Fig.5 is similar to that in Fig.4 due to request collision reduction by the LPRA scheme. It can be observed that the performance of the proposed protocol is better than for the original protocol in terms of average total delay and packet loss rate.
Scheme I Scheme II Modified DQRUMA/MC-CDMA Modified ideal DQRUMA/MC-CDMA
0.01
0.001
IV. Conclusions
References [1]
[2]
[3]
[4]
E. Nikura, A. Toskala, E. Dahlman, L. Girard, and A. Klein, “FRAMES multiple access for UMTS and IMT-2000,” IEEE Personal Commun. Mag., pp. 16-24, Apr. 1998. L. Oritagoza-Guerrero and A. H. Aghvami, “A distributed dynamic resource allocation for a hybrid TDMA/CDMA system,” IEEE Trans. Veh. Technol., vol. 47, pp. 1162-1178, Nov. 1998. A. E. Brand and A. H. Aghvami, “Performance of a joint CDMA/PRMA protocol for mixed voice/data transmission for third generation mobile communication,” IEEE J. Select. Areas Commun., vol. 14, no. 9, pp. 1698-1707, Dec. 1996. A. E. Brand and A. H. Aghvami, “Multidimensional PRMA with prioritized baysian broadcast - a MAC strategy for multiservice
0.0001 60
70
80 90 100 The number of voice mobile calls
110
120
Fig. 2. The video packet loss rate in case of the number of video calls = 5 and the number of data calls = 0.
0.1 Scheme I Scheme II Modified DQRUMA/MC-CDMA Modified ideal DQRUMA/MC-CDMA Voice packet loss rate
A new slotted MC-CDMA protocol is proposed for wireless packet networks. Requests based on LPRA are attempted concurrently with data transmission of other calls, which yields a very low collision probability and considerably reduces the buffer size required in data mobile terminals. Simulation results show that both schemes of the proposed protocol yield better performance than the original DQRUMA/MC-CDMA protocol in terms of average total delay in data traffic and packet loss rate for voice and video traffic. The proposed protocol is adequate for real-time traffic as well as non-realtime traffic.
0.01
0.001 60
70
80 90 100 The number of voice mobile calls
110
120
Fig. 3. The voice packet loss rate in case of the number of video calls = 5 and the number of data calls = 0.
5 Scheme I (20 data calls) Scheme II (20 data calls) Modified DQRUMA/MC-CDMA (20 data calls) Modified ideal DQRUMA/MC-CDMA (20 data calls)
4
Average total delay of data calls [slots]
Average total delay of data calls [slots]
5
3
2
1
0
Scheme I (40 data calls) Scheme II (40 data calls) Modified DQRUMA/MC-CDMA (40 data calls) Modified ideal DQRUMA/MC-CDMA (40 data calls)
4
3
2
1
0 10
20
30
40 50 60 70 The number of voice mobile calls
80
90
100
Fig. 4. The average total delay in case of the number of video calls = 5 and the number of data calls = 20.
10
20
30 40 50 60 The number of voice mobile calls
70
80
Fig. 5. The average total delay in case of the number of video calls = 5 and the number of data calls = 40.
