An adaptive QoS guaranteeing MAC protocol for real-time traffic in TDMA-based wireless ATM networks

Computer Communications 24 (2001) 344±352

www.elsevier.com/locate/comcom

An adaptive QoS guaranteeing MAC protocol for real-time traf®c in TDMA-based wireless ATM networks S. Yoon, S. Bahk* School of Electrical Engineering, Seoul National University, Seoul, South Korea Received 11 October 1999; accepted 20 April 2000

Abstract This paper presents a new media access control (MAC) protocol based on forward error control (FEC), which is appropriate for supporting real-time traf®c with strict QoS requirements in wireless ATM networks. As the channel BER in wireless environments is very high and varying 10 25 ±10 22, previous schemes that use powerful FEC have combated to overcome this noisy channel condition at the cost of valuable bandwidth. As most previous works have been dedicated to maximize the channel ef®ciency, they were not able to meet QoS requirements of real-time applications in wireless networks. A new MAC protocol proposed in this paper is designed to guarantee the throughput requested by a real-time traf®c user while keeping the bandwidth consumption at a minimum. The proposed scheme is for a TDMA system and uses adaptive FEC. We analyze the wireless channel and model it as a two-state error control system to design an ef®cient MAC protocol. We use simulation experiments to show how the proposed scheme provides QoS guarantees, and compare it with the CDMA system in terms of capacity, i.e. the number of users that can be supported. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Adaptive guaranteeing; MAC protocol; TDMA-based wireless ATM networks

1. Introduction It is expected that multimedia services will be provided in broadband networks in the near future [1]. This expectation has brought about a new need for supporting multimedia services in wireless networks, which result in a new problem of compatibility between wired and wireless networks [2,3]. A possible networking scenario to support wireless multimedia services is to use wireless ATM. Therefore wireless ATM networks should maintain the compatibility with currently deployed wired ATM networks, leading to the use of similar notions of the ®xed cell size, VCI routing and QoS provisioning. Error control schemes used in ATM are designed for the channel BER of 10 29. Therefore they are not appropriate for use in time-varying and noisy wireless channels. To provide various QoS guarantees in wireless ATM environments, forward error control (FEC) and ARQ are frequently used. Also adaptive error control schemes have been studied in order to utilize valuable wireless bandwidth ef®ciently [2±6]. They have used various codes such as shortened codes and rate compatible punctured convolutional

* Corresponding author. Tel.: 182-2-880-8414; fax: 182-2-880-8214. E-mail address: [email protected] (S. Bahk).

(RCPC) codes to meet QoS requirements of multimedia services. While these schemes have the advantage of using the same hardware in generating various code rates, they have two serious problems that make it dif®cult to be applied for real-time traf®c in wireless ATM networks. The ®rst problem is due to the transmission of the ®xed sized ATM cell. As adaptive rate codes result in various sizes of data, ¯exible bandwidth allocation schemes should be used to meet the delay requirement of real-time traf®c. This makes the media access control (MAC) protocol design more dif®cult. The second problem occurs due to the noisy channel. By using the ®xed code rate, it is not possible to provide QoS guarantees in the noisy channel. In Ref. [2], an adaptive FEC scheme has been studied to maximize channel throughput, which is not appropriate for supporting real-time traf®c. In Refs. [3±5] were also proposed the adaptive error control schemes that meet the user's various QoS requirements. But the adaptive error control schemes shown in Refs. [3±5] are not appropriate for transmitting the ®xed size of cell ef®ciently because they have been designed without considering channel error conditions. Therefore, we need to design an adaptive error control scheme suited for wireless ATM networks where various throughput guarantees should be supported. Our proposed MAC protocol is based on the adaptive error control

0140-3664/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0140-366 4(00)00238-3

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upper bound of received BER code rate 8/9 code rate 4/5 code rate 2/3

S1 S2 S3

code rate 4/7 code rate1/2 code rate 4/9 code rate 4/10

S4 S5 S6 S7

channel BER Fig. 1. BER according to RCPC codes.

scheme. In Section 2, we analyze the wireless channel, and model it into the two error control state system, and calculate the probability of a mobile terminal being at each error control state. In Section 3, we propose a new MAC protocol that uses the two-state error control and provides QoS guarantees for real-time traf®c. Then we evaluate its performance. The conclusion is given in Section 4.

its probability density function (p.d.f.) is represented by the log-normal distribution. It is possible to keep the received bit error rate at a certain level over a wide range of channel BER by using RCPC [5,9]. The performance of RCPC is measured by the received bit error probability Pb and the ®rst error event probability PE. They are bounded by [9]

2. Classi®cation of wireless channel states

Pb #

…1†

This equation describes the path loss at distance r from the base station. The path loss exponent n varies depending on r [7]. The log-normal distribution is given by [8] 1 p…x† ˆ p exp…2…x 2 m x †2 =2s 2 †: 2ps

…4†

1 1 X a P ; P dˆd d d

…5†

free

We brie¯y review path loss models in micro-cellular environments, and classify the wireless channel into two states to apply an adaptive error control scheme according to the channel BER condition. From long-term fading, the path loss equation is given by [6] PL…r† ˆ 10n log…r† 1 p1 :

1 1 X c P ; P dˆd d d

…2†

mx and s represent, respectively, the mean and standard deviation of the received average power at distance r from the base station. Short-term fading can be represented by Rayleigh fading, which is given by [3] ! x x2 …3† p…x† ˆ 2 exp 2 2 : s 2s p Here V x ˆ …p=2†s is the mean of the received signal x and

PE #

free

where P is the puncturing period, dfree is the free distance, ad is the number of paths at distance d from the transmitted path, cd is the total number of nonzero information bits on all paths at distance d from the transmitted path and Pd is the probability that the decoder selects an erroneous path at distance d from the transmitted path. We can obtain the RCPC rates varying from 8/9 to 4/10 for the original code of (3,1,6). The BER curve is given in Fig. 1. We classify the channel state according to the channel BER for error control. The BER depends upon the SNR at the receiver. The signal power received at a mobile terminal varies according to its mobility. The p.d.f. of the received power signal is used to obtain the probability of the terminal being at each channel state. To obtain the p.d.f. of the received signal power, we assume the mobility pattern to be random. Let the random process P…x; y : t0 † denote the received power at time t0 at position (x,y). P…x; y : t0 † is lognormally distributed if there is no movement. Then the

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Required QoS BER=10 -3

Required QoS BER=10 -2

probability channel BER Required QoS BER=10 -5

Required QoS BER=10 -4

each state

Fig. 2. The probability of a mobile being at each error control state.

average received power at time t0 1 Dt is given by I P…x; y : t0 † £ Ds …x; y† ds; P…x; y : t0 1 Dt† ˆ

…6†

surface

where Ds …x; y† is the p.d.f. of the user mobility. As the mobility pattern is random, the net power variation towards and away from the base station becomes zero because its directional components cancel each other. Therefore, only the power variation on the tangential direction of the circle centered at the base station needs to be considered. The p.d.f. of the average received power is the same as that of the user mobility along the circle centered at the base station. This p.d.f. is log-normally distributed and P…x; y : t0 † is no longer a random process. We also assume the Rayleigh fading channel and coherent BPSK modulation/demodulation. The procedures for calculating the probability of a terminal being at each error control state are as follows. (a) Calculate the received power of a mobile user by using Eq. (1). (b) Calculate the SNR at the cross points of between two adjacent control states based upon the required BER. If the BER, Pe,psk, is given at each cross point, the SNR is given by SNR…i† ˆ

…1 2 2 £ Pe;psk †2 : 4 £ Pe;psk …1 2 Pe;psk †

…7†

(c) Find the cross points of error control states for the lognormal distribution function. The mean value of this p.d.f. is obtained from (a). The cross point Ci is given by sr    p SNR…i†N0 …8† Ci ˆ log10 2 where N0 is the background noise. (d) The probability of the terminal being at error control state i, Pi, is given by Pi ˆ

ZCi 1 1 Ci

f …x† dx

…9†

where f(x) is the log-normal distribution function. The transmission power and the distance from the base station are required to calculate the mean received power in (a). Instead of using these values, we assume several values of the mean received power that are able to represent some positions. For example, when SNR ˆ 7 dB, the channel BER is given as 0.043 in the Rayleigh fading channel. Therefore, the results given in Fig. 2 represent all possible positions within a cell. From Fig. 2, we can conclude that the classi®cation of the wireless channel into two or three error control states according to the channel BER may be suf®cient for all cases. To make the system simple, we selected the two-state

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CSI

Scheduler

Base station

Mobile terminal

Fig. 3. Scheduler.

error control model. States p1 and p2 are combined for the state `quiet' and p3±p7 for the state `noisy'. Since ATM uses the ®xed size of packet, called a cell, for transmission, much attention needs to be paid in selecting code rates. If code rates 4/5 and 4/10 are used for the quiet and noisy states, respectively, the amount of data to be transmitted for the noisy state is twice as much as for the quiet state. By classifying the wireless channel into two states and using the adaptive coding scheme, various BER requirements at the receiver can be met over the wide range of channel BER. 3. Proposed MAC protocol In this section we propose a MAC protocol that allocates extra bandwidth to provide QoS guarantees for users in the noisy state. For the upstream channel, a contention resolution algorithm in Ref. [10] is used. The system operates based on time slots. When a user is in the noisy channel, the system assigns an extra slot for that user, which is twice as many slots as needed for the user in the quiet channel. Considering the small size of a cell covered by a base station, we assume the propagation delay between the base station and a mobile user is negligible [10]. The scheduler given in Fig. 3 has the full knowledge of channel state information (CSI).

Fig. 4 shows the general frame structure that contains allocation slots for notifying which users are eligible for transmission in the next frame [10]. The upstream frame contains signaling mini-slots for the channel access that uses the slotted ALOHA protocol. Active mobile users use queue status mini-slots to feedback their queue information to the base station. Based on the information of the queue status and the channel state, the scheduling algorithm allocates the appropriate number of slots to guarantee the QoS requested by active users. The number of queue status mini-slots is equal to the maximum number of mobile users that can be served within a frame. The message slots are used for the message transmission. The scheduler needs to reserve a certain number of slots for active users possibly in the noisy state. The reserved slots are used for users in the noisy channel to meet their QoS requirements. If some slots are still left unassigned, they will be allocated to new calls. The allocation slots in the downstream frame are used to inform mobile users of which ones are eligible for transmission at the next upstream frame. 3.1. Calculation of the number of reserved slots The scheduler should satisfy the user's requested QoS that has been negotiated during the call setup procedures. Therefore for some users in the noisy state, the scheduler

Upstream frame structure Signaling slots

Queue status slots used for feedback

M message slots

Down stream frame structure M message slots

Allocation slots

Fig. 4. Frame structure.

Signaling slots

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received BER code rate 4/5

c1

code rate 4/10

c6

channel BER Fig. 5. Determination of the channel state according to various QoS.

σ =8

σ =9

Fig. 6. Number of reserved slots.

needs to reserve a certain number of extra slots. To obtain this number, it is necessary to establish the average number of users in the noisy state ®rst. Assume that the cell has a shape of a circle with radius R and that users are uniformly distributed within the cell. The probability that a mobile terminal is located at the distance r from the base station is given by 2 fp …r† ˆ 2 r R

…10†

The determination of the channel state depends upon the required BER as shown in Fig. 5. For example if the QoS of BER 10 26 is requested by a mobile user, it can be guaranteed at the channel BER below C6 by using the 4/5 rates code below C1 and the 4/10 rates code between C1 and C6. Let Sc denote the power at the cross point and P(r) denote the received power at the distance r from the base station. The probability that a mobile user is in the quiet state, pg, is

given by pg …r† ˆ Pr…P…r†uSc † ˆ

Z1 1 p exp…2z2 † dz p a

log Sc 2 log P…r† p : where a ˆ 2 2s Pg in the circular cell is given by ZR fp …r†pg …r† dr: Pg ˆ 0

…11†

…12†

The number of slots that need to be reserved varies according to the scheduling method. Assume that constant bit rate (CBR) traf®c is served by circuit switching and variable bit rate (VBR) 1 traf®c by packet switching. Let Nf, Nr, Nn and Np denote the number of slots in a frame, 1 It represents an on±off source with exponentially distributed call holding time.

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one slot missing ratio reserved slot 2

reserved slot 5(o) reserved slot 4(*)

reserved slot 3

required QoS: BER Fig. 7. One slot-missing ratio.

the number of reserved slots, the average number of users in the noisy state and the maximum number of users that can be served within a frame, respectively. Then Nr can be obtained by an iteration method. Consider the case of CBR traf®c ®rst. (a) Given Nr (b) 2Np ˆ Nf 2 Nr : (c) Nn ˆ Np …1 2 Pg †: (d) Nr ˆ Ceil…Nn †; which takes the smallest integer greater than Nn, and go to (b). For VBR traf®c, let p denote the probability that a VBR call is active and Nan denote the average number of users in the noisy state.

(e) Nr ˆ Ceil…Nan † and go to (b). Fig. 6 shows the number of slots that need to be reserved for the case of Nf ˆ 12; R ˆ 200 m; and the required BER of 10 23. Here, as an example, we consider the packet reservation multiple access (PRMA) scheme that supports on±off sources and the circuit switching scheme. For this scheme, Np is calculated by the method presented in Ref. [11]. The standard deviations of 8 and 9 are considered for log-normal distribution. The results show that 3 slots in

Packet dropping probability

(a) Given Nr (b) Calculate Np by using a scheduling algorithm.

(c) Nn ˆ Np …1 2 Pg †: (d) ! Nn X Nn i p …1 2 p†Nn 2i £ i: Nan ˆ i iˆ0

number of users Fig. 8. Packet dropping probability.

S. Yoon, S. Bahk / Computer Communications 24 (2001) 344±352

Average access delay

350

number of users Fig. 9. Average access delay.

circuit switching and 4 slots in PRMA are required to meet the QoS requirements. 3.2. Performance evaluation In simulations we consider one type of CBR traf®c which requires one slot of bandwidth when the channel is quiet. If too many users are being served, there may not be extra slots that can be used by some users in the noisy state. In this case, QoS cannot be guaranteed for those users. Let us de®ne the one slot-missing ratio as the performance measure. That is the ratio of the number of users in the noisy state who need two slots each to the number of users who fail to receive the requested two slots. For the frame length of 12 slots, the one slot-missing ratio is given in Fig. 7. When 4 slots are used, the acceptable performance was obtained. For the simulation of VBR traf®c, ATM cells arriving from the active calls contend for channel access and the admitted cells are multiplexed for transmission of each frame. Therefore, the order of transmitted ATM cells can vary within the frame. The scheduler shown in Fig. 3 determines which active calls will be transmitted at the beginning of every frame. Cell loss occurs if a cell cannot be transmitted within the two frame time from the arrival. We adopt the packet dropping probability and the average access delay as the performance measures. Fig. 8 shows the packet dropping probability when the requested BER varies from 10 22 to 10 26. The average access delay measured in unit of slots is given in Fig. 9. It shows that if the packet dropping probability is below 10 22, the average access delay is not much greater than one frame time of delay, i.e. 12 slots. Therefore the proposed MAC protocol can be applied for real-time VBR traf®c that requires acceptable delay jitter and guaranteed bandwidth, regardless of the channel state.

3.3. Comparison with a CDMA system We compare our proposed TDMA based system with the CDMA system which uses perfect power control. The maximum number of users that can be served in a micro-cell is considered for comparison. The CDMA system uses more and more power to provide QoS guarantees as QoS requirements requested by users are getting stricter. In Ref. [12], the CDMA system capacity with perfect power control is calculated. When the maximum number of users served in a cell is N, the probability P of the system guaranteeing the requested QoS is given by ! NX 21 N 2 1 12Pˆ ak …1 2 a†N212k k kˆ0   d 2 k 2 0:741N p ; …13† Q 0:234N where a is the call activity factor, d is a temporary variable 2 and Q(x) is the Q function. We assume that a two-array antenna and a maximal ratio combined with coherent detection are used. If there exist L independent paths in Rayleigh fading environments, the crossover probability is given by [13] ! k 21 L 2 1 1 k  h iL LX 1 1 …1 1 m† ; …14† P0 ˆ 2 …1 2 m† 2 k kˆ0 p where m ˆ gc =…gc 1 1† and gc ˆ (SNR at the receiver)/L. If the required BER is Pb, the procedures to calculate the CDMA system capacity are given as follows. (a) Obtain P0 which satis®es Pb by Eq. (4). (b) Obtain gc which satis®es P0 by Eq. (14). (c) Obtain the CDMA system capacity by using SNR ˆ gc £ L in Eq. (13). 2

This is dependent upon the SNR at the receiver.

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351

Fig. 10. System capacity comparison.

The probability P can be interpreted as the packet dropping probability. Let P ˆ 10 26 Then we compare the capacity of the CDMA system with that of the proposed MAC system. The variables used in this comparison are listed below. data rate: 16 Kbps on±off source with exponentially distributed talk and silence spurt durations; mean talk spurt of 1 s, mean silence spurt of 1.3 s; QPSK modulation, coherent detection and roll off factor of 1/4; voice activity factor of 3/8, 1.25 KHz bandwidth, cluster size of 7; synchronization overhead: 10 bits/slots; guard time overhead: 1% of total bandwidth; (3,1,6) convolutional code (original code of RCPC) for the CDMA system; channel state: quiet and noisy states with exponentially distributed duration times; frame length: 12 slots, 24 ms; message slots of 12 slots: 48 bytes 1 12 bytes (RCPC with rate 4/5); up/down stream signaling slots of 12 slots, feedback slots of 20 slots and allocation slots of 12 slots: 4 bytes 1 4 bytes (RCPC with rate 1/2). Fig. 10 shows the simulation results. As the QoS requirement is becoming tighter, the proposed protocol supports more users than the CDMA system. This is because if channel environments are getting worse, the CDMA system uses more and more power for transmission, which increases the interference for users in the neighboring cells. This increased interference exponentially deteriorates the CDMA system capacity as described in Ref. [13].

4. Conclusion We have proposed a new MAC protocol suited for TDMA-based wireless ATM networks where channel environments are time varying and the transmission packet size is ®xed. The wireless channel was ®rstly modeled as a two-state error system in order to facilitate its adaptive control. Then we considered a network supporting one type of traf®c and proposed an MAC protocol that can guarantee the bandwidth requested by a real-time traf®c user. The proposed scheme assigns an additional slot for a user in the noisy state to guarantee the QoS requested by that user. The ATM cells are scheduled for transmission at every frame time according to the channel error state and queue length status. Through the simulations, we showed that our scheme provides acceptable QoS for users by adopting powerful FEC, which overcomes time varying channel conditions. It was also shown that the proposed scheme achieves comparable performance to the CDMA system as the required QoS is getting tighter. For multiple types of traf®c with different QoS requirements, the proposed MAC protocol can be also used with a slight modi®cation. Acknowledgements This work has been supported by KOSEF under the grant number 96-0102-10-01-3. References [1] D. Raychaudhari, N.D. Wilson, ATM-based transport architecture for multiservices wireless personal communication networks, IEEE Journal on Selected Areas in Communications 12 (8) (1994) 1401± 1414.

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[2] Y. Kim, S.W. Bahk, An adaptive error control scheme by using shortened code, in: Proceedings of IEEE Globcom'96, 1996, pp. 2157±2161. [3] D. Moor, M. Rice, Variable rate error control for wireless ATM networks, in: Proceedings of IEEE ICC'95, 1995, pp. 988±992. [4] S.B. Wicker, M.J. Bartz, Type-II hybrid ARQ protocols using punctured MDS codes, IEEE Transactions on Communications 42 (2±4) (1994) 1431±1440. [5] B. Vucetic, An adaptive coding scheme for time-varying channels, IEEE Transactions on Communications 39 (5) (1991) 653±663. [6] H. Lou, A.S. Cheung, Performance of punctured channel codes with ARQ for multimedia transmission in Rayleigh fading channels, IEEE ICC'96, 1996, pp. 282±286. [7] H.H. Xia, et al., Micro-cellular characteristic for personal communications in urban and suburban environments, IEEE Transactions on Vehicular Technology 43 (3) (1994) 743±752. [8] Rappaport, Wireless Communications, Prentice Hall, Englewood Cliffs, NJ, 1996. [9] J. Hanenauer, Rate-compatible punctured convolutional codes and their applications, IEEE Transactions on Communications 36 (4) (1988) 389±400. [10] Y.J. Kim et al., A new medium access control scheme for wireless ATM networks, IEEE VTC (1997) 295±299. [11] S. Nanda, et al., Performance of PRMA: a packet voice protocol for cellular system, IEEE Transactions on Vehicular Technology 40 (3) (1991) 584±598. [12] K.S. Gilhousen, On the capacity of a cellular CDMA system, IEEE Transactions on Vehicular Technology 40 (2) (1991) 303±312. [13] J.G. Proakis, Digital Communications, McGrawHill, New York, 1995.

Sunkeun Yoon: Sunkeun Yoon received his BS and MS degrees from School of Electrical Engineering in Seoul National University in 1996 and 1998, respectively. He has been working at the K and C patent law of®ce as a patent attorney since January 2000.

Saewoong Bahk: Saewoong Bahk received his BS and MS degrees in electrical engineering from Seoul National University in 1984 and 1986, respectively, and his PhD degree from the University of Pennsylvania in 1991. From 1991 through 1994 he was with the Department of Network Operations Systems at AT and T Bell Laboratories as a Member of Technical Staff, where he worked on the projects for the development of network operation systems for AT and T networks. Then he joined the School of Electrical Engineering at Seoul National University and now serves as an associate professor. His areas of interests include performance analysis of communication networks, high-speed network protocol design, routing and resource allocation at wireless/wired networks.