PROPOSAL OF MULTI-HOP WIRELESS LAN SYSTEM FOR QOS GUARANTEED TRANSMISSION

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1 PROPOSAL OF MULTI-HOP WIRELESS LAN SYSTEM FOR QOS GUARANTEED TRANSMISSION Phuc Khanh KIEU, Shinichi MIYAMOTO Graduate School of Engineering, Osaka University 2-1 Yamada-oka, Suita, Osaka, JAPAN ABSTRACT The widespread use of multimedia networking applications has created the need for QoS support in the WLAN systems. The coverage area of WLAN systems is limited, so multi-hop WLAN system is considered to be one of the solutions. However, in home or office environments, due to the multipath propagation effect, the performance of multi-hop WLAN system is degraded. Therefore, QoS guarantee in multi-hop WLAN systems is still difficult. This paper proposes a multi-hop WLAN system for QoS guarantee by using channel estimation to select the appropriate transmission rate and giving optimum transmission opportunity to stations. The numerical simulation results show that by using the proposed system, QoS transmission can be guaranteed over a wide coverage area. Destination STA.1 STA.2 Source I. INTRODUCTION In recent years, IEEE wireless LAN (WLAN) systems have been widely established on campuses, in public places and in indoor environments to provide convenient data transmission between mobile devices and the Internet. The widespread use of multimedia networking applications has created the need for QoS (Quality of Service) guarantee in WLAN systems. The support of video, audio, real-time VoIP and other multimedia applications over WLAN with QoS requirements is the key to success for WLAN in multimedia home networking and future wireless communications. IEEE Working Group is currently developing a new standard called IEEE 82.11e, which enhances the original MAC protocol to support QoS transmission in WLAN systems. IEEE 82.11e adds a new function called HCCA (Hybrid Coordination Function Controlled Channel Access), which is an extension to the legacy controlled channel access mechanism PCF (Point Coordination Function) to support parameterized QoS [1]. With this function, time-sensitive applications such as voice and video, which are typical applications in home networks, can be guaranteed in WLAN systems. However, due to range attenuation, the coverage area of WLAN systems is limited. For the purpose of extending the transmission range, the multi-hop WLAN system is considered. In multi-hop WLAN system, data frames generated from the source station are sent to relay stations before delivered to destination station. Therefore, multi-hop WLAN system is expected to be a solution to extend the coverage area of QoS guaranteed transmission. However, in home or office environments, because of the multipath propagation effect, the performance of multi-hop WLAN system is degraded. In single-hop WLAN system, by using adaptive rate transmission technique Figure 1: Scenario with two Relay Stations. [2], stations can select the appropriate transmission rate according to the propagation condition, thus the performance degradation can be reduced. However, in multi-hop WLAN system, the variation of channel condition of each hop is independent, so it is difficult to apply adaptive transmission technique. In IEEE 82.11e system, a station works as a central coordinator polling every station in the system. But in multi-hop WLAN system, the central coordinator does not exist. Recently, many researchers have shown much interest in developing QoS support in multi-hop WLAN system, but most of them are about prioritized QoS [3, 4]. Parameterized QoS support in multi-hop WLAN system is still a problem and need to be resolved. This paper proposes a multi-hop WLAN system, as a solution to guarantee QoS transmission over a long distance. In this proposed system, the transmitting station of each hop estimates the channel condition to select the appropriate transmission rate used for data transmission of that link. The source station plays a role as a central coordinator to grant the optimum transmission opportunity to all the hops of the WLAN system. II. PROPOSED MULTI-HOP WLAN SYSTEM FOR QOS GUARANTEE Fig. 1 shows the scenario with two relay stations. In this multihop WLAN system, QoS data frames generated at the source station are transmitted to the destination station via two relay stations STA.1, STA.2. We assume that data frames are not generated at the relay stations as well as the destination station. Fig. 2 shows the data frame transmission protocol used in this proposed multi-hop WLAN system. In order to guarantee parameterized QoS, in this proposed system HCCA mech /6/$2. c 26 IEEE

2 82.11e periodic superframe Beacon R Poll Poll End Beacon Source R Poll STA.1 STA.2 TXOP 1 TXOP 2 R Destination TXOP 3 Figure 2: Frame Transmission Protocol. anism of IEEE 82.11e standard has been applied. The HCCA mechanism defines a transmission opportunity (TXOP), during which the transmitting station has the right to transmit multiple frames to the receiving station without contention with other stations [5]. In this multi-hop WLAN system, the source station plays a role as a central coordinator granting the TXOP to all stations in the system. As Fig. 2 shows, at the beginning of each superframe, the source station sends a beacon frame to the relay station STA.1 to start a new superframe with duration T SF. At the legacy WLAN system, the receiving station always sends an ACK frame to the transmitting station after each data frame is successfully received. In this proposed system, to improve the efficiency of the MAC protocol we apply the Block ACK mechanism [6], which is the optional scheme of the IEEE 82.11e standard. During TXOP 1 duration, the source station sends multiple data frames to STA.1 separated by a SIFS (Short Interframe Space) interval. At the end of TXOP 1, the source station sends a Block ACK Request (R) frame to STA.1. STA.1 will respond with a Block ACK () frame containing the acknowledgement status for the previous burst of data frames. After transmitting data to STA.1, the source station polls STA.1 and grants a TXOP 2.DuringTXOP 2, STA.1 forwards the data frames received from the source station to STA.2. In the next step, the source station polls STA.2 via relay station STA.1 and grants a TXOP 3.DuringTXOP 3, STA.2 sends the data frames received from STA.1 to the destination station. After receiving these data frames, the destination station replies the Block ACK frame to the source station via relay stations STA.2, STA.1, respectively. The source station ends the superframe after received this Block ACK frame. The data frame transmission process will be repeated in the next superframe. In every hop, at the end of each TXOP, the transmitting station based on the information from the received Block ACK frame decides whether to retransmit the error frames or not. In transmitting QoS streaming, the delay time of all data frames must be limited to the designated delay bound. In this paper, when received the Block ACK frame, the transmitting station estimates the maximum delay time τ expect of the error data frames in case these frames are correctly retransmitted in the next superframe. Only the error frames that have the delay time τ expect less than the designed bound delay τ target will be retransmitted in the next superframe. Delay time τ expect is estimated based on the delay time τ of the error frame at the time the Block ACK frame is received, as below τ expect = τ + T SF. (1) When an error frame occurs, if the expected delay of the error frame τ expect is smaller than the designated delay bound τ target, the transmitting station will remain the error frame in the buffer and retransmit it in the next superframe. Otherwise, the error frame will not be retransmitted. III. ADAPTIVE TRANSMISSION TECHNIQUE In this proposed system, to guarantee parameterized QoS transmission, the transmission rates and the transmission opportunity granted for each hop must be selected according to the channel condition of that link. This section describes how to decide the transmission rates and the TXOP durations. A. Decision of Transmission Rates In this proposed system, we use Block ACK mechanism to improve the MAC efficiency. However, in Block ACK mechanism all the data frames are transmitted by the same transmission rate in one TXOP. Thus, once the transmission rate does not match the channel condition, the bursty error will occur. In this multi-hop WLAN system, if data frames are not correctly transmitted at any link, the performance of the overall system is greatly degraded. Therefore, to guarantee the parameterized

3 Tx Rx Poll ACK TXOP R Table 2: Simulation Parameters. Parameters Value PHY Layer IEEE 82.11g Standard SIFS 1 µsec Bound Delay 1 msec Size 798 bytes Type MPEG2-TS 4 Multiplex Transmission Power 15 dbm Doppler Frequency 2 Hz Figure 3: Adaptive Transmission Rate Scheme. Table 1: Threshold of Transmission Rates Selection. Estimated SNR Transmission Rate 2.5 db 6 Mbps 4.83 db 9 Mbps 5.12 db 12 Mbps 7.88 db 18 Mbps 1.77 db 24 Mbps db 36 Mbps db 48 Mbps 19.7 db 54 Mbps QoS transmission in multi-hop WLAN system, the transmission rate and TXOP duration must be selected according to the propagation condition of each hop in the system. In the proposed system, we apply an adaptive rate transmission protocol for the multi-hop WLAN system. Fig. 3 shows the adaptive transmission rate selection algorithm. At the beginning of each TXOP, the transmitting station sends an estimation frame to the receiving station estimating the SNR (Signal to Noise Ratio) of the propagation channel at the receiving side. The receiving station based on the estimated SNR selects the transmission rate appropriate to the channel condition. Table 1 shows the threshold of the estimated SNR and the selected transmission rate. This is the SNR that satisfies BER 1 5 (BER: Bit Error Rate) over an ideal AWGN channel. Then the receiving station responds the selected transmission rate to the transmitting station. During this TXOP, the transmitting station sends all data frames using the selected transmission rate. B. Decision of TXOP Durations In the proposed system, to guarantee the delay time of QoS data frames transmitted from the source station to the destination station, the amount of data transmitted over each hop of one superframe should be equal. Therefore, the equation TXOP i Rate i = TXOP j Rate j (i j) (2) is established. The TXOP of each hop should be decided according to the transmission rate of this hop so that satisfy Equation (2). At the beginning of each superframe, the source station must decide the TXOP of each hop in this system due to the channel condition of that hop. As mentioned above, the transmission rates are decided at the beginning of each TXOP according to the channel condition of each hop. So at the time deciding the TXOPs of one superframe, the source station still cannot grasp the transmission rates which will be used for data transmission of hops in that superframe. As a method to decide the TXOPs, the source station can use the transmission rates selected in the previous superframe instead of the transmission rates of hops in current superframe. Under fading environment, because the channel condition changes momentarily, the transmission rate selected in current superframe and the transmission rate in the next superframe may not be the same. Therefore, to reduce the effect of the variation of channel condition, at the end of each TXOP, the transmitting station piggy-backs the channel estimation frame onto the R frame to estimate the channel condition one more time. The receiving station will select the transmission rate Rate i which is appropriate to the newest propagation condition. By sending the Block ACK frame, the receiving station replies the selected transmission rate Rate i to the source station. Based on these Rate i,atthe beginning of each superframe, the source station decides the transmission opportunity for all the hops. Notice that the sum of TXOP i is equal to the superframe duration T SF, that is, N h T SF = TXOP i (3) i=1 where N h is the number of hops. Therefore, from Equation (2) and Equation (3), TXOP i (i =1,..., N h ) durations can be calculated as below T SF TXOP i =. (4) N h Rate 1 i Rate i i=1 As Equation (4) shows, at the beginning of a superframe, the source station decides the transmission opportunity of each hop based on the transmission rates of the previous superframe. Because of the channel condition variation, the transmission rate selected in this superframe and the transmission rate in the next superframe are not always the same. Therefore, if the channel variation is fast, the parameterized QoS transmission may not be guaranteed. Furthermore, compared to single hop system, throughput performance of multi-hop system is degraded. However, in case the request throughput is not too high and the channel variation is slow enough, the QoS guaranteed transmission can be expected in the proposed multi-hop WLAN system.

4 Maximum Throughput (Mbps) adaptive 6Mbps 12Mbps 24Mbps 54Mbps Maximum Throughput (Mbps) hop, 5m 1hop, 3m 1hop, 4m 3hop, 5m 3hop, 3m 3hop, 4m 5hop, 5m 5hop, 3m 5hop, 4m Distance (m) Figure 4: Maximum Achievable Throughput Performance of 3-hop WLAN System. IV. COMPUTER SIMULATION A. Simulation Parameters In this section, we use computer simulation to evaluate the performance of the proposed multi-hop WLAN system. In the following performance evaluation, we assume that the multi-hop WLAN system is composed of one source station, one destination station and some relay stations. Relay stations are placed at even intervals between the source and the destination station. QoS data frames are generated at the source station only. Table 2 shows the simulation parameters employed in this paper. We use the IEEE 82.11g standard [7] as PHY layer. MPEG2-TS 4 multiplex data frame is used as QoS data frame. In the indoor environment, the transmission loss L (db) is decided based on the frequency f (MHz), attenuation coefficient N, distance between transmitter and receiver d (m) and the addition loss L f (n) caused by n pieces of wall, as below L = 2 log 1 f + N log 1 d + L f (n) 28. (5) In this simulation, we consider the housing environment with 2.4GHz-band WLAN system is used. According to [6], attenuation coefficient N =28and addition loss L f =5. B. Maximum Achievable Throughput Performance Fig. 4 shows the maximum throughput versus source destination distance of 3-hop WLAN system in case the adaptive transmission rate protocol is used. For comparison, this figure also shows the throughput performances in case using the fixed transmission rates 6, 12, 24, 54Mbps. In this evaluation, the superframe duration T SF is set to be 1 msec. As Fig. 4 shows, the maximum throughput decreases when source - destination distance increases. This is because the longer the hop distance, the lower the SNR at the receiver. When the SNR is low, the low transmission rate is selected and the packet error rate becomes higher, therefore the system has low performance. On the contrary, when hop distance is short, Superframe (msec) Figure 5: Maximum Achievable Throughput versus Superframe Duration of Multi-hop WLAN System. the high transmission rate is selected, and the packet error rate is small, therefore system has better performance. Also in this figure we see that, compared to the fixed rate protocols, the adaptive rate protocol give the best throughput at every distance. This is due to the fact that, the adaptive rate transmission mechanism can follow the variation of channel state, therefore appropriate transmission rates can be selected at all the links of the multi-hop WLAN system. C. Optimum Superframe Selection From the transmission protocol shown in Fig. 2, we can see that the superframe duration T SF is an important parameter deciding the delay time of data frames. In this system, Block ACK mechanism is used, in one TXOP data frames are transmitted by the same transmission rate, so the throughput performance depends on the TXOP durations. As Equation (4) shows, TXOP durations depend on the superframe duration. For these reasons, the performance of system is closely dependent on the superframe duration T SF. If the superframe duration is too long, TXOP of each hop becomes long. In this case, the adaptive transmission method cannot follow the variation of channel condition, so the performance of system becomes degraded. However, if the superframe duration is too short, the overhead will become large, and the throughput performance also becomes degraded. We use computer simulation to evaluate the maximum throughput performance of multi-hop WLAN system at different superframe durations, and from that investigate the relations between the optimum superframe T SF and other parameters, such as hop number and hop distance. 1) Relation between Optimum T SF and Hop Distance Fig. 5 shows the maximum throughput versus superframe duration of multi-hop WLAN system. The optimum superframe is considered the superframe which gets the highest maximum throughput performance. As Fig. 5 shows, with the same hop number, the shorter the hop distance, the longer the optimum

5 Table 3: Samples of Optimum Superframe. Hops Hop Distance Superframe 1 5 m 1 msec 3 5 m 1 msec 5 5 m 1 msec 1 3 m 1 msec 3 3 m 2 msec 5 3 m 3 msec 1 4 m 8msec 3 4 m 15 msec 5 4 m 21 msec Guaranteed Distance (m) hop 2 hop 3 hop 4 hop 5 hop superframe. This is because when the hop distance is short, the packet error rate is low. Therefore, using long superframe can reduce the overhead, so the throughput performance can be improved. However, when hop distance is long, the optimum superframe tends to become short superframe. This is because when a long superframe is selected, data frames are transmitted by the same transmission rate in a long TXOP. Therefore, the transmission rate can not follow the variation of channel condition, thus burst error occurs for a long duration and the throughput performance of system is greatly degraded. 2) Relation between Optimum T SF and Hop Number From Fig. 5 we see that, with the same hop distance, the larger the number of relay stations, the longer the optimum superframe duration. This is because with the same hop distance, the TXOP duration of each hop is about the same, therefore optimum superframe duration increases with the number of hops, in other words, the number of relay stations. By computer simulation results, the optimum superframe in some cases are shown in Table 3. So, with different hop distances and relay station numbers, an optimum superframe must be decided to give the best performance. D. Performance Evaluation with Optimum Superframe We evaluate the performance of multi-hop WLAN system using optimum superframe. Fig. 6 shows the maximum guaranteed distance versus the offered load of multi-hop WLAN system. As Fig. 6 shows, the larger the hop number, the lower the guaranteed offered load. This is because in multi-hop WLAN system, the larger the hop number, the shorter the transmission opportunity granted for each hop, therefore high throughput cannot be guaranteed in this system. However, when offered load is not too high, the larger the hop number, the longer the guaranteed transmission distance. This is because when offered load is low, despite of the large hop number, the TXOP granted for each hop is long enough, therefore QoS can be guaranteed over a long distance. For example, when SDTV (Standard Definition Television) video stream is transmitted over multi-hop WLAN system, 6Mbps of throughput is required. As Fig. 6 shows, at 1-hop WLAN system, 6Mbps of throughput transmission can be guaranteed over a coverage area of about 3m. However, when using 5-hop WLAN system, the coverage area Offered Load (Mbps) Figure 6: Maximum Achievable Throughput Performance of Multi-hop WLAN System. can be extended to about 11m. So, we can say that by using the proposed multi-hop WLAN system, SDTV transmission can be guaranteed over a long distance. V. CONCLUSION In this paper, we have proposed a multi-hop WLAN system for parameterized QoS guaranteed transmission. We employed a channel estimation method to the proposed system therefore suitable transmission rates can be selected. Also in this proposed system, optimum transmission opportunity is granted to each hop so that throughput performance as well as delay performance is improved. Numerical results have shown that, by using proposed system, when offered load is not too high, QoS transmission can be guaranteed over a long distance. REFERENCES [1] S. Mangold, S. Choi, G.R. Hiertz, O. Klein, and B. Walke, Analysis of IEEE 82.11e for QoS support in wireless LANs, IEEE Wireless Communications, vol. 1, no. 6, pp. 4 5, December 23. [2] S. Tou, S. Miyamoto, and N. Morinaga, Angle diversity reception technique with CNR estimation under man-made noise and multipath fading environments, in Proc. of the 2 International Symposium on Antennas and Propagation, August 2, pp [3] H. Izumikawa, K. Sugiyama, and H. Shinonaga, Scheduding algorithm for fairness improvement among users in multihop wireless networks, in Proc. of the Wireless Personal Multimedia Communications 25, September 25, pp [4] H. Tian, C. L. Law, S. K. Bose, and W. Xiao, A cross-layer approach of QoS support in multihop wireless networks, in Proc. of the Wireless Personal Multimedia Communications 25, September 25, pp [5] S. Mangold, S. Choi, P. May, O. Klein, G. Hiertz, and L. Stibor, IEEE 82.11e wireless LAN for Quality of Service, in Proc. of European Wireless, Florence, Italy, February 22, vol. 1, pp [6] M. Morikura and S. Kubota, High-speed Wireless LAN Textbook, Impress, Tokyo, 25. [7] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications. Amendment 4 : Further Higher Rate Extension in the 2.4GHz Band, IEEE Std 82.11g-23.