In-frame Querying to Utilize Full Duplex Communication in IEEE ax

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1 In-frame Querying to Utilize Full Duplex Communication in IEEE ax Gunhee Lee Pohang, Republic of Korea Hyeongtae Ahn Cheeha Kim Abstract The IEEE standard based Wireless Local Area Networks (WLAN) is the most popular and widely deployed communication protocol in the world. It is anticipated that WLAN will drive the future communication systems in order to provide more data rate for more users. IEEE ax is one of the most promising and advanced standards among the drafts. Full duplex radio with self-interference cancellation is highly anticipated to be incorporated into IEEE ax standard. However, according to the empirical study of WLAN usage patterns showing extreme unbalance between uplink and downlink traffic, there may be plenty of wasted opportunities that naive full duplex MAC protocols cannot utilize. To extend those opportunities, we develop In-frame Querying method to instantly find the candidate stations which are able to support the full duplex capability with AP. We conduct computer simulations to evaluate the performance gain of In-frame Querying method for full duplex communication in IEEE ax. Our results show that In-frame Querying is a simple and robust scheme to improve the throughput significantly compared to the legacy methods in WLAN. I. INTRODUCTION IEEE Wireless Local Area Networks (WLAN) have been a tremendous success for the past 20 years [1]. The proliferation of IEEE devices such as Access Point (AP) and smartphones enabled IEEE WLAN standard as a major Internet access protocol for mobile computing. IEEE WLAN original standard was published in 1999 and reaffirmed in 2003 [2]. As of 2015, The most widely used version of IEEE is IEEE n However, increasing storage capacity of mobile devices and volume of multimedia traffic make even IEEE n insufficient for today s users demand. Therefore, many improvements on IEEE standard have been proposed including IEEE ac, af, and ah [3]. To address highly crowded situation, the High-Efficiency Wireless LAN (HEW) Study Group and IEEE Task Group ax (TGax) are working on a new amendment named IEEE ax-2019 [4]. The goal of ax is to satisfy the forecasted user demands in Future user demands are composed of the high definition audio-visual real-time traffic and the dense WLAN scenario. The dense WLAN scenarios include three key environments for next-generation WLANs, which are a stadium, a train, and an apartment building. These environments can include multiple neighboring WLAN basic service sets (BSSs) and many stations (STAs) under the coverage of each access point. Currently, WLAN standards such as IEEE n/ac embrace inter-frame space (IFS), contention window (CW), and binary exponential backoffs to control the operation of Medium Access Control (MAC) layer [1]. Those were implemented and worked robustly, however, the efficiency and fairness of WLAN were severely criticized by multiple studies [5] [6]. We briefly review two critical problems of IEEE ax draft standard trying to solve. First, Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA) incurs too high collision probability with many STAs. This inefficiency is also amplified by the naive assumption of the number of STAs in a BSS. The initial value of contention window, CW min of IEEE n/ac is 15. The dense scenarios such as a stadium and a train almost always guarantee the collision and multiple backoffs in this case. Also binary exponential backoff algorithm always favors the last successfully transmitted station because a successful transmission resets the CW of the sender STA to CW min while a failed transmission doubles CW. If there were negligible number of transmission failures in a BSS then it might not be a problem. However, we can expect much more transmission failure in dense scenarios and legacy MAC protocol can lead to huge throughput loss. Second, non-collision frame loss is not taken into account by the standard. Every transmission failure is treated as a same, thus doubling the CW of sender. Current half duplex (HD) radio devices cannot infer the reason of frame loss. However, as the IEEE TGax discussed [7], there is a possibility of ax standard adopting the full duplex (FD) devices to drastically increase the performance and efficiency of future WLAN. FD devices can transmit and receive simultaneously, therefore infering the reason of frame loss and trying to detect collision rather than avoid is much more viable strategy. TGax is considering many novel technologies including uplink(ul) MU-MIMO, orthogonal frequency division multiple access (OFDMA), overlapping basic service sets (OBSSs) interference handling, and FD radio. In the following section, we briefly introduce FD radio which is relevant to our proposed method /15/$ IEEE 194

2 STA AP AP STA STA Opportunity Window DL heavy Fig. 1. DL heavy cases occupy 90% of WLAN use cases. To utilize STA opportunity window, the information of hidden terminal relationship between STAs is required. Recently, FD radio with bi-directional communication became one of the most promising technique to improve the efficiency of RF spectrum use [8]. Various researchers reported the successful experimentation of their prototype devices [9] [10] [11]. Especially, self-interference cancelling (SIC) FD transceivers enabled the practical realization and implementation of small and low-cost FD devices. Selfinterference is caused by the simultaneous transmission and reception at the single frequency channel. The relatively strong transmitted signal interferes directly the received signal. SIC FD transceivers can suppress this effect by several methods, and the SIC mechanism based on linear processing is one of the most practical methods [12]. Therefore, the importance of advanced MAC protocols for FD enabled AP and STAs is immense. A. Asymmetry of UL and DL traffics Another important issue we should consider is the asymmetry of uplink (UL) and downlink (DL) traffic volume and frequency. According to the empirical study of Palit et al. [13], the amount of UL packets is about 80% of the amount of the DL packets for Social Networking Service (SNS) browsing. In case of random web browsing, it is in the range of 70% to 85%. Also, the size of the packets in the UL traffic are much smaller than those of the DL traffic. About 90% of the UL packets are smaller than 100 bytes, while most DL packets have the size of Maximum Transmission Unit (MTU), which is generally 1500 bytes. We can interpret these facts into these two statements. First, the UL frames are less frequent than the DL frames. Second, the UL frames are shorter than 6.7% of the DL frames. The dense deployment scenario of IEEE ax should fully consider this traffic model. By using these facts, we can construct more realistic FD traffic model as Figure 1. In real-life traffic model, about 90% of transmissions are DL heavy cases and these traffics bring STA opportunity window to the interest of IEEE ax standard. STA opportunity window is defined as the empty channel time caused by the unbalance of the size between UL and DL frames. B. Hidden terminal problem The hidden terminal problem of WLAN is a well-known problem and measures including Request-To-Send/Clear-To- Send (RTS/CTS) and busy-tone were considered to overcome this problem [5]. However, traditional research of the hidden terminal problem consists of purely preventive viewpoints. WLAN with HD radio could not utilize the hidden terminal relationship, thus the reasoning was valid in the past. Though, with advance of SIC FD radio, the way we should handle the hidden terminal problem has been changed. The STA opportunity window in Figure 1 can be utilized only by the STAs that are hidden to current transmitting STA. To explain this problem, we present Figure 2. Let s call a STA hidden to STA r as STA x and a STA exposed to STA r as STA z. If STA r s UL transmission frame is shorter than AP s DL transmission frame, then STA x can grab the STA opportunity window to use the channel without interfering AP STA r transmission. AP can cancel its own signal via self-interference cancellation and decode STA x s UL transmission. STA r cannot hear STA x s signal because of the hidden terminal relationship. However, STA z cannot grab the STA opportunity window in the same situation, because STA z s signal will reach STA r and interfere AP STA r transmission. STA y is a possible competitor against STA x. Both STAs are candidate STAs and exposed to each other. Therefore the need of discovering hidden terminal relationship between STAs emerges. The problems are how AP should know whether a given STA is hidden to STA r and how AP should choose which STA to transmit in the STA opportunity window. In this paper, we propose a method called In-frame Querying to solve these problems. II. RELATED WORK In WLAN, FD MAC is relatively less researched topic than traditional HD MAC. To the best of our knowledge, we are the first to recognize and to use the STA opportunity window caused by the asymmetry of UL and DL traffic in FD enabled WLAN. Choi et al. proposed a FD multi-channel MAC protocol for Cognitive Radio Networks [14]. However, their method requires at least two transceivers at each node to enable FD radio capability, and it does not recognize the STA opportunity window. In this paper, we focus on improving the performance of one-hop WLAN BSS, and our focus and method are entirely different from theirs. Goyal et al. studied the distributed FD MAC design based on IEEE DCF [15]. As we described earlier, legacy DCF has several problems for dense WLAN scenarios. Their three node FD transmission considers a similar traffic model as our work, but their MAC can only handle the situation with additional overhead generated from a handshaking process to determine the hidden terminal relationship. Compared to that, our In-frame Querying method does not alter existing MAC frame structure and does not require control messages to determine the hidden terminal relationship between STAs. In [10] and [11], Jain et al. and Duarte et al. mainly explored the physical nature of SIC and FD radio. Thus their MAC protocols are simple and robust, but do not consider the traffic model derived from empirical study and do not try to improve the MAC of WLAN standard itself. Also, exploiting the emerging new opportunities from densely deployed WLAN situation was not their main concern. III. SYSTEM MODEL We set up an example BSS as Figure 2 to explain the problem and proposed solution more briefly and clearly. Every device in this BSS is supposed to have FD capability. 195

3 STA r STA z AP STA x STA y Transmission Hidden Exposed In-frame Querying AP STA r (a) STAx AP Hidden (STAx) Fig. 2. Example IEEE ax BSS setup. AP and STA r are transmitting and receiving simultaneously. STA x is hidden to STA r, while STA z is exposed to STA r. STA y is hidden to STA r but exposed to STA x. In addition, our BSS uses FFT size of 64 to maintain the consistency with IEEE n/ac. Among the 64 subcarriers, 52 subcarriers are allocated as data subcarriers, 8 subcarriers are allocated as guard subcarriers, and 4 subcarriers are allocated as pilot subcarriers in n-2009 standard [16]. Frame aggregation scheme can coexist with our proposed method, because in the case of A-MPDU or A-MSDU each subframe can perform In-frame Querying without any problem. A-MPDU aggregation is mandatory in IEEE ac and IEEE ax is highly anticipated to incorporate frame aggregation scheme also. The Block ACK scheme can be used with our proposed method without any alteration. Our proposed method does not assume any kind of medium access control (MAC). With any MAC, we may consider four different cases. First, if AP controls the channel and STA have UL data to send, then In-frame Querying is applied. Second, if AP controls the channel and STA does not have UL data to send, then STA is enforced to send a dummy frame to find hidden terminals to the STA. Third, if STA controls the channel and AP has DL data to send, then we have the same case as the first case. Finally, if STA controls the channel and AP does not have DL data to send, then the FD communication cannot occur. Finally, we assume that every STA and AP in the BSS adheres to the regulation of corresponding country s ISM band TX power. For example, 5GHz band rules and restriction by FCC in the United States restricts the TX power of devices up to 30 dbm. The reason of this is to keep the symmetric property of hidden terminal relationship. Otherwise, if a STA illegally amplifies TX power by a great margin, asymmetric hidden terminal relationship can happen and lead to collision. IV. IN-FRAME QUERYING To utilize the STA opportunity window in Figure 1, we need to find out the hidden terminal relationship between the STAs. Especially, as we explored the hidden terminal problem thoroughly in the previous section, a candidate STA trying to access the channel must be hidden to receiving STA. Otherwise, the traffic from AP to receiving STA (AP STA r in Figure 3) will be undecodable due to the interference. Note that we denote the receiving STA as STA r. That is STA r can be any STA which is receiving a DL frame at a given time. The procedure of detecting hidden terminals is well illustrated in Figure 3. If a STA can decode AP s DL frame without problem, then it is hidden and safe to participate in In-frame Querying. On the contrary, if a STA cannot decode AP s DL Undecodable In-frame Querying AP STA r (b) Exposed (STAz) Fig. 3. Diagrams illustrate basic idea how to detect hidden terminals. (a) If STA r is hidden to STA x, then STA x can decode the DL frame of AP without interference. Because STA r s UL frame cannot reach STA x, STA x receives only DL frame from AP. (b) If STA r is exposed to STA z, then STA z cannot decode the DL frame of AP because of interference. The interference is caused by STA r s UL frame. Even with FD radio, STA z cannot receive two frames simultaneously. frame, then it is not hidden and not safe to participate in Inframe Querying. Each STA that has an UL traffic to send and is hidden to STA r is a candidate STA. Candidate STAs know that they can participate in In-frame Querying process. However, STA y is exposed to STA x. If both STAs try to transmit in STA opportunity window at the same time, then both transmissions will collide. In-frame Querying does not rely on previous knowledge or history data to detect current hidden terminal relationship between STAs. It is to eliminate the necessity of overhead inducing control messages or hidden terminal relationship tables. AP does not take the responsibility of creating, reading, updating, and deleting additional information to utilize the STA opportunity window in our method. In-frame Querying needs one subcarrier in DL for its operation. A reserved subcarrier is allocated as a query subcarrier instead of data subcarrier. The potential disadvantage here is a loss of DL throughput, but if we consider that there are 52 data subcarriers then the loss is less than 1/52 of DL throughput. As we can see in the evaluation section, this is barely noticeable and the benefits from In-frame Querying is much bigger compared to the maximum 1.9% throughput loss. Also, in the proposed method, STAs receive a 12-bit STA ID from AP in the time of association. The 48-bit IPv4 address is used throughout the communication purpose. 12-bit STA ID is only needed for In-frame Querying purpose and adopted for the efficiency purpose. Total 4096 STAs can be allocated by 12-bit STA ID, so it can support densely deployed scenario of IEEE ax sufficiently. A. Collision detecting contention Initiation of the transmission is as same as existing WLAN standards. By using any kind of MAC protocol, either AP or STA will control the channel. The bi-directional communication enabled by FD radio follows. In the DL heavy case, at the end of the UL transmission, STA opportunity window starts. Every candidate STA in the BSS listens to AP s DL transmission. The data subcarriers can be ignored, but pilot 196

4 TABLE I. SIMULATION PARAMETERS FOR EXAMPLE AX BSS Downlink Query subcarrier Uplink STAr AP Query Start... x x 3 AP STAr x 3 Announce Winner (12-bit) STAx AP Name Explanation Value SIFS SIFS interval 16µs SYMBOL An OFDM symbol duration 4µs R data Physical maximum data rates 150 Mbps R control rates for control packets 6 Mbps UL/DL Uplink to downlink frame size ratio L data Maximum data length for a frame bits L pre Preamble length for a frame 20µs L ACK ACK frame length 112 bits N ST A Number of STAs var. F F T size Number of subcarriers in OFDM 64 TX range Transmission range for each device 100m Fig. 4. Collision detecting contention of In-frame Querying. The behavior of candidate STAs follows binary exponential backoff. The winner of contention(sta x) is notified by the Announce Winner frame. subcarriers and a query subcarrier should not be ignored. Pilot subcarriers are used as a synchronization mechanism, and those are essential to participate in the contention. The query subcarrier conveys Query Start frame to notify the candidate STAs the end of STA r s UL transmission. AP begins the In-frame Querying process only if the DL frame is sufficiently longer than the UL frame. The decision is simple, because AP knows the length of both frames. First, AP sends Query Start frame on the query subcarrier immediately when the STA opportunity window happens. Query Start frame means that AP is receiving STA IDs for selection. Each slot is as long as an OFDM symbol duration, which is 4µs using 800ns OFDM Guard Interval. If a given BSS is using 400ns Short Guard Interval scheme, then each query slot is 3.6µs. Candidate STAs which know the hidden terminal relationship by overhearing procedure described in Figure 3 send their 12-bit STA ID immediately. Even in the BPSK modulation, 51 data subcarriers contain 51 bits. Thus sending 12-bit information in one OFDM symbol duration is not a problem. Candidate STAs do not use query subcarrier for this purpose. If AP receives decodable 12-bit STA ID in any query slot, then AP announces the Winner STA ID immediately by Announce Winner frame. Announce Winner frame contains 12-bit STA ID of Winner STA. The STA ID signal received after the Announce Winner frame is ignored by AP. The Winner STA listened to Announce Winner frame should immediately start its UL transmission. If AP cannot decode the signal in given query slot due to collision, AP simply does not transmit anything on query subcarrier. Then the candidate STAs know that their transmission of STA ID had collided and follow the binary exponential backoff behavior. The candidate STAs choose one random integer from the interval of [0, 2 k 1 1] where k is the number of transmission attempt. This backoff counter is the number of slot for which a candidate STA needs to wait. This sequence is illustrated in Figure 4. By using binary exponential backoff scheme, In-frame Querying can utilize STA opportunity window without any control messages or additional information. V. EVALUATION We conducted computer simulations to evaluate the performance of In-frame Querying method. We constructed simulations using Visual C on Windows 7 operating system. Given the undecided details of IEEE ax draft standard in PHY layers and parameters, our evaluation had to be based on the extension of IEEE n/ac PHY layer. Notable simulation parameters are presented in Table I. We excluded the MAC part from the simulation, because In-frame Querying method does not presume a certain MAC protocol. We measured system-wide aggregated throughput to compare the performances between three methods, which are half duplex (HD), full duplex (FD), and full duplex with Inframe Querying (FD with IFQ) in Figure 5. The simulation environment was 200m 200m two-dimensional space, and the STAs positions were uniform randomly distributed. The AP and STAs always had data to send, thus our simulation was conducted in the saturated condition. The simulation was performed n = 10 times, and the average values were used in the results. In all setups, the ratio of hidden STAs compared to exposed STAs were from 10% to 30%. Standard deviations of the results ranged from to As we can see in Figure 5, in the UL/DL = and N ST A = 70 setup, FD with IFQ recorded throughput increase of 196.8% compared to HD, and 43.9% compared to FD. The number of hidden STAs is directly related to the number of candidate STAs, therefore we can confirm that FD with IFQ has greater throughput gain when the number of STAs increases. Although the UL frame size is smaller than the DL frame size, the throughput is increased by filling the empty channel time called STA opportunity window in Figure 1 with as many UL frames as possible. As the number of STAs increases, the number of candidate STAs with UL frame also increases. Therefore FD with IFQ can transmit more UL frames compared to FD in the densely deployed situation. Because of the abundance of short ( bytes) data frames of real-world traffic model, our method improves the performance of IEEE ax WLAN significantly. In the densely deployed scenario of IEEE ax with realistic traffic model, the FD with In-frame Querying showed significant throughput increase. 197

5 Throughput(Mbps) UL/DL = Number of STAs HD FD FD with IFQ Fig. 5. Simulation results of half duplex (HD), full duplex (FD), and full duplex with In-frame Querying (FD with IFQ). The throughput in relation to the number of STAs was plotted. UL/DL ratio of from the empirical study was used. VI. CONCLUSION To cope with ever increasing demands of WLAN users and dense deployment scenarios, IEEE ax WLAN standard is discussed. In this paper, we revealed a significant phenomenon of asymmetry of UL and DL traffic in FD enabled WLAN. To utilize the STA opportunity window, we proposed In-frame Querying method to instantly find candidate STAs. We did computer simulations to evaluate the throughput gain of In-frame Querying. In-frame Querying demonstrated significant throughput increase compared to HD and FD protocols in the DL heavy real-world traffic model. We consider that our method is simple, robust and efficient improvement for future IEEE ax standard. ACKNOWLEDGMENT This work was supported by Institute for Information & communications Technology Promotion(IITP) grant funded by the Korea government(msip) (No.R , Development of full duplex based MAC protocol for IEEE ax with densely deployed devices). [6] Y. Fukuda, Unfair and inefficient share of wireless LAN resource among uplink and downlink data traffic and its solution, IEICE transactions on communications, vol. 88, no. 4, pp , [7] W. Sun, O. Lee, Y. Shin, S. Kim, C. Yang, H. Kim, and S. Choi, Wifi could be much more, Communications Magazine, IEEE, vol. 52, no. 11, pp , Nov [8] D. Korpi, L. Anttila, V. Syrjala, and M. Valkama, Widely linear digital self-interference cancellation in direct-conversion full-duplex transceiver, Selected Areas in Communications, IEEE Journal on, vol. 32, no. 9, pp , Sept [9] J. I. Choi, M. Jain, K. Srinivasan, P. Levis, and S. Katti, Achieving single channel, full duplex wireless communication, in Proceedings of the Sixteenth Annual International Conference on Mobile Computing and Networking, ser. MobiCom 10. New York, NY, USA: ACM, 2010, pp [Online]. Available: [10] M. Jain, J. I. Choi, T. Kim, D. Bharadia, S. Seth, K. Srinivasan, P. Levis, S. Katti, and P. Sinha, Practical, real-time, full duplex wireless, in Proceedings of the 17th Annual International Conference on Mobile Computing and Networking, ser. MobiCom 11. New York, NY, USA: ACM, 2011, pp [Online]. Available: [11] M. Duarte, A. Sabharwal, V. Aggarwal, R. Jana, K. Ramakrishnan, C. Rice, and N. Shankaranarayanan, Design and characterization of a full-duplex multiantenna system for wifi networks, Vehicular Technology, IEEE Transactions on, vol. 63, no. 3, pp , March [12] M. Duarte, C. Dick, and A. Sabharwal, Experiment-driven characterization of full-duplex wireless systems, Wireless Communications, IEEE Transactions on, vol. 11, no. 12, pp , December [13] R. Palit, K. Naik, and A. Singh, Anatomy of wifi access traffic of smartphones and implications for energy saving techniques, International Journal of Energy, Information and Communications, vol. 3, no. 1, pp. 1 16, [14] N. Choi, M. Patel, and S. Venkatesan, A full duplex multi-channel mac protocol for multi-hop cognitive radio networks, in Cognitive Radio Oriented Wireless Networks and Communications, st International Conference on, June 2006, pp [15] S. Goyal, P. Liu, O. Gurbuz, E. Erkip, and S. Panwar, A distributed mac protocol for full duplex radio, in Signals, Systems and Computers, 2013 Asilomar Conference on, Nov 2013, pp [16] IEEE standard for information technology local and metropolitan area networks specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Amendment 5: Enhancements for Higher Throughput, IEEE Std n (Amendment to IEEE Std as amended by IEEE Std k-2008, IEEE Std r-2008, IEEE Std y-2008, and IEEE Std w-2009), pp , Oct REFERENCES [1] D.-J. Deng, K.-C. Chen, and R.-S. Cheng, IEEE ax: Next generation wireless local area networks, in Heterogeneous Networking for Quality, Reliability, Security and Robustness (QShine), th International Conference on. IEEE, 2014, pp [2] IEEE standard for information technology telecommunications and information exchange between systems local and metropolitan area networks specific requirements part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE Std (Revision of IEEE Std ), pp , March [3] M. X. Gong, B. Hart, and S. Mao, Advanced wireless lan technologies: IEEE ac and beyond, SIGMOBILE Mob. Comput. Commun. Rev., vol. 18, no. 4, pp , Jan [Online]. Available: [4] Y. Asai, Advanced progress in IEEE WLAN standardization, in Microwave Conference (APMC), 2014 Asia-Pacific, Nov 2014, pp [5] Z. Chang, O. Alanen, T. Huovinen, T. Nihtila, E. H. Ong, J. Kneckt, and T. Ristaniemi, Performance analysis of IEEE ac DCF with hidden nodes, in Vehicular Technology Conference (VTC Spring), 2012 IEEE 75th, May 2012, pp