A New Full Duplex MAC Protocol to Solve the Asymmetric Transmission Time

A New Full Duplex MAC Protocol to Solve the Asymmetric Transmission Time Jin-Ki Kim, Won-Kyung Kim and Jae-Hyun Kim Department of Electrical and Computer Engineering Ajou University Suwon, Korea E-mail : {kjkcop, wk53, jkim}@ajou.ac.kr Abstract Recently, the full duplex communication has been spotlighted as one of key technologies of the future wireless local area network (WLAN). The full duplex MAC protocols are actively researched because the feasibility of full duplexing is achieved because of the technology evolution. The full duplex MAC protocols can be categorized as two types. One is centralized type and the other is distributed type. In most of distributed types, primary and secondary transmission time are different. To solve the problem occurred by the asymmetric transmission time, the busy tone is used in traditional full duplex MAC protocol. However, the usage of busy tone causes performance degradation as the busy tone has no data but consumes resource. Therefore, we propose a new full duplex MAC protocol which doesn t use busy tone. The simulation results show that throughput of the proposed full duplex MAC protocol is better than traditional full duplex MAC protocol or half duplex as maximum MAC protocol data unit (MPDU) size is bigger. Additionally, in terms of power consumption, the proposed full duplex MAC protocol shows better performance compare to others. Keywords Full duplex; MAC protocol; WLAN; I. INTRODUCTION Due to the increase of devices like smart phones or tablet PCs, volume of data traffic is rapidly growing [1]. However, the growth of the volume causes degradation of QoS. In May 2013, IEEE 802.11 has started HEW SG (High Efficiency WLAN Study Group) to achieve improvement of WLAN performance. In May 2014, 802.11ax TG (Task Group) started standardization in earnest [2]. The full duplex communication is one of key technologies to improve performance of WLAN. The full duplex communication means that the one single station do the transmission and reception simultaneously at same time and frequency. Fig. 1 shows that the difference between half duplex and full duplex communication. Thus, the capacity can be improved up to 2 times theoretically. The full duplex communication could not be implemented because of self-interference. If receiver transmits data when it receives data from other station, this signal affects receiving antenna as interference. This is called self-interference. However, the full duplex communication can be used recently because of the technique of self-interference cancellation. Some researches show that the full duplex communication can be implemented [3]-[7]. New protocol in MAC layer is (a) Fig. 1. (a) Half duplex communication (b) Full duplex communication necessary for full duplex communication because full duplex transmission has been possible in physical layer. After development of self-interference cancellation technology, a research related to full duplex MAC protocol has been actively conducted. There are two major kinds of full duplex MAC protocols which are centralized and distributed type. The centralized type is that the access point (AP) in the network schedules every full duplex data transmission. On the other hand, the distributed type is that the data can be transmitted through the competition. Most of distributed type full duplex MAC protocol adopts the carrier sense multiple access with collision avoidance (CSMA/CA). Full duplex MAC protocols of centralized type which have been proposed are as follows. The authors of [8] proposed centralized type full-duplex MAC protocol, FD-MAC, which builds on IEEE 802.11 with three new mechanisms. The first one is shared random backoff, another is header snooping, and the other is virtual backoff. The authors of [9] proposed Janus scheme. It is typical centralized type of full duplex MAC protocol. The AP in the network schedules every transmission to maximize throughput. The stations which have the data to transmit send information about data size and interference to the AP. After that, the AP schedules the transmission based on the received information. In the centralized type, the AP collects information to schedule every transmission. It is reason of network performance degradation because information to schedule can be overhead and time is wasted to gathering information. For (b) 978-1-4673-9526-7/15/$31.00 2015 IEEE

Start ST i and ST j communicate with full duplex Neighboring stations of station 1 Neighboring stations NS j and NS i freeze their Transmission of ST j complete first? ST i transmits packet first ST j transmits flag packet DS_NS i is bigger than remaining DS i? NS i freeze their Finish NS i transmits data to ST i ST j transmits packet first ST i transmits flag packet DS_NS j is bigger than remaining DS j? NS j freeze their NS j transmits data to ST j these reasons most of proposed full duplex MAC protocols adopted distributed type. The ContraFlow scheme is the one of distributed type [10]. This paper refers that the problem occurs in asymmetric dual link. Therefore, the authors of this paper propose the method that primary receiver to be secondary transmitter. Also, this paper solves the problem related to fairness. The other paper related to distributed type, proposed the concept of full duplex acknowledgement and transmission flag [11]. The full duplex acknowledgement indicates the type of full duplex transmission. The transmission flag blocks the transmission from the neighboring node. In case of distributed type, the transmission time of primary and secondary transmission are different due to variable size of data and the channel condition. The station which finishes data transmission should transmit busy tone until the other station finishes its transmission. If not, other station would start its transmission and it could collide with packet. Since busy tone does not contain any user data, it wastes channel resource and power so that it reduces performance of entire network. For these reasons, we propose a novel full duplex MAC protocol which resolves asymmetric transmission problems without using busy tone. Instead of transmitting busy tone, station which received full data sends packet first and Fig. 2. Flow chart of the proposed full duplex MAC protocol Station 1 Station 2 Fig. 3. System model allows neighboring stations to transmit their data to itself. Then, the sender transmits flag packet instead of busy tone. To evaluate the performance of the proposed protocol, we performed the simulation and compared throughput and power consumption performance of the proposed MAC and that of existing ones. The rest of this paper is organized as follows. The proposed full duplex MAC protocol is introduced in Section II. In Section III, we show that the performance evaluation of the proposed full duplex MAC protocol by using simulation. Finally, in Section IV, we summarize this paper. II. PROPOSED FULL DUPLEX MAC PROTOCOL Fig. 2 shows that the flow chart of proposed full duplex MAC protocol. In fig. 2, ST i and ST j denote station i and j. NS i and NS j denote neighboring stations of station i and j. DS i and DS j denote data size of station i and j. DS_NS i and DS_NS j denote data size of neighboring stations of station i and j. At first, ST i and ST j communicate with full duplex. NS i and NS j sense that channel is busy. Therefore, they stop decreasing their. If transmission of ST j finishes first, ST i transmits packet and ST j transmits flag packet. On the other hand, if transmission of ST i finishes first, ST j transmits packet and ST i transmits flag packet. The neighboring stations which are received packet, compare their data size and remaining data size of ST i or ST j. After that, if their data size is smaller than remaining data size of ST i or ST j, they decrease their. Fig. 3 shows that the system model. The station 1 and the station 2 communicate with full duplex. In addition, there are some stations near the station 1 and station 2. Fig. 4 shows that the example of traditional full duplex MAC protocol [10]. The busy tone is used to solve asymmetric transmission time in traditional full duplex MAC protocols. At first, station 1 transmits PLCP, MAC header and data to station 2. Neighboring stations of station 1 can also receive PLCP and MAC header. However, the destination address in MAC header is different from their address. Therefore, they freeze their. When station 2 receives PLCP and MAC header, it takes role of secondary transmitter. Thus, it transmits PLCP, MAC header and data to station 1. Neighboring stations freeze their because they sense that channel is busy. Even if station 1 finished its transmission, it should send busy tone to neighboring stations to receive packet safely from station 2. The neighboring stations which receive busy tone from station 1 will not decrease their backoff

Station 1 Station 2 1 0 PLCP MAC Data 1 2 B/T Backoff counter : 0 9 8 PLCP MAC Data 1 2 PLCP MAC PLCP MAC B/T Neighbor station 13 12 11 10 9 8 7 6 5 4 Backoff counter freeze Fig. 4. The traditional full duplex MAC protocol Station 1 Station 2 1 0 PLCP MAC Data 1 2 Backoff counter : 0 9 8 Flag PLCP MAC PLCP MAC PLCP MAC Data 1 2 Flag PLCP MAC Data ot her 2 Neighbor station 13 12 11 10 9 8 7 6 5 4 Backoff counter freeze 3 2 1 0 PLCP MAC PLCP MAC Data ot her 2 Fig. 5. The proposed full duplex MAC protocol counter because they sense that the channel is busy. After finished data transmission, every station can decrease their. Thus, in traditional full duplex MAC protocol, channel resources and power will be wasted because the data is not contained in busy tone. Therefore, we propose a new full duplex MAC protocol to solve this problem. The proposed full duplex MAC protocol do not use busy tone. Fig. 5 shows that the example of proposed full duplex MAC protocol. The procedure of proposed full duplex MAC protocol is equal to it of traditional full duplex MAC protocol until end of stations 1 s transmission. After station 1 finishes its transmission, station 2 stops its transmission and transmits packet to stations 1 first. packet includes information related with remaining data size which is transmitted to station 1. The station 1 transmits flag packet to its neighboring stations after receiving packet. Fig. 6 shows format of flag packet. This packet includes information of remaining data size which station 1 should receive from stations 2. For this reason, stations which received flag packet freeze its until station 1 finishes its reception. Neighboring stations can receive packet besides station 1. Each station which received packet from station 2 compares size of their data and size of station 2 s remaining data which is transmitted to station 1. If size of their data is smaller than size s data, neighboring station can transmit its data to station 2. Therefore, its is decreased. On the other hand, if size of their data is bigger than size s data, they cannot transmit their data because station 2 cannot receive packet from station 1. Therefore, they freeze their backoff counter. Likewise, without using busy tone, the proposed full duplex MAC protocol can prevent data transmission of neighboring Frame Control Duration DA FCS 2bytes 2bytes 6bytes 4bytes Fig. 6. Flag packet format stations. In addition, network performance will be improved by allowing station 2 to receive data in the network. In the traditional full duplex MAC protocol, T T_busy which stands for the time that neighboring stations cannot transmit data. It is as follows, T T_busy = 2 T P + 2T M + T DS2 + T SIFS + T + T DIFS (1) T P denotes the time of PLCP. T M denotes the time of MAC header. T DS2 denotes the time of the data. T SIFS denotes the time of SIFS. T denotes the time of. T DIFS denotes the time of DIFS. In the proposed full duplex MAC protocol, T P_busy which stands for the time that neighboring stations cannot transmit data. It is as follows, T P_busy = T P + T M + T DS1 + T SIFS + T + T DIFS (2) Therefore, the gain, G, increases in the proposed full duplex MAC protocol as follows, G = T T_busy - T P_busy = T P + T M + (T DS2 - T DS1 ) (3) However, this is not valid for every situation. We can obtain gain only when data size of neighboring station of station 2 is smaller than G. If there is no smaller than G among data size of neighboring stations, the network

Fig. 7. Network topology TABLE I. SIMULATION PARAMETER Parameter Data rate PLCP size MAC header size FCS size size Flag size SIFS size DIFS size Value 39 Mbps 40μs 36 bytes 4 bytes 14 bytes 14 bytes 16μs 34μs 1 slot size 9μs Maximum MPDU size Simulation time Transmission power of station 3895, 7991, 11454 bytes 15 minutes 23 dbm (200mW) performance of proposed full duplex MAC protocol is degradation. In the proposed full duplex MAC protocol, transmission time is longer than the traditional full duplex MAC protocol. It takes 2T sifs +T ack more than the traditional full duplex for station 2 to finishes its transmission because when station 1 finishes its transmission, station 2 stops its transmission and transmits packet. III. PERFORMANCE EVALUATION To evaluate the performance of the proposed full duplex MAC protocol, we compared this with half duplex and traditional full duplex MAC protocol in terms of throughput and power consumption in the simulation. MATLAB is used for simulation. The simulation is conducted 5 times for each parameter. A. Simulation environment Fig. 7 shows the network topology. Each station can communicate with other stations which is connected by dashed line. The simulation is based on 802.11ac [12]. For each scenario, MPDU length is set with 3895, 7991 and 11,454 bytes, data is randomly generated with a uniform distribution form each station. Contention window size is set [0, 1023] and Fig. 8. Network throughput according to maximum MPDU size Fig. 9. Consumption power according to maximum MPDU size initial backoff stage is assumed as 3. Other simulation parameters are shown in table 1. B. Simulation results Fig. 8 shows the network throughput according to the maximum MPDU size. As shown in Fig. 8, throughput of the full duplex is high in comparison with throughput of the half duplex. In addition, as the maximum MPDU size increases, throughput of the proposed full duplex MAC protocol is higher than it of traditional full duplex MAC protocol. The possibility that station would generate asymmetric data increases as the maximum MPDU size gets bigger. Therefore, higher gain will be more achievable when we use the proposed full duplex MAC protocol. When the maximum MPDU size is 3,859 bytes, the traditional full duplex MAC protocol achieves higher throughput than the proposed full duplex MAC protocol. In full duplex communication, when the maximum MPDU size is small, the difference of data size which is generated at each

station is small. Thus, the gain which is obtained by using the proposed full duplex MAC protocol will be decreased. In other words, when the maximum MPDU size is small, the overhead generated by transmitting flag packet affects performance of the proposed full duplex MAC protocol. Fig. 9 shows power consumption while transmitting 1kbits. As the maximum MPDU size increases, power consumption to transmit 1kbits decreases because as the maximum MPDU size becomes greater, overhead such as PLCP or MAC header decreases. Among three MAC protocol, half duplex consumes the least power. In other words, it means that there would be least collision occurs. We can assume that because CSMA/CA protocol had designed suitable for half duplex, it has least collision with same power. On the other hand the traditional full duplex MAC protocol consumes the biggest amount of power. It is because of busy tone, which is used to prevent other neighboring stations to start transmission to the station which finished its transmission. Because busy tone has no data, it only wastes power consumption. During asymmetric transmission time, proposed full duplex MAC protocol can transmit data without using busy tone. Therefore, the proposed full duplex MAC protocol consumes more power than half duplex, but it consumes less than the traditional full duplex MAC protocol. Thus, in terms of power consumption, the proposed full duplex MAC protocol shows better performance than the traditional full duplex MAC protocol. IV. Conclusion In full duplex communication, the end of transmission time may differ due to difference between data size of each station. The traditional full duplex MAC protocol uses busy tone to solve problem caused by asymmetric transmission time. However, it is waste of resource and power because busy tone does not contains any data. This is the reason of degradation of network performance. Therefore, we proposed the full duplex MAC protocol without using busy tone. In the proposed full duplex MAC protocol, packet and flag packet is used instead of busy tone. The simulation results show that throughput of the proposed full duplex MAC protocol is better than the other two MAC protocols for bigger maximum MPDU size. In addition, in terms of power consumption, the proposed full duplex MAC protocol shows better performance. Even if the proposed full duplex consumes more power than half duplex, it consumes less than the traditional full duplex MAC protocol as shown in simulation results. In the future, we will enhance the performance of proposed full duplex MAC protocol. This protocol is weak for collision because there are no packet such as RTS/CTS. Therefore, we will improve this protocol to prevent collision without degradation of network performance. 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