A MAC Protocol based on Dynamic Time Adjusting in Wireless MIMO Networks

212 7th International ICST Conference on Communications and Networking in China (CHINACOM) A MAC Protocol based on Dynamic Time Adjusting in Wireless MIMO Networks Yang Qin*, Xiaoxiong Zhong, Li Li, Zhenhua Su,Xuebing Li Harbin Institute of Technology Shenzhen Graduate School, Shenzhen, China *Corresponding author, email: yqinsg@gmail.com Abstract Multi-Input Multi-Output (MIMO) technology can improve the throughput of wireless networks, and many Media Access Control (MAC) protocols have been proposed to support simultaneous transmissions. But there also exist some limitations or drawbacks in those MAC protocols. They can not utilize the communication resources efficiently, such as MIMA-MAC, - MAC, etc. In this paper, we propose a MAC protocol based on dynamically adjusting the waiting time of CTS packets in MIMO ad hoc networks called -MAC. It can allocate the antenna resources optimally by dynamically adjusting the priority of CTS. The shorter the waiting time of CTS is, the higher the priority of CTS is. We analyze the performance of -MAC and evaluate it through extensive simulations. The simulation results show that -MAC can enhance the throughput of wireless networks compared with -MAC. Keywords- MIMO; MAC; throughput I. INTRODUCTION In recent years, with the rapid development of wireless communication technology, different types of wireless terminals connected to each other by way of point to point links, form Mobile Ad hoc network (MANET) [1]. However, the throughput of non-infrastructure point to point link is very low. It is well known that spectrum resources are scarce, and an effective way of improving throughput can improve the spectrum efficiency. Now Multi-Input Multi- Output (MIMO) technology is considered as a key technique for future wireless communications which can greatly increase the date rate of wireless networks without increasing transmitting power and bandwidth. However, most of the Media Access Control (MAC) protocols are based on a single antenna, and do not support the simultaneous transmissions, so these transmission strategies also do not apply to the networks using MIMO technique in physical layer. Therefore, many MAC protocols using MIMO have been proposed [2]-[9]. These protocols, using MIMO technology, can be divided into two categories generally. In the first one, they transmit the same signal, but weighted differently, out of each antenna element and exploit the array not to increase the throughput of a single transmit/receive pair but to enable reuse across pairs through proper beamforming of the MIMO links. NULLHOC [4] and SPACE-MAC [5] belong to this type of protocols. NULLHOC, which is based on MIMO type beamforming, allows multiple data streams at the same time in the same collision domain, thereby increasing overall capacity of a network. But it divides the link layer bandwidth into two separate sub-channels for data and control, which limits the link layer bandwidth. SPACE-MAC overcomes the drawback of NULLHOC, without splitting the channel into two subchannels, and performs better than NULLHOC. The main drawback of SPACE-MAC is the use of silence period. MA- MAC [6] eliminates dependence on the silence period, so it improves the throughput and delay performance. Another type of MIMO technology uses Spatial Multiplexing(SM) technology. The data is multiplexed into N distinct streams and then each stream is transmitted out of a different antenna with the same power at the same frequency. MIMA-MAC [7] uses two contention slots constituted by an slot and a CTS slot in a fixed-size frame, and is capable of simultaneous data reception from two transmitters. The main drawback of MIMA-MAC is that the stream allocation is not based on the transmission demand of neighbor nodes, and the number of transmitted streams must be half of the number of the antennas. So the transmitted stream is always not maximized. When taking into account rich link adaptation capabilities in MIMO systems, MIMA-MAC's link throughput may be lower than that of single link MAC. So Zhao [8] presents a design that can adaptively switch between single link scheme and concurrent link scheme to improve the link throughput by simultaneously guaranteeing each link's throughput to be no less than its single link scheme's counterpart. The principle of the MAC protocol with Parallel Processing (-MAC) [9] is that the receiver receives up to two continuous packets, then determines the maximum allowable number of data streams and responds with different CTS packets. It can guarantee the optimal distribution of stream. However, because the receiver of -MAC does not immediately return a CTS packet after receiving an packet, but chooses to wait for the next packet to determine the distribution of stream. In some special cases, especially in areas with high node density, -MAC cannot guarantee that the antenna resources of all nodes are utilized effectively, and some of the resources are wasted. Moreover, as technology evolves, the nodes will have more and more antennas, the optimal number of divisions will increase, while the -MAC is not able to adapt to this change well. In this paper, we propose a MIMO-based dynamic adjusting CTS waiting time MAC protocol (-MAC). It allocates the antenna resources effectively by dynamically adjusting the waiting time of CTS. Our simulation results show that -MAC has a good scalability, no matter how many antennas the nodes have and what the optimal number of divisions is, moreover it can achieve the global optimum allocation of resources, and enhance the network throughput 53 978-1-4673-2699-5/12/$31. 212 IEEE

compared with -MAC. -MAC assumes that there are four antennas on each node, the communication range of each node is one hop, that is, each node can only communicate with its neighbors, carrier sense range is two hops which means that each node can confirm whether there were nodes that are sending message in the range of two hops, and the optimal number of divisions is two. For convenience to compare with it, our proposed protocol uses the same conditions as -MAC. The rest of the paper is organized as follows. Section II discusses the motivation behind pursuing this work. Section III provides detailed description of the proposed protocol (-MAC). Section IV contains a performance evaluation. Section V we present the simulation results. Section VI is the conclusion of this paper. II. MOTIVATION Though -MAC is a MAC protocol which can enable simultaneous communications and improves the throughput of wireless networks, there also exist some drawbacks, which provide the foundations for our further study. In Fig. 1a), there are only two nodes A and B in the collision domain, the arrow indicates the direction of communication for the two nodes. Fig. 1b) shows the process of the communication, the number in the bracket denotes the number of stream allocated for some node. Node A sends an packet in the first time slot, and node B doesn't immediately return a CTS-S packet after receiving the packet, but to wait for the second packet. In the second time slot, nothing will be done due to no other transfer requests. Node B sends CTS-S packet in the third time slot, which contains the number of available flow for node A, and then node A starts to send data using four antennas. Here, it is not necessary to wait a whole time slot for node B, and it should reply a CTS-S packet in this time slot, because it means that there is no more packet that will reach in the time slot, if the backoff time has been expired. A a) Two nodes in a collision domain b) -MAC wastes one time slot c) The case that Node E is an external node B A B C D E CTS-D CTS-S DATA(2) DATA(2) ACK ACK d) Timing Diagram that conflict at Node C Figure 1. Limitations of -MAC In addition, when the node density is high and the number of nodes that have transfer request in the same collision domain becomes larger, the topology in Fig. 1c) often occurs. We can know that the optimal stream allocation is to allocate two streams for each transmitting node, that is, node A sends data to node B with two antennas, node C sends data to node D with two antennas, node E also uses two antennas to send data to node D. However, the procedure of -MAC protocol is shown in Fig. 1d). Firstly, node A and E send in the first time slot, and no interference will happen. Then, node C sends its in the second time slot. Now both node B and node D have receive two, so they will send their CTS-D or CTS-S at the same time, which contain the allocated stream number for A and C, or C and E. So the conflict will happen, which results in the case that node C can not receive the packet successfully and will not send data in next time slot. We can see that there are a total of four streams which are allocated for node A and E. In such case, -MAC protocol does not achieve the optimal allocation of data streams, which does not reach the maximum utilization of the antennas. Motivation is derived from these observations to improve resource utilization of the antennas and the throughput of the whole network. Next section will give detailed description of the proposed protocol (-MAC). III. PROPOSED PROTOCOL A) Description of protocol In this paper, we propose a MAC protocol called - MAC, and the core idea of the protocol is that it dynamically adjusts the waiting time of CTS, to change their priorities. The shorter the waiting time of CTS is, the higher its priority is. Thus, it will not distinguish and CTS time slot. When a node wants to send data, it sends an packet beforehand, and the node that receives for the first time will modify the waiting time of its CTS to the maximum backoff time of, which means that the priority of CTS is lowest. If there is no another request in the next time slot while the backoff time is expired, then the node will send CTS in this time slot, without having to wait until the next time slot, thus saving a time slot. If it receives another in the next time slot, two cases will happen. Firstly, if the packet is not sent to itself, the waiting time of CTS will be modified to, the priority of CTS will be the highest, that means at the beginning of the next time slot the node will immediately send CTS. Secondly, if the packet received is sent to itself, then the backoff time of CTS will be changed to one time slot (the priority is still higher than the node that just receives an packet, and it is higher 531

than the priority of ). Otherwise, the CTS priority of the node will not be changed. In addition, we should note that if the node receives a CTS packet whose priority is the highest, then it will wait a time slot to send its data, because other nodes will send CTS in this time slot. In the example below, we will illustrate this point. For other conditions, the nodes will send their data in the next time slot while receiving CTS without waiting a time slot. After sending the data, the order to send ACK is as follows: the priority of ACK is higher than, but according to the priority of CTS packet, the priority of ACK is also different. The higher the priority of CTS is, the lower the priority of ACK is. B) Examples of protocol operation -MAC protocol is suitable for various network topologies. It can perform well in the topologies where - MAC performs badly. Here are some examples. Through them, we can know how the -MAC operates and why it works better than -MAC. 1) Different collision domains, same receiving node: There are two different transmission requests to the same receiving node in different collision domains, as shown in Fig. 2a). Node A sends after the backoff time randomly selected. Node B adjusts its CTS backoff time to the maximum backoff time after receiving the. And then, Node C sends a packet in the second time slot. Node B adjusts its CTS backoff time to one after it receives the second packet, and sends CTS in the next time slot. The CTS packet contains IDs of node A and node C and their maximum number of available flows. Then node A and C will send data at the beginning of the next time slot. Finally, node B will send an ACK packet. a) Topology for the example b) -MAC timing diagram Figure 2. Different collision domains, same receiving node 2) Different collision domains, different receiving nodes: This is a more complex situation, as shown in Fig. 3a). This example explains why node A should wait a time slot while it receives the CTS from B, this issue mentioned in the previous section. The procedure for this communications is shown in Fig. 3b). a) Topology for the example b) -MAC timing diagram Figure 3. Different collision domains, different receiving nodes According to the protocol, node A first selects a random number of backoff time slots, after that, sends a packet, only node B receives this, and node C only senses that a node is sending data but cannot successfully receive. While node B receives the, it adjusts its CTS waiting time to the maximum backoff time. Then node C sends a in the next time slot and this is successfully received by node B and node D. When node D receives this, it adjusts its CTS waiting time to the maximum backoff time. When node B receives the, it adjusts its CTS waiting time to. In the third slot, node B first sends CTS, and node D sends CTS in the next time slot. These CTS packets contain the allocated stream number for each node, respectively. Then, node A and C send data at the same time in the next time slot. Note that if node A does not wait a time slot, node A and C can not be coordinated. At the end of transmission, node D sends ACK first, and in the next time slot node B also sends an ACK. These topologies have been resolved by the protocol - MAC, and also gain the optimal flow allocation in the whole networks. When node density increases, -MAC cannot achieve the optimal allocation, but our proposed protocol can also achieve the optimal flow allocation. For the topology shown in Fig. 1c), -MAC does not work well, but our proposed protocol can get better performance. In Fig. 1c), nodes A, C and E have communication requests. Node A and E first send in the first time slot and there is no interference. Node B and D both receive one packet, so they adjust their CTS waiting time to the maximum backoff time, and then node C sends a in the next time slot. At this time, node B adjusts its CTS waiting time to ; node D adjusts its CTS waiting time to one time slot. So in the next time slot, node B has a higher priority. It sends CTS first, and node D sends CTS in the next time slot. It should be noticed that node A should send data after waiting a time slot when it received node B's CTS, we have explained it above. Finally node D and B will reply ACK respectively. We can see that there are a total of six parallel data flows and it gains the optimal flow allocation in the whole networks. 532

The probability that some node successfully sends packets in some time slot when there are z nodes which want to send packets simultaneously is 3 z n 1 z 1 (3) Pss ( z) = z((1 p ) ) p (1 p ) n 1 sl sl = sl Figure 4. Procedure for -MAC in the topology shown in Fig. 1c) C) Data frame format of -MAC Fig.5 shows the frame format of -MAC. The difference between -MAC and -MAC is that we no longer distinguish and CTS time slot, all named as contention time slot. The competition includes not only the competition between and, but also between and CTS. In each control slot, nodes can send or CTS. This is implemented by dynamically adjusting CTS waiting time to change the priority of CTS. These mini time slots are used to avoid the collision of, CTS, etc. The use of time slots in -MAC is more flexible than that in -MAC, and it reinforces competition between and CTS. Backoff Mini slots 1 -MAC Frame Backoff Mini slots 2/CTS1 2/CTS1 -MAC Frame 2/CTS1 Backoff -MAC Frame DATA ACK1 ACK2 Contention Contention Contention Contention DATA slot ACK slot Slot1 Slot2 Slot3 Slot4 Figure 5. Data frame format of -MAC IV. PERFORMANCE ANALYSIS In this section, we present the performance analysis of -MAC protocol. We first make the following assumptions: the number of nodes is fixed [1], the channel is ideal (packet transmission won't be failure caused by the noise), and at any moment the sending queues of all nodes are not empty, which means that any node always has data to be transmitted, and the probability that each node sends packet per time slot isτ. In addition, we also assume that each node has four antennas, and the largest number of divisions is two. Based on above assumptions, we can easily obtain the probability that no packet will be sent in one time slot in the case of n nodes is P ( n) = (1 τ ) n (1) idle The probability that there are z nodes sending packets in some time slot is z n z z () = (1 ) (2) P send z C n τ τ Where n is the total number of the nodes. Where mini-slot. psl is the probability that a node sends packets in a So using (2) (3), we can obtain the probability P s that a node sends packets successfully in a time slot. n Ps ( n) = P ( z) Pss ( z) z = 1 send Therefore, we can approximately describe the number of divisions, and we use P (1) and P(2) to represent the probability that the number of the divisions is one or two respectively. (4) P(1) = Ps ( n)( P idle ( n 1) + (1 Ps ( n 1))) (5) P(2) Ps ( n) Ps ( n 1) = (6) In addition, the probability that in a time slot there are no nodes sending packets or the transmission fails is p fault P fault = P idle ( n) + (1 Ps ( n)) (7) Based on the above derivation, we can easily calculate the system throughput, using S to represent the system throughput, then E[The average size of transmission load in unit time slot] (8) S = E[The average length of time slot] It means that system throughput is equal to the expected value of load information size transmitted in unit time slot [11]. We use E 1 ( P ) and E ( P) to denote the average size of payload 2 when the division is one or two respectively, σ to denote the slot time, T 1 and 2 s T to denote the time that packet is s successfully sent for different divisions, then we can obtain: (9) S P (1) E 1 ( P ) + P ( 2 ) E 2 ( P ) = 1 2 P fa ult σ + P (1 ) Ts + P ( 2 ) Ts V. SIMULATION RESULTS In this section, we evaluate our protocol through extensive simulation in MATLAB and give the performance comparison of -MAC and -MAC in different topologies. The main parameters of protocol are summarized in Table I. According to the formula (9) and the parameters which are in Table I, we can calculate the throughput and the result is shown in Fig.6. We can find that with the interval of sending packet decreasing, the throughput of the entire network rises sharply at the beginning. The reason for this phenomenon can be explained as follows. While the interval of sending packet is long, the packets sent into the network are fewer, so the throughput is lower. But with the interval decreasing, the 533

packets sent into the network increase and so the throughput gets higher. TABLE I. PARAMETERS USED IN SIMULATION Parameter Value Parameter Value Packet 228bytes Channel Bit Rate 2Mbit/s payload MAC header 272bits Propagation Delay 5μs PHY header 128bits Slot Time 2μs Throughput (kbps) Throughput ()kbps) ACK 4 3 3 2 2 1 4 3 3 2 2 1 2bytes+PHY header 24bytes+PHY header ACK_Timeout CTS 2 5 1 5 3 1 Figure 6. Performance of -MAC protocol 2 5 1 5 3 1 1876μs 24bytes+PH Y header MIMA Figure 7. The comparison of three protocols in the first topology (as shown in Fig.1a)) As shown in Fig. 7, it is the comparison with the - MAC, MIMA-MAC and -MAC protocol in the topology shown in Fig. 1a). Compared with MIMA-MAC, a major improvement of -MAC is that when there is only a pair of nodes transmitting data within the collision domain, MIMA-MAC can only use half of the antennas to send data, which wastes another half antenna resource, and -MAC can use the entire antenna resources to transmit data in this situation. Therefore, the throughput is greatly improved compared with MIMA-MAC. -MAC is an improvement of -MAC, aimed at the shortcoming that -MAC must wait an empty time slot. So the throughput of -MAC is also greatly improved. In Fig.7, the abscissa denotes the packet sending interval; the ordinate denotes throughput. Decreasing the packet sending interval indicates that the packet sending rate increases. When sending a packet every ms, the throughput of three protocols is the same, and it is relatively small. Because at this time there are only a few packets sent into the network, basically they do not conflict or lose. Similarly, the throughput of three protocols is the same until the interval of 1ms. Starting from 1ms, as the packet sending rate increases, the efficiency of MIMA-MAC declines obviously, the reason for the decline is MIMA-MAC can only use two antennas to send data in this topology, it takes more time to send the same size packet, therefore, in the same time, the number of packets sent is much less than the other two protocols, and the throughput is also much smaller. The throughput of -MAC is almost same as -MAC until the time of 5ms, but when the packet sending intervals are more frequent, the maximum throughput of -MAC achieves 3.5Mbps, while the maximum throughput of -MAC can achieve 3.7Mbps. The reason is that -MAC will not waste a blank slot, therefore, the average length of a frame is shorter, that is, more packets will be sent per unit time, and the throughput will be greater. Troughput ()kbps 4 4 3 3 2 2 1 2 5 1 5 3 1 Figure 8. The performance comparison between -MAC and - MAC (for the topology shown in Fig.2a)) Thoughput ()kbps 4 3 3 2 2 1 2 5 1 5 3 1 Figure 9. The performance comparison between -MAC and - MAC(for the topology shown in Fig.3a)) For the topology shown in Fig. 2a) and the topology shown in Fig. 3a), -MAC has achieved the maximum antenna utilization, at this moment, -MAC maintains this efficiency. As shown in Fig. 8 and Fig. 9, in the topology 534

shown in Fig. 2a), the maximum throughput of the entire network can achieve 4.413Mbps; in the topology shown in Fig. 3a), the maximum throughput of the entire network can achieve 3.785Mbps and the performance of these two protocols are the same. In the topology shown in Fig. 1c), if there is no external node E as interference node, the performance of -MAC and -MAC are the same. However, if there is an external interference from node E, -MAC will fail. In this case, -MAC results in wasting part of antenna resources after receiving CTS. But -MAC will still be able to maximize the use of antenna resources. That is, in this network topology, the existence of such an interference node E will result in only two antennas being used to send data by - MAC, which the overall throughput reduces by half. Nevertheless, -MAC still can guarantee the performance unchanged, as shown in Fig. 1. Throughput (kbps) Throughput (kbps) 4 3 3 2 2 1 2 5 1 5 3 1 Figure 1. The performance of -MAC and -MAC (for the topology shown in Fig.1c)) 4 3 3 2 2 1 2 5 1 5 3 1 Figure 11. The performance comparison between -MAC and - MAC in the average situation However, in the actual network structure, the probabilities of various topologies are random. If simply considering the probability of all the four topologies is the same, the average throughput of -MAC and -MAC is shown in Fig. 11. As shown in Fig. 11, with the sending packet interval decreasing, the throughput of these two protocols is gradually increasing and the throughput of -MAC is always higher than -MAC. That is, in some topologies, - MAC has better performance; in other topologies, - MAC maintains the same performance as -MAC. The performance of -MAC will not be worse than - MAC, so it means that the overall performance of - MAC is better than that of -MAC. In the case of the probabilities of all topologies are equal, the best throughput of -MAV can achieve 3.762Mbps, while the best throughput of -MAC can only achieve 3.31Mbps. VI. CONCLUSION In this paper, we have proposed a MAC protocol based on dynamically adjusting the waiting time of CTS packet for MIMO ad hoc networks called -MAC, which can make the allocation of antenna resources more rational, and can improve the performance of wireless networks in some topologies where -MAC cannot make full use of the antenna resources. Our simulation results show that - MAC has a higher throughput, and can optimally allocate antenna resources in any topologies. REFERENCES [1] S. Corson and J. Macker, Mobile ad hoc networking (MANET): routing protocol performance issues and evaluation consideration, IETF RFC251, Jan, 1999. [2] K. Sundaresan, R. Sivakumar, M. A. Ingram and T.-Y. Chang, Medium access control in Ad Hoc networks with MIMO links: optimization considerations and algorithms, IEEE Transactions on Mobile Computing, vol.3, no.4, pp.35-365, Oct.-Dec. 24. [3] J. S. Park and M. Gerla, MIMOMAN: a MIMO MAC protocol for Ad Hoc networks, 4th International Conference on Ad Hoc Networks and Wireless (AD HOC NOW 25), pp.27-22, Oct. 25. [4] J.C. Mundarath, P. Ramanathan and B.D. Van Veen, NULLHOC: a MAC protocol for adaptive antenna array based wireless Ad Hoc networks in multipath enviroments, In Proceedings of IEEE GLOBECOM 24. [5] J. Park, A. Nandan, M. Gerla, and L. Heechoon, SPACE-MAC: enabling spatial reuse using MIMO channel-aware MAC, IEEE ICC 25, 25. [6] D.J. Dechene, K.A. Meerja, A. Shami, S. Primak, A novel MIMO- Aware distributed media access control scheme for IEEE 82.11 wireless local area networks, Local Computer Networks, 27. LCN 27, pp.125-132. [7] M. Park, S.H. Choi and S.M. Nettles, Cross-layer MAC design for wireless networks using MIMO, Proceeding of GLOBECOM 25, vol.5, pp.287-2874, Nov.-Dec. 25. [8] P.K. Zhao, B. Daneshrad, An optimal single/concurrent link MAC scheme for a single-hop MIMO network, MILITARY COMMUNICATIONS CONFERENCE, 211. [9] M. Shirasu and I. Sasase, "A MAC protocol for maximum stream allocation depending on the number of antennas and received packets in MIMO ad hoc networks," in Proc. IEEE International Conference on Communications, pp. 3295-33, June 27. [1] IEEE standard for wireless LAN medium access control (MAC) and physical layer (PHY) specifications, Nov. 1997. P82.11. [11] G. Bianchi, Performance analysis of the IEEE 82.11 distributed coordination function, IEEE Journal on Selected Areas in Communications, Vol. 18, March 2. 535