Coordinated Dynamic Physical Carrier Sensing based on Local Optimization in Wireless Ad hoc Networks

Transcription

1 2013 IEEE Wireless Communications and Networking Conference (WCNC): MAC Coordinated Dynamic Physical Carrier Sensing based on Local Optimization in Wireless Ad hoc Networks XinMing Zhang, Guoqing Qiu, Zhilong Dai School of Computer Science and Technology University of Science and Technology of China Hefei, P.R.China Dan Keun Sung Department of Electrical Engineering Korea Advanced Institute of Science and Technology Daejeon, Korea Abstract Carrier sensing schemes have been recognized as a key knob for improving network performance. The distributed coordination function (DCF) and its modifications focus on the feature of the ongoing transmission links to enhance link throughput, however, a lack of coordinated adjustment of the surrounding nodes may reduce the aggregate throughput of entire network. In this paper, we are concerned with the delayed transmissions from exposed terminals, and interference and collisions from hidden terminals. We classify the neighboring areas for a given transmission link into three areas: a hidden area, an exposed area and an overlapped area, and then propose a coordinated dynamic physical carrier sense (CDPCS) scheme in which the carrier sensing threshold (CST) values of the neighbors are adjusted at the same time to achieve local optimization by utilizing the exchanged information and states of neighboring nodes. Simulation results show that the proposed schemes can work effectively. I. INTRODUCTION In wireless multi-hop ad hoc networks, IEEE DCF has been widely adopted as a main medium access control (MAC) protocol. The DCF protocol utilizes carrier sensing to detect the status of the medium and determines either to transmit or to defer transmission to mittigate a collision problem. There are two type of carrier sensing: physical carrier sensing (PCS) and virtual carrier sensing (VCS). A node can initiate a transmission if it senses the signal strength below a given carrier sensing threshold (CS th ) value in the PCS [1], while it exchanges request-to-send (RTS) and clear-to-send (CTS) frames and sets its network allocation vector (NAV) to the duration of a successful transmission in the VCS, which has been included in the IEEE standard. Many studies [1] [4] focused on spatial reuse in the PCS environment, which coordinate concurrent transmission nodes in different collision domains in order to improve channel utilization and network throughput. These studies showed that there is a trade-off among carrier sensing range, throughput, and the hidden/exposed node problems. Both hidden nodes and exposed nodes affect the network performance This work was supported in part by the National Natural Science Foundation of China under Grant No & No & No [5]. Conventional RTS/CTS schemes which effectively set the receive range to the data transmission range can partly eliminate the hidden terminal and exposed terminal problems in transmission range. But the node outside the transmission range may cause interference to the receiver, which is called the extended hidden-terminal problem [6]. In order to solve the interference problems, we can extend the RTS/CTS reception range. However, the extended RTS/CTS transmission range can also cause to reduce the number of concurrent transmissions while increasing the number of exposed terminals, therefore, the conventional RTS/CTS schemes have a shortage in solving the interference problems. In many cases, transmission nodes block the transmission of the surrounding neighbor nodes even though the blocked nodes would not induce sufficient interference to corrupt its reception at the receiver. These problems are mainly attributed to an inherent property that the DCF mechanisms excessively pay attention to the ongoing transmission between the sender and receiver. In this paper, the initial motivation of our proposed scheme is as follows: the nodes which are around the transmission link, such as hidden nodes and exposed nodes, acquire the information and states of transmission nodes by the exchange of simplified RTS/CTS (SRTS/SCTS) control frames in which the transmission power is enough to let interference nodes to correctly receive and decode RTS/CTS signal, and then adjust their carrier sensing threshold values accordingly. Meanwhile, the proposed coordinated dynamic physical carrier sense (CDPCS) scheme employs different carrier sensing threshold values for different location areas: hidden node area, exposed node area and overlapped node area. Each node adjusts its CS th according to the optimization of the performance of local network around transmission links and can improve the overall network performance. II. RELATED WORK Most of previous studies on carrier sensing problems focused on the spatial reuse improvement of networks [2], [4], [7] [9]. Kim et al. [2] showed that the spatial reuse depends only on the ratio of the transmit power to the carrier sensing /13/$ IEEE 398

2 threshold while only a set of discrete data rates are available and mentioned that there exists a tradeoff between the level of spatial reuse and the data rate that can be sustained by each transmission. Ma et al. [7], Wong et al. [10] and Deng et al. [11] argued that the carrier sensing threshold is a tunable parameter that can significantly affect the MAC performance. Besides numerical analysis, Ma et al. [12] and Zhu et al. [13] proposed centralized heuristic algorithms for adjusting the carrier sensing threshold based on the network performance, which can not be implemented in distributed ad hoc networks. Park et al. [14], Ven et al. [1] and Zhu et al. [3] proposed distributed adaptive schemes to dynamically adjust the CS th value to eliminate the likelihood of collisions from hidden terminals in order to enhance spatial reuse. However, these schemes did not consider the delay problems of exposed nodes, and also did not distinguish the nodes located within different areas, such as the sensing area and the interference area. It has been a hot technical issue to solve the hidden and exposed terminal problems in wireless ad hoc networks. However, a simple RTS/CTS mechanism is not sufficient enough to resolve these problems completely. A tradeoff between collisions (hidden nodes) and unused capacity (exposed nodes) was investigated in [5]. Alawieh et al. [4] showed that selecting a small carrier sensing threshold value would severely impact the spatial reuse, whereas setting a large CS th value would cause excessive interference among concurrent transmissions. Nadeem et al. [8] incorporated location information in DCF frame (RTS/CTS) exchange sequences so that stations sharing the communication channel are able to make better interference predictions and blocking channel access. In [14], the sender exploits such correlation using an automated on line learning algorithm and makes a decision about the channel availability at the intended receiver. There have been other approaches to increase the number of concurrent transmissions in networks. Ye et al. [9] suggested to change the timing of the steps within the synchronized states among one-hop area neighbors and RTS-CTS-DATA- ACK frame sequence. Acharya et al [16] observed that, in an overactive RTS-CTS situation where the RTS-CTS exchange affects more surrounding stations than needed. Yang et al. [17] proposed a spatial backoff algorithm to adjust the channel contention level, but this algorithm is operated when a collision occurs or a transmission fails. III. LOCAL OPTIMIZATION OF CARRIER SENSING WITH COORDINATION A. Transmission Area Analysis The hidden and exposed terminal problems can not be resolved completely in recent carrier sensing schemes [1], [3] because of a lack of consideration about the delay problem of exposed nodes. The nodes located within different node areas such as hidden, exposed, and overlapped areas should be distinguished and analyzed to employ different carrier sensing adjusting strategies to meet different node states. Thus, we first divide the nodes into three different node areas, and then analyze the feature of nodes in each region. Fig. 1 shows three different node areas in a local transmission area. Here, node A starts its transmission to node B in the beginning, while node C can not sense the ongoing transmission between node A and node B because node C is outside of node A s sensing range. In this case, node C senses the channel idle and naturally initiates its own transmission causing to interfere with the transmission of node A. In fact, node C does not know that its transmission corrupted the transmission from node A, and the region where node C is located is called the hidden area. Node D is an exposed node. After node A starts transmission to node B, node D detects the activity of node A and then blocks its own transmission. Unfortunately, the transmission of node D does not corrupt the current transmission because receiver B does not receive interference from node D. This exposed problem causes unnecessary delay, and the region where node D is located is called the exposed area. The overlapped node area refers to the overlap node area of the sensing range of a transmitter, such as node A, and the interference range of a receiver, such as node B. For example, node E is in the overlapped node area. The nodes located in the overlapped node area are close to the transmission link and mitigate the contention from a sender by tuning the CS th, and also can mitigate the conflict from a receiver by sensing the interference range. Fig. 1. Three different areas in a local transmission area B. Carrier Sensing Tuning Strategy based on Local Optimization Based on the above problems, the conventional PCS schemes have the following two disadvantages: lack of coordinated carrier sensing tuning in surrounding nodes and a lack of different adjustment in different node areas. We propose a new carrier sensing tuning strategy based on local optimization to solve the above problems. 1) SRTS-SCTS exchange In order to achieve local optimization in a distributed network, surrounding nodes need to exchange some information and states, such as transmission distance, transmission power and interference range, and then obtain the optimal carrier sensing threshold to cooperate the local network carrier sensing scheme. For the minimal overhead of information exchange, we use simplified RTS/CTS control frames, called SRTS/SCTS, to carry the exchanged information and states of nodes. 399

3 SRTS/SCTS control frames cancel the NAV scheme used in the conventional RTS/CTS exchange scheme, and nodes which receive SRTS/SCTS frames only read the carried information without blocking its transmission. Fig. 2 shows the sequence diagram of SRTS-SCTS exchange. The sender node first sends an SRTS frame which delivers the information to surrounding neighbors, and then the surrounding nodes receive the SRTS frame and save the received information. The intended destination of the SRTS frame replies an SCTS frame to the sender and broadcasts the SCTS to the surrounding neighbors. After receiving the control frames and reading the carried information, the nodes set their carrier sensing thresholds by an appropriate scheme. Through the exchange of SRTS/SCTS control frames, the nodes around the transmission link can be aware of the information and states of the current transmission. Fig. 2. Sequence diagram of exchanging SRTS-SCTS frames In order to achieve the goal of local network optimization, each node needs to collect the information of all on-going neighboring transmissions. Each node maintains two data structures of vector type: V-SRTS and V-SCTS, which store the calculated carrier sensing threshold values from the control frames, and then deletes the corresponding record value after the transmission is finished. Vector V-SRTS and V-SCTS consist of < I, d > and < I, d >, respectively where I is the index of SRTS and SCTS frames and d is the calculated carrier sensing range value. After receiving the control frames, the node determines the located node area, which includes the overlapped area, hidden node area and exposed node area, under the stored vector V-SRTS and V-SCTS. The node in the overlapped area has a record of a pair of transmission (node i, j) in V-SRTS and V-SCTS, respectively. The node in the hidden node area has a record in vector V-SCTS, while the node in the exposed area has a record in vector V- SRTS. Generally, each node may be located in different node area at the same time, and the carrier sensing tuning strategy needs to be selected according to the priority of each area. The overlapped node area has the highest priority, and a less priority is assigned in hidden nodes, and the exposed area has the lowest priority. The strategies to determine the located node areas is presented in Algorithm 1. Based on the decisions of Algorithm 1, we can have a different carrier sensing tuning strategy according to the distinguishing features in different node areas. 2) Carrier sensing tuning strategy in different node areas Each node calculates the carrier sensing threshold according Algorithm 1 The Strategy to Determine the Located Areas 1: After Receiving SRTS/SCTS Frame 2: if V-SRTS< i > in V-SRTS corresponding V-SCTS< j > in V-SCTS then 3: Node is in Overlapped node Area of transmission i to j; 4: else { V-SCTS is not NULL } 5: Node is in Hidden node Area; 6: else { V-SCTS is NULL AND V-SRTS is not NULL } 7: Node is in Exposed node Area; 8: else 9: No Transmissions around; 10: end if to the V-SRTS and V-SCTS. In the overlapped area, nodes are close to transmission link, and, thus, they adjust the carrier sensing threshold value only to sense the transmission of the sender, and the carrier sensing range is quite small, which corresponds to a large threshold value in order to improve the spatial reuse level. The nodes in the hidden node area should sense the transmission of the sender, and then the carrier sensing range is set to a large value to mitigate interference. For improving the number of concurrent transmissions, a small carrier sensing range is set so that nodes in the exposed node area are encouraged to transmit their frames. The final carrier sensing range is obtained based on the following equation: r cs = min{max{d in V SRT S}, r max } when in the overlapped node area min{max{d in V SCT S}, r max } when in the hidden node area max{min{d in V SRT S}, r min } when in the exposed node area where r max and r min is the maximum and minimum bounds of carrier sensing range in order to ensure the safety of adjustment, respectively. IV. PROPOSED ALGORITHM Based on the above analysis, we propose a new scheme called a coordinated dynamic physical carrier sensing (CD- PCS) scheme. In this section, we introduce our underlying concept of this proposed scheme and also present it in pseudocode. A. Coordinated Dynamic Physical Carrier Sensing (CDPCS) Scheme The proposed CDPCS scheme mainly includes three parts: exchange of SRTS/SCTS control frames, determination of located node area and carrier sensing threshold tuning. The first two parts was presented in the previous section, and then the last part focuses on the dynamic carrier sensing tuning after receiving SRTS or SCTS frame for coordination. In wireless ad hoc networks, the signal P r received at the receiver is a decreasing function of the distance d. Let P s be the transmit power of the sender, and θ is the path-loss (1) 400

4 exponent, ranging from two (line-of-sight free space) to four (indoor) [18], then P r is: P r = P s d(s, r) θ. (2) When node D receives an SRTS frame from other node, e.g. node A, node D is able to calculate the distance (d AD ) to node A as follows: d A,D = ( P t,a P r,d ) 1 θ, (3) where θ is the path-loss exponent, and we typically choose 3 in our scheme. P t,a is the transmit power of node A and P r,d is the received signal strength of node D. Node D saves the d A,D into the V-SRTS as V-SRTS< I, d >, where, I is the index of transmitter and d is the d A,D. Node D calculates the new CS th value based on d A,D to compare with the CS th value of D. If the new CS th value is larger and D does not receive any SCTS, the new CS th value is set to encourage its transmission. After receiving the SCTS frame, node B is able to calculate d AB by using Eq. 3. Secondly, the receiver (node B) calculates its interference range: d B,I = ( P t,a P t,a ) 1 θ, (4) ς d θ data N B AB where ς data represents the SINR threshold for data frames. N B is the current threshold of noise measured at node B. Next, node B obtains the necessary information about the transmitter A (such as d A,B and d B,I ) from the SRTS frame. In order to enforce the interfering nodes of receiver B to defer their transmission, an SCTS frame, including d A,D and d B,I, is sent to make sure that all neighboring nodes in node B s interference range receive the SCTS frame. Thus, the transmit power of node B should be set to be large enough to transfer the SCTS frames availably. Some nodes outside of the interference range of node B still receive the SCTS frames. Accordingly, node C calculates the d B,C, and then compares the d B,C with the value of d B,I in the received SCTS frame. If d B,C is larger than d B,I, node C does not to adjust the CS th value and discards the SCTS frames. Otherwise, node C detects its location in the interference, and adjusts its CS th value to ensure that its carrier sensing range includes node A. Now, we obtain the following inequality: d c,c = d A,C d A,B + d B,C, where d c,c denotes the carrier sensing range of node C; the minimum carrier sensing range is d c,c = d A,B + d B,C, and the minimum carrier sense range corresponds to the maximum CS th, η max,c. If the CS th value is lower than the η max,c, it limits its threshold. If the CS th value is higher than the η max,c, node C can not sense the receiver B. Thus, the minimum carrier sensing range is equal to the silence range of node A. The maximum carrier sensing threshold is calculated as follows: P η max,c = (d A,B + d B,C ) 3. (5) In multi-hop networks, a node may receive many SRTS frames from more than one pair of nodes. The maximum carrier sensing node of all neighboring node pairs is η max,c = min{ P (d }. i cs B. Algorithm )3 Each node maintains two vectors: V-SRTS< I, d i CS > and V-SCTS< I, d i CS >. When node I receives a control frame, the CDPCS scheme starts as in Algorithm 2. Lines 2 to 10 represent the processing for SRTS and SCTS control frames. After receiving the SRTS frame, the node stores the distance between the sender and receiver from the SRTS frame as d A,D in vector V-SRTS. When the node receives the SCTS frame, the distance of interference link, d B,I, adding the distance of transmission link, d A,B, is stored in V-SCTS. When receiving an ACK frame, it means that the data transmission has finished, and the transmission records in vectors should be deleted, and then the algorithm jumps to line 11. Lines 11 to 17 represent the processing of carrier sensing range (r cs ) selection. If the vector V-SRTS and V-SCTS in a node stores the record of a pair of SRTS and SCTS transmission, which indicates that the node is located in the overlapped node area of a link, the maximum d cs in V-SRTS is selected as the carrier sensing range. If the V-SCTS is not null, which means that the node is located in a hidden node area, the maximum d cs in the V-SCTS is selected as the carrier sensing range in order to mitigate the interference from hidden nodes. If the V-SRTS is null and V-SCTS is not null, which means that the node is located in an exposed node area, the minimum d cs in V-SRTS is selected as the carrier sensing range to decrease the delayed concurrent transmissions. In order to guarantee the safety of carrier sensing range adjustment during the processing of signal reception and numerical calculation, we utilize the r max and r min as the boundary of carrier sensing range tuning, and r max is set as the transmission radius of node, while r max is set to the treble transmission radius. After data transmission, the receiver sends an ACK to the sender. Neighboring nodes which receive the ACK frame delete the current distance records from two vectors: V- SRTS< I, d i CS > and V-SCTS< I, di CS >. Meanwhile, every record in vectors maintains a timestamp. After certain time (maximum transmission time of the data frame), the node checks the timestamp of each record and deletes the timeout records in vectors to ensure the real-time characteristics of vectors. V. PERFORMANCE EVALUATION In this section, we present a simulation study to evaluate the performance of the proposed scheme. we compare the performance of the proposed scheme with that of two other schemes: IEEE scheme and the spatial backoff scheme proposed in [17]. The simulation is carried out using NS-2. In the simulation of grid topology and random dynamic topology, we adopt a two-ray model as a radio propagation model. The dynamic Source Routing (DSR) routing protocol is considered as a routing protocol. The transmission radius is 250m, and the initial carrier sensing range is 450m. The SRTS/SCTS 401

5 Algorithm 2 CDPCS 1: After Receiving SRTS/SCTS Frame 2: if Frame type is SRTS then 3: distance=d A,D ; Put distance into V-SRTS; 4: end if 5: if Frame type is SCTS then 6: distance = d A,B + d B,I ; Put distance into V-SCTS; 7: end if 8: if Frame type is ACK then 9: Delete records of sender in V-SRTS / V-SCTS; Goto Line 11; 10: end if 11: if V-SRTS< i > in V-SRTS corresponding V-SCTS< j > in V-SCTS then 12: r cs = min{max{d in V-SRTS}, r max }; 13: else {V-SCTS is not NULL} 14: r cs = min{max{d in V-SCTS}, r max }; 15: else {V-SCTS is NULL AND V-SRTS is not NULL} 16: r cs = max{min{d in V-SRTS}, r min }; 17: end if Fig. 4. Collision probability versus the number of CBR flows transmission range is set to 1.6 times the distance of data transmission. The simulation time for each simulation scenario is 500 seconds. In the results, each data point represents an average of 10 trials of experiments. The confidence level is set to 95%, and the confidence interval is shown as a vertical bar in figures. The NAV scheme is discarded in the proposed mechanism. A. Grid Topology In this subsection, we consider 25 nodes placed on a 5 5 grid at unit distance (200m) from each other. The CBR (Constant Bit Rate) flows randomly choose end-to-end source and destination pairs in this topology, and the transmission rate is 1Mbps. Fig. 3. Throughput versus the number of CBR flows Fig. 3 shows the throughput performance of the following three schemes: IEEE basic scheme, spatial backoff scheme, and the proposed scheme, for varying the number of CBR flows. The network has maximum throughput when the number of CBR flows is 7. The proposed scheme can improve the throughput of network because the CS th values of the Fig. 5. Throughput versus distance to adjacent node neighboring nodes are adjusted at the same time. Fig. 4 shows the collision probability of the three schemes for varying the number of CBR flows. The proposed scheme yields the lowest collision probability and interference. In order to achieve the optimization of local network, the CS th values of surrounding neighbors are adjusted at the same time, and then the number of collisions can be reduced effectively. In order to further investigate the relationship between node density and throughput, we consider 25 nodes placed on a 5 5 grid, the distance of adjacent nodes is set from 120m to 240m and the topology is gird. The number of CBR flows maintains 7, and other experiment parameters are the same as above parameters. Fig. 5 shows the throughput of the three schemes for varying the distance to adjacent node. We can observe from the figure that the proposed CDPCS scheme has the best result when the node density is high.when the distance between each node is small, hidden nodes and exposed nodes around the transmission links cause conflicts and interference. Meanwhile, the performance of the proposed CDPCS scheme is not particularly outstanding because it can not utilize the advantages of local information exchange when the node is sparse. B. Random Dynamic Topology We consider a randomly generated network with 100 nodes which are randomly distributed inside an m 2 with a maximal moving speed of 5 m/s. As the transmission rate increases, the interference range increases. Thus, higher transmission rates do not help to improve network throughput. 402

6 Fig. 6 shows the throughput of the three different schemes for varying transmission rates. The network achieves the maximum throughput when the transmission rate is set to 25Mbps. In the random network topology, this scheme still demonstrates good performance. In the case of random dynamic topology, the neighboring nodes are constantly moving, however, the proposed CDPCS scheme is based on the exchange of control frames, and then the topology change does not affect the performance considerably. Thus, the proposed CDPCS scheme is able to adapt to random or dynamic topology. As the transmission rate increases, the network reaches its maximum capacity, and the throughput is maintained at a slow growth until the transmission rate is higher than 25Mbps. Fig. 6. Throughput versus transmission rates Fig. 7. Throughput versus number of nodes To test more general topologies, we place nodes randomly in an m 2 area with 9 pairs of randomly chosen nodes acting as sources and destinations with the same transmission rate, 25Mbps. We investigate the throughput by varying the number of nodes from 20 to 60. Results are shown in Fig. 7, where the X-axis is the number of nodes and the Y-axis is the throughput in Kbps. In general, the proposed CDPCS scheme achieves the better performance in a dense network. VI. CONCLUSION In this paper, we mainly focused on the physical carrier sensing scheme to enhance the performance of DCF-based multi-hop wireless networks. We are concerned with the solution of the hidden terminal and exposed terminal problems and propose a coordinated dynamic physical carrier sense (CDPCS) scheme. The proposed CDPCS scheme enables a node to sense the busy medium and block its transmission whenever it falls in the interference range of the receiver, while it enables nodes in the carrier sensing range and not in the interference range encourage to transmit their packets.through theoretical analysis and experimental simulation, the proposed CDPCS scheme can improve network performance by increasing the number of concurrent transmissions of exposed nodes and reducing the collision probability and interference caused by hidden nodes in networks. REFERENCES [1] P. M. van de Ven, A. J. E. M. Janssen, and J. S. H. Van Leeuwaarden. Optimal tradeoff between exposed and hidden nodes in large wireless networks, Proc. ACM SIGMETRICS, pp.14-18, [2] T-S. Kim, H. Lim and J. C. Hou, Understanding and Improving the spatial reuse in multihop wireless networks, IEEE Transactions on Mobile Computing, Vol. 7, No. 10, pp ,october [3] J. Zhu, X. Gao, L. L. Yang, W. S. Conner, S. Roy and M. M. 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