Bi-Code Channel Access Method for Ad Hoc Networks

Transcription

1 Bi-Code Channel Access Method for Ad Hoc Networks Jarmo Prokkola, Timo Bräysy Centre for Wireless Communications (CWC), University of Oulu, Oulu, Finland Abstract Ad hoc networks operate conventionally on a contention based common channel where harmful collisions bring problems with increasing traffic load. The use of code division multiple access (CDMA) increases the capacity by allowing multiple successful transmissions simultaneously within the limits of multiple access interference (MAI). Most of the studies on CDMA ad hoc networking so far, however, concentrate on complex schemes like transmitter based orthogonal coding. In this paper, a cross-layered design approach for ad hoc networking is taken and a CDMA-based bi-code channel access method is proposed. In this method, two channels are tracked at the receiver. One is used as a dedicated receiver based channel for robust data transmission, and the other, as a common access channel. It is shown that this method outperforms traditional methods and still is easy to implement over different platforms, robust for the design parameters and different conditions while maintaining the original idea of ad hoc networking. Index Terms BCCA, MAC, performance, ad hoc, CDMA, spreading code. INTRODUCTION Ad hoc network is a self-organizing mobile network without centralized control where each node acts also as a0 router to attain coverage over multiple hops. In this kind of network configuration, it is obvious that the routing protocol plays very important role. Consequently, the main interest of the research has been focused on routing protocols []. The idea of ad hoc networking differs from the traditional network operation where a centralized control, fixed channel assignment and duplexing usually exist and the quality of service (QoS) can be guaranteed. Therefore, in addition to routing, issues in other OSI-model layers need to be considered. In this paper we focus on L and L2 layer (the physical layer and the MAC layer (Medium Access Control)) issues on terms of ad hoc networking. Typically, ad hoc networks operate on a common channel, which in the case of spread spectrum (SS) communications is equal to the use of common spreading code (C-code) for all the nodes. This introduces the need for contention of the common transmission medium. Lots of different random access methods have been proposed (e.g., a good overview in [2]). The fundamental problem of the use of common channel is that all the nodes in the neighborhood (i.e., within the effective radio range) of the transmitting node are locked on to that transmission (Figure a). This is required, because simultaneous transmissions on the same channel can cause collisions where all the information could be lost. In SS systems, it is possible to increase effective capacity by using different codes. This enables the possibility for multiple successful transmissions at the same time, frequency and space (Figure b) within the limits of MAI caused by non-zero cross-correlations of the spreading codes. (a) (b) Figure. Example: When operating with common codes, all receivers are locked to A s transmission (figure a). Using different codes (e.g., receiver based), simultaneous transmission can occur (figure b) and the capacity is effectively doubled in this special case. In this paper a CDMA approach is taken in order improve ad hoc network performance. CDMA-based MAC protocols and channel access methods have been studied with respect of ad hoc networking before (e.g., [3], [4]). However, in most of the studies, the approach has been based on optimality and throughput maximization. This usually introduces the use of transmitter based spreading codes (T-codes) or pair wise codes [5]. This code assignment makes the receiver architecture very complex, because the receiver must be able to track (receive) multiple spreading codes simultaneously. Also, the goal of optimal performance requires the use of orthogonal spreading codes for all possibly colliding transmissions. But, the orthogonal functionality requires careful synchronization [6], which is contrary to the idea of ad hoc networking where even coarse network synchronization should not be assumed. Because of the limited number of orthogonal codes and limited code tracking capability of feasible receivers, a scheme of recoding has to be applied. Efficient recoding algorithms have been proposed (e.g., [7]), but they all need considerable amount of control traffic. The basic functionality of ad hoc network already uses a great deal of potential data capacity for control traffic, so extra control traffic is not welcome [8], [9]. With increasing mobility, the need for routing control and also for recoding multiplies. This would easily lead to a situation where the amount of total control traffic would be intolerable. In our cross-layered approach, the goal is not to produce an optimal CDMA solution, but one which could be easily implemented, utilized and it still would have high performance over * This work was supported by Finnish Defence Forces

2 conventional ad hoc networks. The goal is also a robust solution, which is not sensitive to optimization parameters, operational environment or non-idealities, while the original idea of ad hoc networking is maintained. THE SYSTEM DESCRIPTION A. The functionality of the receiver From the fundamental idea of ad hoc networking, it is clear that a common channel, which is known to all nodes, should exist. On the other hand, the use of CDMA would increase the performance. Therefore, we propose a method where twochannel receiver capable of tracking and receiving transmissions with two different spreading codes is used. One code is used as a common access channel and the other, as a dedicated receiver code (R-code) channel. Common channel is used for route search, topology and connectivity maintenance, while the R-code channel is used for all directed (data) transmissions. We call this method bi-code channel access (BCCA). Notice that this channel access method differs totally from hybrid spreading code protocols ( [0]) where physical layer packet is split with two different codes. With R-codes, it is possible to enhance the performance by making multiple successful simultaneous transmissions. The advantage over T-code methods is that the receiver does not have to be able to track several spreading codes simultaneously. A disadvantage is that there still exists a slight possibility for collisions. This kind of approach could, in fact, be taken in any domain, say time, frequency or code domain. Nevertheless, in the case of ad hoc networking, the use of code domain is the most straightforward. Figure 2 clarifies the idea of using BCCA. Direct sequence (DS) spreading is preferred in order to avoid modifications to the RF/IF -block of the receiver, which would be necessary for example in the case of frequency hopping (FH). In the transmitter, it is only required that the used spreading code can be changed by message (packet) basis. The idea-level structure of the possible receiver is shown in Figure 3. The implementation of BCCA will only require extra despreading module with matched filter (MF) or correlator, detector and, of course, a code synchronization unit. Received channel samples are buffered in order to separate messages received with different channels (codes), since common and dedicated data can arrive simultaneously. The rest of the receiver is common to both channels and usually includes demodulation, channel decoding, de-interleaving, etc. Buffering could introduce some minor delay, if the traffic to both channels is high, because the arrival of the whole packet has to be waited before releasing the received samples forward. This delay can be avoided if the receiver structure after despreading is also separated through physical layer to MAC. B. On the spreading code usage With BCCA, it is possible to maintain the original idea of ad hoc networking and still get high performance via R-coding compared to pure C-coding. While the performance of R- coding does not achieve the performance of T-coding in ideal case, its implementation is simple and the functionality is robust. Also, the amount of extra control traffic is minimized, because in common case, there is no need for recoding. Aside from orthogonal coding, the use of CDMA always involves MAI i.e., transmissions interfere each other. In our proposal this fact is accepted and no effort is put to optimization of spreading code usage. Recommended properties for the spreading code family are well-controlled cross-correlation properties (e.g., Gold sequences []). In this performance study, we use a simplified assumption that spreading code cross-correlations always have the value of /N c where N c is the length of the spreading code. Receptions with different codes (MAI) are effectively modeled as Gaussian noise. Receiver dedicated code Frequency Code Common code Time Figure 2. A Time-frequency-code-division view of the proposed method. RF/IF -block, ADC Common code MF (+sync) Receiver code MF (+sync) Buffer Rest of PHY Figure 3. An idea-level view of the possible receiver structure of BCCA. THE SIMULATION MODELS The simulation models are built to OPNET 8. network simulator and they are very much the same that were used in our earlier work in the field (e.g., [8], [2]). C. The physical layer and MAC One advantage of BCCA is that it can be easily realized to existing systems. Here, BCCA is realized to well-known IEEE 802. (latter referred as 80.) [3], [4]. It might not be an optimal solution, but it is a good starting point for further development. Also, a lot of performance studies already exist for 802., so the comparison to BCCA can be done easily. The model of 802. used here is originally developed by OPNET Technologies Inc. A modification was needed so that the transmitter spreading code can be changed when instructed to do so (by routing protocol), and of course, an additional receiver channel was needed to physical layer. The RTS/CTShandshaking (Request-to-Send / Clear-to-Send) of 802., does not fit to our idea of BCCA, and is therefore, turned off. Thus, the contention in Distributed Coordination Function (DCF) mode is handled by np-csma (non-persistent Carrier Sense Multiple Access) with exponential back-off algorithm. Carrier sensing is done by power level measurement with a threshold. Sensing may also react to transmissions with unknown codes, because of non-zero cross-correlation of the codes. This certainly reduces MAI, but it might also decrease throughput from

3 the optimum in cases where MAI would not be strong enough to cause packet loss even with multiple simultaneously occurring transmissions. Nevertheless, the optimization of carrier sensing threshold is not considered here. At the physical layer, channel data rate is set to Mbit/s. In 802. DS-mode, N c = chip spreading code is used to spread signal per bit basis. Therefore, the processing gain is directly (0.4 db). D. The Network Layer The networking protocol is minimized in order to keep the model simple enough to emphasize the performance characteristics of the proposed method. The network layer is based on the AODV (Ad hoc On-Demand Distance Vector) routing protocol operation. AODV is reactive, pure on-demand type routing protocol where routes are formed and updated only when needed. A detailed description of AODV can be found e.g., from [6]. The used AODV simulation model for OPNET 8. is based on [5]. Some modifications to this model, which include additional statistic collection capabilities and bug fixings, have been made. The original model had some deficiencies in active route updating and some bugs in route management. AODV hello-messages and passive listening at MAClayer are not used. Network layer data packet header contains 32 bit IP-like (internet protocol) source and destination addresses, Type- (8 bits), Hop count- (8 bits) and Datagram length (6 bits) -fields. These fields are needed for the operation of AODV, and can be therefore calculated as routing overhead. E. The functionality of BCCA with AODV To enable R-type communications, information from neighboring node spreading codes are needed. Figure 4 presents an example of BCCA usage with AODV. In the initial state, the network topology is completely unknown. When node A wants to communicate with node F, it starts route discovery by broadcasting a route request (RREQ) packet with C- code like it is done normally in AODV. When the desired destination node F gets RREQ, it replies with a route reply packet (RREP). Now, the RREP is unicasted upstream along the reverse route which was created by the propagation of RREQ. The next hop nodes are now known for the intermediate nodes in the reverse route and therefore R-codes can be used for RREP transmission as well as for subsequent data communications along the active forward route. Always, when the next hop node is dedicated and known, receiving node s R-code is used for transmission. C-code is used for broadcasting and also, if the next hop node is not known. The scenarios of this study are quite simple and therefore it is assumed that the R-type spreading codes are fixed to nodes and known a priori. Although this kind of assumption is not always realistic, it can be reasonable for example in military environment and we do not lose the generality of the results. Moreover, the algorithm for distributing the spreading codes can be quite easily implemented to AODV and no extra control packets are needed. The spreading code distribution can be handled in route discovery state, since in BCCA, the nodes only need to know the codes of the neighboring nodes, which belong to an active route. The method for spreading code distribution is already under development. (a) (b) Figure 4. An example of node A searching a route to node F. RREQ is broadcast through the network with C-code (3.a) and RREP is unicast along found reverse route upstream to source A with R-codes used between hops (3.b). SIMULATIONS F. Simulation scenario Simulations are carried out in order to explore the performance of the proposed BCCA method. The performance is studied as a function of offered data traffic, which is normalized to the physical layer data rate. Several performance metrics are used (see more details from [8] and [9]). We have selected a small network of 0 active nodes in order to focus the attention to lower layers. Of course, the whole network performance is in the main interest, but to reveal the performance of BCCA, a scenario where the performance is not limited by other issues (routing protocol) is preferred. A simplified radio interface, with nominal radio range of 250 m is used and the antennas are assumed to be omni-directional ( bidirectional links). To guarantee satisfactory connectivity, the area is chosen to be 400 m 400 m. From to 3 hops occur in this kind of scenario with an average path length of about.4 hops. At the application layer, we use variable bit rate traffic source with Poisson arrivals (VBR-M), in which packet interarrival times are exponentially distributed. Packet length is set to a constant value of 52 bits. Exponential distribution is of form x f ( x) = λe λ, () where /λ is the expectation value of the distribution. Modern data traffic is found to be highly variable and therefore heavytailed distributions (e.g., Pareto distribution) are used in modeling [8]. Heavy tailed distributions are not used here, because they also introduce their own effects to the performance, which is not in the focus of this paper. Poisson-process still is a good reference point and it models for example voice traffic accurately and it also fits quite well to some data application modeling [7]. In simulations, each offered traffic value point is averaged over 90 iterations with on the average of packets per iteration. A session model of dynamic connections is used here. In this model, source node chooses randomly the destination node to communicate with, and the connections are changed during iteration. Session duration is exponentially distributed with

4 expectation value of 00 packets. Ad hoc network must also support mobility, so mobile scenarios are also included to the study. The mobility is based on the random waypoint model used in several other studies [9], [20]. The speed of the network nodes is set randomly from 0.2 to.5 m/s in order to model pedestrian movement. When mobility is enabled, it is continuous with no pause time. G. Simulation results In all figures, cases with pure 802. and 802. with BCCA enhancement, both with, no mobility and continuous mobility, are presented. Figure 5 depicts the average data packet delay as a function of offered data traffic. Typical network behavior can be clearly seen, i.e., with increasing traffic, the delay remains low from low to moderate traffic load, but after a certain point the delay starts to significantly increase. At this point the network starts to collapse due to congestion. Extra delay is caused by numerous retransmissions, buffer fulfillments, route maintenance and route re-discoveries. As it is seen from the figure, the performance enhancement of BCCA is obvious. For example, at data traffic load of 0., the delay of BCCA in no mobility case is only about 5 ms, while in the case of pure 802. it is over twofold (about 2 ms). Avg. data packet delay [s] 0, 0,0 0 node placement in used scenario is about 97.7 %. Due to slow speed of the nodes, the mobility only reduces the connectivity problems, for the possible bad connections are not permanent. Also, the path lengths are reduced, because the random waypoint mobility model is known to exhibit properties where the node density is larger at the center of the operational area than at the boundary areas [2]. Of course, if the speed of the nodes would be much higher, the routing protocol would not be able to adapt to rapid topological changes and the performance would be worse (see e.g., [8]). Data throughput S 0,4 0,35 0,3 0,25 0,2 0,5 0, 0,05 0 Figure 6. Data throughput. Packet loss ratio Ideal random channel access,0e+00 0,0 0,,0E-0,0E-02 0,00 Figure 5. Average data packet delay. From the delay figure, the effect of the routing protocol can be also seen. The performance deteriorates (delay increases) at very low traffic load. The main reason for this are route breakages due to too small route timeout parameter of AODV. We have already increased it up to the value of 20 s, but it is still insufficient for such a small traffic loads of < Further increment of the timeout parameter is not desirable, because it would cause problems in heavy traffic and high mobility situations. The optimal performance is got with moderate traffic load, because the used, mainly default, AODV parameters are optimized to traffic loads of about 0.0 to 0.. In Figure 6, normalized network throughput is presented. Pure 802. gets a maximum normalized throughput of about 0.26, while BCCA achieves about 5 % better (0.30) performance in the case of no mobility. With continuous mobility, the ratio of the performance difference is about the same. It is interesting to note that the performance is better, when the network scenario mobile rather than static. This can be explained by the problem of connectivity, which still exists even though the network is quite small. With external simulator, we discovered that the probability for full connectivity with random,0e-03,0e-04 Figure 7. Packet loss ratio. The packet loss ratio is shown in Figure 7. The performance differences are clearly seen with increasing traffic. For example, if % packet loss is assumed to be tolerable for some application, pure 802. can handle traffic loads of about 0.06, while BCCA can handle over 50 % more in no mobility case. With low traffic loads, the benefit of BCCA is negligible, because the packet loss is caused by connectivity problems rather than the used channel access method. Note that in the case of no mobility, packet loss ratio is saturated to %, while with continuous mobility, low packet loss ratios of order of 0.05 % are reached. In Figure 8, normalized total network control load is presented. We point out that the use of BCCA does not increase the need for control traffic. In fact, at high traffic load, the need for control traffic is higher in pure 802., because it starts to collapse earlier. Naturally, if the spreading codes would be unknown in the initial state, the spreading code distribution algorithm would increase the needed control traffic modestly in the beginning of the simulation iteration. With static nodes, the need for control traffic is slightly higher with all traffic loads

5 than with continuous mobility. This is due to performance enhancing feature of decent mobility. The increased control overhead at low traffic load, caused by AODV route timeout, can be also seen. The relative amount of control information is high because of the used small data packet. Normalized total network control load [bit/bit] 2,2 2,8,6,4,2 0,8 Figure 8. Norm. total network control load calc. in bits. In Figure 9, average number of collisions in correctly received packets is illustrated. It is noticed that BCCA is highly tolerant against collisions as compared to pure This highlights the potential benefits of using CDMA, since multiple simultaneous successful transmissions can occur. Average num of collisions in packet. 0, 0,0 0,00 Figure 9. Average number of collisions in packet (MAC layer). SUMMARY A new method for channel access in ad hoc networks was presented. Proposed BCCA uses two-channel approach to enable simple and robust receiver based CDMA networking capability without forgetting the need for a common access channel in ad hoc environment. A simulation test environment was set with AODV routing protocol and IEEE BCCA was realized with simple modifications to IEEE It was shown that BCCA outperformed pure 802. especially in heavy traffic situation where the problems are the greatest and still the use of BCCA does not increase the need for control traffic. It has to be noticed that no optimization what so ever was made to BCCAenabled 802., which, as a platform designed for the operation on a common contention channel, is definitely not the best for the purpose of BCCA. With optimization, the performance difference is likely to be much greater. As a by-product, the study also suggested that there still exists optimization work to do also in AODV parameters with variable traffic load. This was noticed especially at low traffic load where proactive routing protocols would probably have worked better. Also, the connectivity problems of ad hoc networking were observed. The idea of BCCA can basically be used in many kinds of networks, but it is designed especially for the needs of ad hoc networks. BCCA functionality in ad hoc networking is an example of the benefits of cross-layer design in enhancing the performance of wireless systems. REFERENCES [] E.M. Royer and C.-K. Toh, A Review of Current Routing Protocols for Ad-Hoc Mobile Wireless Networks, IEEE Personal Communications, vol. 6, no. 2, Apr.999. [2] R. Rom and M. Sidi, Multiple Access Protocols, Performance and Analysis, Springer-Verlag, New York, 990, pp [3] A. Muqattah and M. Krunz, CDMA-Based MAC Protocol for Wireless Ad Hoc Networks, MobiHoc 03, June [4] S. Lal, E. S. Sousa, Distributed Resource Allocation for DS-CDMA - Based Multimedia ad hoc Wireless LAN s, IEEE J-SAC, Vol. 7, No. 5, May, 999, pp [5] L. 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