Time Synchronization of Cognitive Radio Networks

Transcription

1 Time Synchronization of Cognitive Radio Networks Jari Nieminen, Riku Jäntti Department of Communications and Networking Helsinki University of Technology, Finland {jari.nieminen, Lijun Qian Department of Electrical and Computer Engineering Prairie View A&M University, USA Abstract In this paper, a novel synchronization protocol is proposed especially for Cognitive Radio (CR) networks called CR-Sync. In a CR network, time synchronization is indispensable because of the requirements for coordinated and simultaneous quiet periods for spectrum sensing, as well as the common understanding of time frame/slot in many CR MAC designs. The proposed CR-Sync achieves network-wide time synchronization in a fully distributed manner, i.e., each node performs synchronization individually using CR-Sync. Contrary to many existing synchronization protocols that do not exploit CR attributes, the proposed protocol takes advantage of the potential multiple spectrum holes that are discovered by CR and distributes the synchronization of different pairs of nodes to distinct channels and thus reduces the synchronization time significantly. Detailed analysis of synchronization error and convergence time are provided. Results show that the proposed CR-Sync out-performs other protocols such as TPSN in CR networks. I. INTRODUCTION Cognitive Radio (CR) technology has gained a lot of attention recently due to its capability of dynamically capturing the unoccupied spectrum [1]. CRs are expected to be an important technology driver for wireless networks of the future. They will enable wireless devices to flexibly create many different kinds of communication links depending on required performance and spectrum/interference constraints [2]. In order to realize the benefits of CRs, spectrum awareness is a must. Typically this is done by cooperative sensing, i.e., all CR nodes in the network sharing the same quiet period and sensing the spectrum simultaneously. For example, in the presence of shadow fading, reliable detection is only possible by fusing sensing results from multiple sensors [3]. This implies that all the clocks in a CR network should be synchronized in time. Otherwise, time differences can cause nodes to sense the spectrum at different times. This may lead to incorrect sensing results and the presence of primary users may be neglected. Moreover, time synchronization enables the use of Time Division Multiple Access (TDMA) techniques that are generally considered to be more efficient than contention based Media Access Control (MAC) layer techniques. Many existing CR MAC schemes use time frame/slot structures and therefore require precise time synchronization for efficient operation, e.g. [4] and [5]. Some applications may also require strict time synchronization such as tracking. Hence, the importance of time synchronization in CR networks motivated us to propose a novel CR-Sync for this purpose. Synchronization in communication systems includes physical layer synchronization and network time synchronization. Physical layer synchronization is required for successful transmissions between two radios and it is considered for CR networks, for example, in [6]. However, physical layer synchronization offers only phase synchronization between two radios and therefore cannot provide time synchronization across the entire CR network. Network Time Protocol (NTP) [7] has been widely used in the Internet to provide time synchronization in accuracy of milliseconds. However, CR networks require more strict time synchronization for efficient operations. Furthermore, NTP is not designed for rapidly deployable distributed wireless networks. Global Positioning System (GPS) can provide time synchronization, but it is not cost-efficient and may suffer from availability problems (such as indoors). Time synchronization in wireless sensor networks has been widely studied and several protocols have been proposed such as Reference Broadcast Synchronization (RBS) [8] and Timesync Protocol for Sensor Networks (TPSN) [9]. In RBS, all the receivers time stamp a synchronization packet from the same transmitter individually and then exchange receive time stamps with neighbors. This scheme offers only relative time synchronization among neighboring receivers, not time synchronization with the transmitter. TPSN utilizes classical two-way synchronization between a master and a slave node. In the beginning, synchronization hierarchy is formed and after that masters and slaves exchange messages periodically to achieve time synchronization. In case of Mobile Ad Hoc Networks (MANET), time synchronization is not as critical as in CR networks since the demand comes from event time stamping. In [10], the time difference between the transmitter and the receiver is calculated so that time stamps from the transmitter can be mapped to correspond to receiver s clock. However, networkwide time synchronization is not provided. Furthermore, none of the above synchronization protocols support the use of multiple channels, thus the full capacity and advantages of CR networks are not exploited. In this paper, a novel synchronization protocol is proposed especially for Cognitive Radio (CR) networks called CR-Sync. The main contributions are: (1) Using the proposed CR-Sync, network-wide synchronization is achieved in a fully distributed manner; (2) The proposed protocol takes advantage of the potential multiple spectrum holes that are discovered by CR

2 and distributes the synchronization of different pairs of nodes to distinct channels and thus reduces the synchronization time significantly. Detailed analysis of synchronization error and convergence time are provided. Results show that the proposed CR-Sync out-performs other protocols such as TPSN in CR networks. The paper is organized as follows. The network model and assumptions are given in Section II. Section III introduces our proposed CR-Sync. Performance analysis is provided in Section IV. Section V contains the concluding remarks. II. NETWORK MODEL AND ASSUMPTIONS First of all, we assume that the CR network in question is a distributed network without any preplanned or centralized architecture. Secondly, each CR device has one (dedicated) receiver that listens to a Common Control Channel (CCC) in addition to one or more transceivers. The MAC layer protocol is expected to be able to successfully negotiate and select used channels in addition to assigning transmission schedules. In this work, we consider only low mobility nodes so that the propagation delays are approximately equal and the network topology remains the same during the synchronization process. Before starting the synchronization process, a root node is selected that will initialize the synchronization process and eventually all other nodes in the network should synchronize to the time reference provided by this node. Root node selection can be considered similarly to a cluster head selection problem. In MANETs cluster head selection can be based on power, location, credit etc [11]. However, in case of CR-Sync, it is desirable to have the root node located in the center location such that the convergence time could be minimized. The root node selection can be done by using [12] for example. The protocol is suitable for all MAC layer protocols that use periodic beaconing and thus it is feasible to utilize beacons to deliver synchronization information. The underlying MAC protocol can resolve beacon collisions by using Request to Send and Clear to Send (RTS/CTS) messages. We do not want several randomly chosen nodes to start the synchronization process simultaneously in order to prevent the creation of synchronization islands that have different time references, instead the root node will always start the synchronization process. It is also assumed that all nodes in the network have similar clocks so that there is no need to compare clock attributes of different nodes. III. SYNCHRONIZATION PROTOCOL FOR COGNITIVE RADIO NETWORKS CR-Sync is a master-slave protocol where all slave nodes synchronize to a predefined root node. The synchronization protocol has three phases, Hierarchy Discovery (HD), Synchronization Negotiation (SN) and Synchronization Execution (SE). First, HD phase is used to create a synchronization hierarchy and keep the hierarchy up to date in order to cope with topology changes and node mobility. SN and SE phases Figure 1. An example of the general CR-Sync operations always follow HD phase so that two-way synchronization can be carried out on a negotiated channel. An example of the general CR-Sync operations in a CR network is illustrated in Figure 1. τ is the synchronization slot length. T 1 means the end of the Beacon Interval (BI) and T 2 stands for the total time that it takes to finish the synchronization process. When the network is set up for the first time, the nodes are not synchronized and simultaneous Quiet Period (QP) is not possible. For this, before the actual operation of CR-Sync starts the nodes will carry out coarse synchronization process. The root node will send a beacon on the CCC that includes only a send time stamp since nodes do not have information about available channels. After this, all nodes that receive this beacon will use the send time stamp as time reference and create and send a similar message that includes a new send time and so forth. Now, coarsely synchronized nodes can sense the spectrum and CR-Sync will proceed to the periodic operations shown in Figure 1. In the beginning of HD phase, a selected root node will broadcast a Hierarchy Beacon (HB) message during the Beacon Interval (BI) that includes root node s ID, synchronization level 1 and a list of available channels in addition to a send time stamp. All the nodes that receive this beacon message will set their synchronization level to 2 and broadcast a similar HB message with a new send time stamp. At this point the nodes at level 2 will set the root node as their master and synchronize to it in a coarse manner by using the time stamps they received. This process goes on until every node in the network has found out its level in the hierarchy and broadcasted a HB message. Since HD phase can be included into CR MAC protocols that utilize periodic beaconing, overhead can be minimized and only the addition of node s synchronization level to a beacon is required. After creating a synchronization hierarchy, CR-Sync proceeds to Negotiation Interval (NI). In NI phase we have three different messages, Synchronization Negotiation Message (SNM), Synchronization Execution Message (SEM) and Data

3 Figure 3. Synchronization Interval (SI) Figure 2. Synchronization Execution (SE) Negotiation Message (DNM). SNM and DNM messages are similar and should be defined by the MAC layer protocol. NI phase starts with the root node announcing on the CCC that it is ready to start the synchronization process. After that all its slaves will contact it and negotiate a channel for synchronization. After agreeing on the used synchronization channel, both master and slave will tune to the synchronization channel and carry out the actual time synchronization as illustrated in Figure 2. Each node has to wait until they have synchronized themselves to an upper level node before synchronizing others in order to prevent distribution of false synchronization information in the network. After a slave has been synchronized it will announce on the CCC that it is ready to be a master for other nodes, if necessary, and all its slaves will contact it and negotiate synchronization channels. Later on in the case study we will show more precisely how this works. The exact operations depend on the number of transceivers per node and the available channels as well as network topology. Synchronization execution (SE) phase is initiaited by a slave and it consists of two messages, Synchronization Request (Sreq) and Synchronization Response (Sres). The slave first transmits a Sreq message to the master, including slave and master ID, and send time stamp (T1). Send time stamps are taken at the MAC layer in order to mitigate send and access delays. At the receiver side, the master should time stamp (T2) the incoming Sreq message as close to the physical layer (PHY) as possible, even before opening the packet, to diminish the impact of receive delay variation to synchronization accuracy. Synchronization errors will be discussed more precisely in Section IV-A. After receiving the packet, the master will create and transmit a Sres message which contains master ID, slave ID, T1, T2 and T3. Send time stamp (T3) is again taken at the MAC layer. The slave will time stamp (T4) the incoming Sres message. It is important to take both send and receive time stamps at the same place in both transceivers, respectively, in order to mitigate delay variations. After collecting all the time stamps, {T1,T2,T3,T4}, the slave node can be synchronized to the master and the propagation delay (Δ) and the clock offset (δ) can be calculated as follows (T 2 T 1) + (T 4 T 3) Δ= (1) 2 (T 2 T 1) (T 4 T 3) δ =. (2) 2 In this case since the response from the master comes instantly, it is safe to assume that clock drift will be constant during the synchronization procedure. The entire synchronization procedure is executed periodically so if a node moves or a new node wants to join, they will find their place in the synchronization hierarchy quickly. However, synchronization does not have to be performed every beacon interval and therefore, we introduce a design parameter M (a positive integer) that determines how often synchronization is carried out. In Figure 3 the idea of Synchronization Interval (SI) is depicted. In the figure M is set to four so synchronization is done every fourth beacon round. This way the synchronization overhead can be reduced and the operation of the protocol can be optimized in respect to the desired synchronization accuracy. Nodes will use the information recorded from the BI to determine whether they may have slaves or not. If a node did not hear any HBs after sending one, it can start negotiation for data transmission immediately after it has been synchronized since it will not have any slaves. However, if a node hears a HB after sending one, after it has been synchronized it has to announce on the CCC that it is ready to act as a master for other nodes. Furthermore, masters have to wait for one slot after synchronizing all their slaves before starting the data negotiation to ensure that synchronization negotiation is prioritized over data negotiation. Because the wireless channel may have deep fade and suffer severe packet losses, a node may not be synchronized during the synchronization process but it may still be able to negotiate for data transmissions after the channel recovers from deep fade. In this case, we suggest a back up synchronization execution, where two-way synchronization messaging is carried out before data transmissions. It is worth noting that CR-Sync is locally executed at each individual node, i.e., all the definitions including BI and NI, are local to each node because of the fact that a node only needs to consider all the nodes within its transmission range. For instance, times T 1 and T 2 are not fixed and may be different for different nodes. Hence, each node acts individually and the proposed protocol is fully distributed. Therefore, the NI phase will start propagating after the root node has sent out the HB

4 Figure 4. Network topology of the case study scenario Figure 5. Negotiation Interval (NI) (1 transceiver) and noticed that all of its neighbors have sent out their HBs as well. In other words, synchronization process moves on as a wave from the root node, which means that momentarily synchronization only impacts the nodes that are two-hop away. A. Case Study In order to clarify the detailed operation of CR-Sync, we present a case study according to the example scenario shown in Figure 4. We discuss two cases where each node has one transceiver in the first case and two transceivers in the second case. The focus will be on NI since it is the most important part of the protocol. The scenario consists of 9 nodes and the root node is node 1. In the figure available channels for each node are presented next to each node. It is assumed that the required time for SN is larger than the time for SE. First in the beginning of the HD phase the root node will send a HB in order to start the synchronization process. Nodes {2,3,4} will set their level as 2 and the root node will be their master. Now, all the nodes on level 2 send a HB and nodes {5,6,7,9} will find out that they are on level 3 and send a HB as well. Finally, node 8 will receive a HB from node 7 and the hierarchy is completed. However, node 8 still has to broadcast a HB since there may be additional nodes. After this, the protocol proceeds to the NI phase. At this point all the nodes have found out their levels in the synchronization hierarchy as described before. The root node starts NI phase by announcing that it is ready to proceed with synchronization. After receiving this announcement, all the nodes on level 2 will negotiate with the root node and schedule a channel to perform synchronization. Up to this point, the protocol operation has been the same regardless of the number of transceivers per node. However, in the next step, we will take advantage of multiple transceivers. It is noticed that all nodes should listen to the CCC during NI in order to prevent overlapping allocations. Let us first consider the one transceiver plus one receiver case presented in Figure 5. All nodes tune their receivers to the CCC and listen to that all the time. When a node negotiates transmissions it has to take into account the number of transceivers it has. Naturally, in case of one transceiver, a master can synchronize only one slave at a time. In our example, node 2 will first synchronize with the root node on channel 1. After this, node 2 will announce in the third slot that it is Figure 6. Negotiation Interval (NI) (2 transceivers) ready to synchronize its slave nodes and negotiate with node 5 on which channel to use, and the root node will synchronize another slave (node 3) on channel 2 simultaneously. Similarly, in the fourth slot, nodes 1 and 4 will synchronize on channel 3 and nodes 2 and 5 will synchronize on channel 1. At the same time node 3 will negotiate with nodes 6 and 7. So in the fifth slot, node 4 will announce that it is ready to serve as a master. Node 5 does not need to announce on the CCC since it did not hear any HBs after sending one. Same applies for nodes 6,8 and 9 as well. In the sixth slot, node 3 will also synchronize node 7 since it was unable to synchronize both, nodes 6 and 7, in the fifth slot. Finally, the last node to be synchronized is node 8. After the synchronization process, negotiations for data transmissions begin. Again, the ending of the synchronization for each node, T 2, could be different. In this case, T 2 for node 5 is four slots and for nodes 1,2 and 6 five slots. Negotiation interval for two transceiver case is presented in Figure 6. Now a master can synchronize two slaves simultaneously so the convergence time of the synchronization process becomes smaller and it is possible to utilize multiple frequency bands even better. For instance, the root node can synchronize nodes 2 and 3 at the same time in the second slot, and in the fourth slot, three pairs of nodes can synchronize simultaneously. In this case, nodes 2 and 3 are spatially separated so they can share the SN slot. Compared to the one transceiver case, the entire NI has reduced from eight slots to six slots.

5 IV. ANALYSIS In this section we analyze the performance of CR-Sync in respect to the synchronization error and convergence time of the synchronization process. We derive formulas for synchronization errors and show in general that the accuracy of two-way synchronization in the proposed CR-Sync is much better than that of simple one-way synchronization. We also show that the convergence time of the synchronization process with CR-Sync is much less than that with serial two-way synchronization, such as TPSN [9]. Since all the nodes in CR networks have to periodically change over to the CCC and negotiate for communications, many existing MAC layer protocols (such as [5]) use beacons to carry time synchronization information. As a result, in many cases the node that will send the first beacon is randomly selected [5], we call this type of approach random beacon synchronization. Beacon messages are time stamped at the transmitter before transmission and time stamps are embedded into beacons. Random beacon synchronization has serious drawbacks compared to CR-Sync. First of all, random beacon synchronization may create synchronization islands in the network because it is quite possible that not all the nodes are in transmission range of each other, such as in multi-hop networks. With CR-Sync this problem does not exist since the root node always initializes the synchronization process. Secondly, since beacon synchronization offers only one-way synchronization, it has significantly worse accuracy as shown next. A. Error Analysis Packet delays in communication systems consist of various deterministic and non-deterministic latencies, which are caused by different parts of the system and can be divided into categories as follows Send Time (T s ) Access Time (T a ) Transmission Time (T t ) Propagation Time (T p ) Reception Time (T rp ) Receive Time (T rc ) Send, access and transmission delays are induced by the transmitter. We include interrupt handling and encoding into send times. From these delays encoding and transmission delay can be mainly removed due to their deterministic natures. Encoding delay refers to the time it takes from a transmitter to encode and transform a packet to electromagnetic waves. Transmission time is the time it takes to actually send a message bit by bit and therefore, it can be estimated from the length of the packet and bit rate. However, send time still includes processing delay and interrupt handling. The impact of access delay can be mitigated with MAC layer time stamping, i.e. time stamps are taken at the MAC layer after the packet has gained transmission permission. Especially in contention based MAC schemes access delay can be a significant source of synchronization error in case of congestion. Signal s traveling time from the transmitter to the receiver is called propagation delay. At the receiver side, reception delay corresponds to transmission delay and can be calculated similarly. Receive time includes decoding, byte alignment and interrupt handling. Decoding delay is similar to encoding delay and means the time while the receiver transforms the signal back to binary data and decodes it. Byte alignment delay is usually considered as deterministic since it can be calculated from the bit offset and rate. In addition to these, there may be non-deterministic processing delay and interrupt handling at the receiver side as well. In N-hop communications with one-way beacon synchronization, if time stamping is carried out at the MAC layer, the synchronization error is E beacon = N T s,i + T p,i + T rc,i + δ i (3) i=1 where δ i corresponds to the error introduced by each oscillator on the path. On the other hand, in N-hop communications with our proposed two-way synchronization protocol (again time stamping is carried out at the MAC layer), the synchronization error would be E cr sync = N i=1 Δ Ts,i +Δ Tp,i +Δ Trc,i + δ i 2 where Δ p is the difference in propagation delays and Δ Ts and Δ Trc are differences in send and receive times, correspondingly. Note that by using MAC layer time stamping, access delays are eliminated and most of the send delays as well. When using one-way beacon synchronization, the error is cumulative and includes all the delays as shown in Equation 3. Thus the error grows strongly as a function of hop count and distance. On the contrary, when using the proposed two-way synchronization, the synchronization error includes only the differences in various delays. B. Convergence Time Analysis The overall time duration of the synchronization process in CR networks depends on various design parameters. First of all, the number of transceivers per node, denoted by Trx, determines how many slave nodes one master can synchronize simultaneously provided that the number of available channels is large enough. This leads to second critical parameter, the number of available channels (N). Furthermore, network topology has an impact as well since it determines the number of levels in the synchronization hierarchy (L). Another critical parameter is the maximum number of slave nodes that a master node may have (S), which is closely related to the node density in the network. In general if the number of available channels is large enough, i.e. N >> S, the convergence time of a typical synchronization process of CR-Sync can be approximated as (4)

6 Overhead (number of slots) TPSN (serial) CR Sync (1 Trx) CR Sync (2 Trx) Number of slave nodes (per level) Figure 7. Time duration as a function of number of nodes (Three synchronization levels (L=3)) Overhead (number of slots) TPSN (serial) CR Sync (1 Trx) CR Sync (2 Trx) Number of slave nodes (per level) Figure 8. Time duration as a function of number of nodes (Six synchronization levels (L=6)) follows O(S, L, T rx) = L + S (L 1) (5) Trx Now we compare the convergence time of CR-Sync to a serial two-way synchronization scheme, TPSN. In serial twoway synchronization, all nodes are synchronized on the CCC so CR attributes are not exploited. Synchronization accuracies are expected to be the same since a similar two-way message exchange is carried out in both schemes. The convergence time of the serial two-way synchronization can be approximated by O(S, L) =L + L S. (6) Hence the saving in convergence time by using the proposed CR-Sync equals to LS(1 1/T rx)+s/t rx. As we can see, in one transceiver case the gain compared to TPSN is directly the amount of slave nodes S. The convergence time (in slots) of CR-Sync and serial two-way synchronization as a function of slave nodes (S) is shown in Figures 7 and 8. In both cases the convergence time of CR-Sync is less than that with serial twoway synchronization, regardless of the number of transceivers. Furthermore, the gain achieved with CR-Sync depends on the number of layers L, number of transceivers per node Trx and average number of slave nodes S. V. CONCLUSIONS In this paper we presented a novel protocol for time synchronization in cognitive radio networks, named CR-Sync, and explained the operation of the protocol with a detailed case study. The unique features of the proposed CR-Sync include achieving network-wide synchronization using a fully distributed protocol and exploiting multiple channels to reduce convergence time. In addition to CR networks, the protocol could be utilized with any system that supports the use of multiple frequency bands. We also analyzed the performance of CR-Sync in terms of synchronization error and convergence time. We showed that CR-Sync outperforms other existing solutions in a CR network. In our future research, the optimization of CR-Sync operation will be performed in respect to various design parameters, different scenarios and required accuracies. Furthermore, how to select the root node and the impact of the selection will also be studied. ACKNOWLEDGEMENT This research work is supported in part by TEKES (Finnish Funding Agency for Technology and Innovation) as part of the Wireless Sensor Systems in Indoor Situation Modeling program and by the U.S. Army Research Office under Cooperative Agreement W911NF REFERENCES [1] I. Mitola, J. and J. Maguire, G.Q., Cognitive radio: making software radios more personal, IEEE Personal Communications, vol. 6, no. 4, pp , [2] GDD-06-14, Technical document on overview - wireless, mobile and sensor networks, Tech. Rep. [3] K. Ruttik, K. Koufos, and R. Jantti, Distributed power detection in shadowing environment and with communication constraint, Personal, Indoor and Mobile Radio Communications, PIMRC IEEE 18th International Symposium on, pp. 1 5, [4] Y. Kondareddy and P. Agrawal, Synchronized mac protocol for multihop cognitive radio networks, Communications, ICC 08. IEEE International Conference on, pp , [5] C. Cordeiro and K. Challapali, C-mac: A cognitive mac protocol for multi-channel wireless networks, Proceedings of IEEE DySpan, [6] W. Suwansantisuk, M. Chiani, and M. Z. Win, Frame synchronization for variable-length packets, IEEE Journal on Selected Areas in Communications, vol. 26, pp , [7] D. L. Mills, Internet time synchronization: The network time protocol, IEEE Transactions on Communications, vol. 39, pp , [8] J. Elson, L. Girod, and D. Estrin, Fine-grained network time synchronization using reference broadcasts, Proceedings of the Fifth Symposium on Operating Systems Design and Implementations, pp , [9] S. Ganeriwal, R. Kumar, and M. B. Srivastava, Timing-sync protocol for sensor networks, Sensys 03, pp , [10] K. Römer, Time synchronization in ad hoc networks, MobiHoc 01: Proceedings of the 2nd ACM international symposium on Mobile ad hoc networking & computing, pp , [11] O. Younis and S. Fahmy, Heed: a hybrid, energy-efficient, distributed clustering approach for ad hoc sensor networks, Mobile Computing, IEEE Transactions on, vol. 3, no. 4, pp , [12] S. Basagni, Distributed clustering for ad hoc networks, Parallel Architectures, Algorithms, and Networks, (I-SPAN 99) Proceedings. Fourth InternationalSymposium on, pp , 1999.