CH-CSMA/CA: A MAC Protocol for Asynchronous ...

CH-CSMA/CA MAC protocol for Cognitive Radio Networks

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CH-CSMA/CA: A MAC Protocol for Asynchronous Channel Hopping Rendezvous in 802.11 DCF based Cognitive Radio Networks Quan Liu*, Xiaodong Wang and Xingming Zhou College of Computer Science, National University of Defense Technology, Changsha, Hunan 410073, China E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] *Corresponding author Abstract: Blind rendezvous paradigm recently expedites the approach to the control channel establishment problem in cognitive radio networks. As a result, many novel asynchronous channel hopping (CH) rendezvous schemes are proposed for realizing blind rendezvous in these years. However, how to integrate these asynchronous CH rendezvous schemes with CSMA/CA MAC has not been researched so far. In this paper, we propose a CH rendezvous based CSMA/CA MAC (named CH-CSMA/CA) to work with existing asynchronous CH rendezvous schemes by tailoring mechanisms of 802.11 DCF to the slotted operation manner of these rendezvous schemes. Moreover, we identify that the channel busy-time inconsistency problem unique to CH-CSMA/CA MAC design is caused by virtual carrier sensing (VCS) mechanism of 802.11 DCF, and an enhanced virtual carrier sensing (EVCS) mechanism is proposed to alleviate its impact on networking performance (i.e., channel access delay and throughput). Extensive simulation results verify the effectiveness of our MAC design. Keywords: Cognitive Radio Networks; Blind Rendezvous; Asynchronous Channel-Hopping Rendezvous; Control Channel Establishment; 802.11 CSMA/CA MAC; Channel Busy-time Inconsistency Problem; Enhanced Virtual Carrier Sensing. Reference to this paper should be made as follows: Quan Liu, Xiaodong Wang and Xingming Zhou. (xxxx) ‘CH-CSMA/CA: A MAC protocol for Asynchronous Channel Hopping Rendezvous in 802.11 DCF based Cognitive Radio Networks’, International Journal of Autonomous and Adaptive Communications Systems, Vol. x, No. x, pp.xxx–xxx. Biographical notes: Quan Liu received his B. Eng. degree in 2007 from Central South University (CSU) and finished his master program in 2009 at National University of Defense Technology (NUDT), both in China. His research interests are in distributed coordination, MAC protocol design and cognitive networking in cognitive radio networks. He is a student member of ACM and China Computer Federation (CCF). Xiaodong Wang received the BE, ME, and Ph.D. degrees in computer science in 1994, 1998, and 2001, respectively, all from the National University of Defence Technology (NUDT), Changsha, China. His main research interests are wireless networks and mobile computing, including mobile ad hoc networks and wireless sensor networks. He is a member of the IEEE Communication Society, IEEE, and China Computer Federation (CCF). Xingming Zhou has been with National University of Defence Technology (NUDT) since 1978, where he is currently the head of scholar committee of National Key Laboratory on Parallel and Distributed Processing. He has been the academician of Chinese Academy of Science (CAS) since 1992. He has been the co-chair of many international conferences and workshops on parallel computing, network computing, and mobile networks. His current research interests include highperformance computing and network computing. He is a senior member of China Computer Federation (CCF).

Int. J. of Autonomous and Adaptive Communications Systems, Vol. x, No. x, 2013

1 Introduction Cognitive radio (CR) (Haykin, 2005) is regarded as the most promising technique to resolve spectrum scarcity problem by introducing the novel concept of dynamic spectrum access (DSA) (Zhao and Sadler, 2007). According to the rationale of CR technique, secondary users (SUs) explore and exploit the unoccupied licensed bands (i.e., spectrum opportunity), but have to vacate the used licensed bands immediately to avoid harmful interference with primary users (PUs) when they return. Just as there is no free lunch in the world, CR technique improves the efficiency of spectrum utilization at the cost of encountering the distinctive problem of dynamic channel availability in cognitive radio networks (CRNs), which makes enormous challenges in traffic coordination among SUs and keeps the establishment of control channel still an open problem in MAC design for CRNs (Liang et al., 2011). Like in conventional multi-channel networks, MAC design in CRNs also encounters a fundamental issue termed “rendezvous” which refers to how a sender can find its intended receiver among multiple channels. The most intuitive and well-accepted solution is preassigning a dedicated channel (a.k.a, dedicated common control channel, DCCC) for control negotiation. Due to its simplicity, most distributed MAC protocols (De Domenico et al., 2012) for CRNs employ DCCC to handle the rendezvous problem. However, in addition to control congestion and spectrum waste, DCCC scheme has some distinctive problems in CRNs, e.g., existence problem and availability problem (Lo, 2011). Without relying on central controller or DCCC, blind rendezvous paradigm (Theis et al., 2011) is preferred in practice for distributed CRNs. As a result, channel hopping sequence (CHS) based rendezvous schemes are studied as the most representative technique to realize blind rendezvous in these years. As it is difficult to reach global clock synchronization in distributed CRNs, asynchronous CH rendezvous schemes are prevailing in research on CH based rendezvous schemes. The key idea of asynchronous CH rendezvous schemes (e.g., GOS (DaSilva and Guerreiro, 2008), JS (Liu et al., 2012a), CRSEQ (Shin et al., 2010), DRSEQ (Yang et al., 2010), and ETCH_ASYN (Zhang et al., 2011), etc) is to design periodic CHSs with rotation closure property (Bian et al., 2009) which guarantees any two neighboring SUs achieving rendezvous within a finite time without any requirement of synchronization. Besides details of the CHS structure design and the evaluation of related rendezvous delay, researchers of these proposed asynchronous CH rendezvous schemes have just briefly mentioned that 802.11 DCF is used for channel access in each hop slot, but the MAC details and the related networking performance (i.e., channel access delay and throughput) are not presented. To the best of our knowledge, how to integrate asynchronous CH rendezvous schemes with CSMA/CA MAC has not obtained in-depth study so far, and we are the first to study this topic. In this paper, we detail the design of a CH based CSMA/CA MAC (named CH-CSMA/CA) to work with existing asynchronous CH rendezvous schemes and evaluate the related networking performance. CH-CSMA/CA MAC tailors 802.11 DCF to the slotted operation manner of

Copyright © 2013 Inderscience Enterprises Ltd.

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existing asynchronous CH rendezvous schemes and enhances the virtual carrier sensing mechanism against the channel busy-time inconsistency problem to improve networking performance. The effectiveness of our design is verified through extensive simulations. The remaining part of this paper is organized as follows. We summarize the related work in Section 2. In Section 3, we outline the designed CH-CSMA/CA MAC, in which the network model and considered networking performance are presented. The modified mechanisms of 802.11 DCF are detailed in Section 4, following which we extensively simulate the performance of our CH-CSMA/CA MAC by integrating the latest asynchronous CH rendezvous schemes with it in Section 5. The paper is concluded in Section 6.

2 Related Work Multi-channel MAC design and analysis is still a hot and attractive topic in MAC study for CRNs, especially for singleradio multi-channel (SRMC) CRNs. In SRMC CRNs, SUs are equipped with a single capability-constrained radio which only works on one channel at a time. Though this type of CRNs have the advantage of low requirement on physical device, they face serious challenges in traffic coordination among SUs for MAC design. Existing asynchronous CH rendezvous schemes are designed to solve the control channel establishment problem in SRMC CRNs for traffic coordination. Pawelczak et al. (2009) and Park et al. (2011) comprehensively analyze the performance of four types of multi-channel MAC in SRMC CRNs by employing a discrete markov chain model. However, the MAC protocol they consider is S-ALOHA MAC. Su and Zhang (2008b) analyze their random channel hopping based MAC protocol by using a discrete markov chain model, but the MAC protocol they consider is also S-ALOHA MAC. The S-ALOHA MAC can make their proposed analytical model tractable, but it has limits in practice, i.e., synchronization and low channel utilization. As IEEE 802.11 CSMA/CA MAC is now the most widespread MAC protocol in practical deployment, researchers have commenced the study on introducing 802.11 CSMA/CA MAC into CRNs recently. Hoang et al. (2009), Bae et al. (2010) and Bae et al. (2011) analyze the aggregate throughput of SUs by employing CSMA/CA MAC for channel access. More interesting, Khabazian et al. (2012) and Kumar et al. (2013) study an interesting problem about what mutual effect is between secondary networks and primary networks when they both use CSMA/CA MAC. However, the scenarios they considered are single-radio single-channel (SRSC) CRNs, in which rendezvous problem does not exist. Hasan and Murshed (2012) and Kim et al. (2013) propose their CSMA/CA MAC for SRMC CRNs, but they require the equipped radio for each SU to have wide-spectrum capability or possess channel bonding/aggregation techniques, which reduces the considered SRMC CRNs to SRSC CRNs. In the following sections, we will detail our CHCSMA/CA MAC designed for SRMC CRNs and present the necessary modifications of 802.11 DCF for slotted operation manner of existing asynchronous CH rendezvous schemes.

CH-CSMA/CA MAC protocol for Cognitive Radio Networks

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Figure 1 Overview of CH-CSMA/CA MAC for a rendezvous pair (take GOS scheme for example) clock offset

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3 Overview of CH-CSMA/CA MAC 3.1 Network Model We consider a distributed CRN which has no preassigned DCCC exclusively used for control negotiation among SUs. All SUs have the same available channel set with N channels which are labelled as 0, 1, · · · , N , and each SU is equipped with a single radio which only works on one channel at a time. SUs hop among all the channels in a slot-by-slot manner by following their respective CHSs which are generated by a specific asynchronous CH rendezvous scheme. When a sender and its intended receiver (i.e., rendezvous pair) hop on a common channel in the same hop slot, we say they achieve rendezvous (the grey blocks in Fig.1). The common channel is called the rendezvous channel and the hop slot when the rendezvous pair achieve rendezvous is termed rendezvous slot. Each hop slot with duration Ts is subdivided into three parts: channel switching period tcs , spectrum sensing period tss and contention-transmission period tct . Each SU detects the signals of PUs on the scheduled channel in tss and decides to access the channel by employing 802.11 DCF in tct when the channel is detected unoccupied by PUs. We assume PUs communicate according to a synchronous slot structure and the PU activity on each channel is characterized by a twostate markov chain model (Su and Zhang, 2008a), which is the same as those assumptions in existing work of asynchronous CH rendezvous schemes. To establish a communication link between a rendezvous pair, the sender has to detect its intended receiver for rendezvous attempt by sending probing packets in each hop slot. In 802.11 standard (Gast, 2006), there are two access schemes, i.e., basic access scheme and RTS/CTS access scheme. We employ RTS/CTS packet as the probing packet in the CH-CSMA/CA MAC to reduce overhead, and then RTS/CTS access scheme is mandatory in our MAC. For fairness problem discussed in (Hoang et al., 2009), each SU in our MAC transmits a fixed-size packet to its intended receiver when they achieve rendezvous and succeed in channel contention.

3.2 Performance Metric 3.2.1 Failure Probability of seizing transmission opportunity When a rendezvous pair achieve rendezvous on a unoccupied channel, they need to employ 802.11 DCF for channel

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contention to seize the transmission opportunity (i.e., the rendezvous channel). As other SUs may compete with them on the same channel, the rendezvous pair may fail in competing with other SUs in the whole duration of the rendezvous slot and miss the transmission opportunity. As a result, they have to take extra hop slots called inter-rendezvous interval (IRI) to achieve the next rendezvous (shown in Fig.1). IRI denotes the time interval between two successive rendezvous events for a rendezvous pair. The metric, failure probability of seizing transmission opportunity, is used to evaluate how efficiently the MAC protocol can help a rendezvous pair seize their transmission opportunity in a rendezvous slot.

3.2.2 Effective Expected Time to Rendezvous Existing asynchronous CH rendezvous schemes use the metric, expected time to rendezvous (ETTR), to evaluate the average delay of achieving rendezvous for a rendezvous pair. ETTR denotes the average time to rendezvous (TTR) from the instant when a sender initiates a transmission to the instant when it achieves rendezvous with its intended receiver for the first time (see in Fig.1). It is obvious that ETTR is unable to reflect the channel access delay for the rendezvous pair, as there are two causes for rendezvous failure (the grey blocks without circle in Fig.1): channel unavailability (with probability α) due to activities of PUs and failure in seizing transmission opportunity (with probability β) due to constant collisions in the whole rendezvous slot. Thus, we propose the metric, effective expected time to rendezvous (eETTR), to differ from ETTR and evaluate the average channel access delay for a rendezvous pair. 1 − 1)(1 + EIRI) (1 − α)(1 − β) (hop slot) (1)

eET T R = ET T R + ( + εrs

For the expression of eETTR in formula (1), ETTR denotes the average time to achieve the first rendezvous, EIRI means the average value of IRI, and εrs equals the average delay for a rendezvous pair to reach successful transmission in a rendezvous slot. As ETTR and EIRI are dominated completely by asynchronous CH rendezvous schemes and α is related to PUs activities, modifications of 802.11 DCF should not only meet the operation manner of asynchronous CH rendezvous schemes but also enable to improve networking performance by reducing β and εrs .

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Figure 2 Backoff counter control in CH-CSMA/CA MAC Tg

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4 CH-CSMA/CA MAC for Asynchronous CH Rendezvous Schemes

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Figure 3 The inconsistency problem of channel busy time due to false collision  

• To avoid the consequence of packet truncation and the resulting interference with PUs, the tagged SU freezes its backoff counter till the end of current hop slot (see Fig.2) if the remaining time of the hop slot is not more than tg = trts + 3SIF S + tcts + tdata + tack . tg equals the time required for a successful transmission. At the beginning of the next hop slot, the tagged SU resumes its backoff counter immediately if the channel it jumps into is sensed unoccupied. Thus, tg performs as a guard time to protect the integrity of each packet transmission.

4.2 Enhanced Virtual Carrier Sensing Most 802.11 packets have a duration field which is used to reserve channel by notifying neighbors the duration of channel busy state. In IEEE 802.11 standard, the timer named network allocation vector (NAV) is used to realize the virtual carrier sensing (VCS) mechanism, and it is assigned the value in the duration field of the received packet. When packets arrive at




   



 



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• In conventional backoff counter-down process, the tagged SU will transmit packets in despite of the channel state when its backoff counter reaches zero. In order to avoid interference with PUs, the backoff counter of the tagged SU has to be frozen for the whole duration of a hop slot when the channel is sensed occupied by PUs (see Fig.2).

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In this section, we will present the details of CHCSMA/CA MAC, which includes expanded backoff counter frozen mechanism in backoff counter control and enhanced virtual carrier sensing mechanism against channel busy-time inconsistency problem.

Backoff counter control is the key function of 802.11 DCF to support random access and collision avoidance. When a tagged SU (i.e., a SU selected arbitrarily) has packets to transmit and its scheduled channel in current hop slot is available and sensed idle for DIFS, the tagged SU selects a value randomly from [0, CWmin − 1] to set its backoff counter and enters the backoff counting-down process. CWmin denotes the minimum size of contention window. In conventional backoff counting-down process, backoff counter is frozen only when the channel becomes busy. While in CH-CSMA/CA for CRNs, the backoff counter frozen mechanism has to be expanded to protect PUs from harmful interference and packets from being truncated by slot boundary as follows.

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the MAC layer, MAC protocols estimate whether the channel is idle by checking whether NAV reaches zero first. Although VCS can help MAC protocols designed for conventional wireless networks to estimate channel busy state excellently, it causes time inconsistency between the actual duration of channel busy state and the value of NAV in the design of CH-CSMA/CA MAC, which is termed channel busy-time inconsistency problem. Consider a single-hop CRN example in Fig.3. When SU S wants to communicate with its intended receiver SU R, it first sends a RTS packet to probe SU R for rendezvous attempt. In the duration field of the RTS packet, the value trts vcs in formula (2) is used to reserve the channel for the following packet transmission. In CHCSMA/CA MAC, there are two causes for failure of this RTS transmission, i.e., true collision and false collision. In true collision case, the RTS packet sent from SU S is corrupted by RTS packets sent from other SUs in the same backoff mini-slot. As neighboring SUs (e.g., r1 and r2 ) on the same channel of SU S can not extract the value trts vcs in the corrupted RTS packet, the channel busy-time inconsistency problem doesn’t occur. In false collision case, SU S sends the RTS packet successfully but SU R fails to receive it for the reason that it does not achieve rendezvous with SU S. Since the neighboring SUs (e.g., r1 and r2 ) on the same channel of SU S can extract the value trts vcs in the received RTS packet, they will stay idle for trts vcs even though the channel is not busy actually (see in Fig.3). Thus, the channel busy-time inconsistency problem emerges. It is worth noting that this problem may happen in conventional

CH-CSMA/CA MAC protocol for Cognitive Radio Networks wireless networks when the channel is error-prone, but it can be relieved by transmitting control packets at basic PHY rate. trts vcs = 3SIF S + tcts + tdata + tack

(2)

The fundamental cause for the channel busy-time inconsistency problem is that senders set the value in the duration field of their RTS packets by acquiescing in having achieved rendezvous with their intended receivers. However, a rendezvous pair does not achieve rendezvous for most time in a period of their CHSs. The effect of the channel busy-time inconsistency problem will increase the failure probability β of seizing transmission opportunity for a rendezvous pair. Assume SU r1 and SU r2 in Fig.3 have achieved rendezvous over the same channel of SU S, and SU r1 enters the backoff counting-down process for communication with SU r2 . If the backoff counter of SU S reaches zero before SU r1 , SU S transmits a RTS packet prior to SU r1 to its receiver SU R which may not achieve rendezvous with SU S. After overhearing the RTS packet sent from SU S, SU r1 freezes its backoff counter and sets its NAV trts vcs . The consequence is that SU r1 has to suffer additional delay trts vcs during its backoff counting-down process. As SU r1 has high probability of encountering the channel busy-time inconsistency problem in each backoff mini-slot, the cumulative delay will be severe. Due to the limited duration of each hop slot, the extra delay injected into backoff counting-down process will reduce the number of rendezvous attempt between a rendezvous pair in a rendezvous slot. Intuitively, any rendezvous pair using conventional VCS mechanism suffer high failure probability of seizing transmission opportunity and large channel access delay. We propose an enhanced virtual carrier sensing (EVCS) mechanism to alleviate the impact of the channel busy-time inconsistency problem on channel access delay performance. The EVCS mechanism works like VCS mechanism in the scenario of data fragmentation burst where each data fragment just reserves the channel for the next one data fragment transmission. In the EVCS mechanism (see Fig.3), the value in the duration field of RTS packet trts evcs in formula (3) is equal to the transmission time of the next CTS packet, but the transmission time of data and ACK packets are not included. The duration field of replied CTS packet has value tcts evcs = tcts = 2SIF S + t + t for the next data and ACK data ack vcs transmission. Each data packet has duration value tdata evcs = SIF S + tack equal to the ACK transmission time. Once false collision occurs, the SU victims postpone to resume their backoff counters with additional delay trts evcs which is much less than trts by comparing formula (2) with formula (3). As vcs the extra delay injected into backoff counting-down progress is reduced, a rendezvous pair can do more times of rendezvous attempt in their rendezvous slots. Thus, the two parameters β and εrs in formula (1) which are dominated by 802.11 DCF can be decreased and the channel access delay eETTR can be improved. trts evcs = SIF S + tcts

(3)

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Table 1 Protocol parameters of PHY layer and MAC layer

PHY Layer PLCP preamble (pphy ) PLCP header (hphy ) Basic rate (rbasic ) Data rate (rdata ) Backoff mini-slot (σ)

144 bits 48 bits 1 Mbps 11 Mbps 20 µs

MAC Layer MAC header (hmac ) MAC payload (ldata ) SIFS DIFS RTS (lrts ) CTS (lcts ) ACK (lack ) CWmin (W0 ) Backoff stages (m) Spectrum Sensing Period (Tss ) Hop slot (Ts )

272 bits 8184 bits 10 µs 50 µs 160 bits 112 bits 112 bits 32 3 2 ms 12 ms

5 Simulation Evaluation We develop a C/C++ event-driven simulator to simulate our CH-CSMA/CA MAC by integrating it with the latest four asynchronous CH rendezvous schemes, i.e., CRSEQ, DRSEQ, JS and ETCH_ASYN. The performances, failure probability β of seizing transmission opportunity, channel access delay eETTR and network throughput, are evaluated in terms of channel number N , SU density η per channel and three different virtual carrier sensing mechanisms, i.e., VCS, EVCS and OVCS. OVCS denotes the optimal VCS in which SUs are assumed to be capable of identifying false collisions during their rendezvous processes and its resulting performance is regarded as the upper bound for EVCS and VCS. A singlehop CRN is constructed by setting a 100m×100m topology with randomly deployed SUs with 250m transmission range and all SUs are involved in saturated transmission. The probability α of channel unavailability is set to 30% by adjusting the two-state markov chain model of PU activity. As existing asynchronous CH rendezvous schemes can guarantee fair channel access to each channel, we use SU density η per channel instead of the total number of SUs to indicate collision level. Channel switch period tcs is neglected due to its insignificance with respect to the duration of hop slot. The average value for each simulation result is derived from 20 repeated experiments and each experiment lasts for 300 seconds. To indicate reliability of simulation results, the confidence intervals with 95% confidence level are given for each simulation result. The protocol parameters of PHY layer and MAC layer are listed in Table 1. By using the symbols parenthesized after parameters in Table 1, the transmission p +hphy +ldata + hmac and time of data packet is tdata = phy rbasic rdata the transmission time of control packets are trts,cts,ack = pphy +hphy +lrts,cts,ack . rbasic

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Figure 4 Failure probability of seizing transmission opportunity in a rendezvous slot (α = 0.3)

Figure 5 Channel Access Delay (α = 0.3) CH-CSMA/CA (CRSEQ)

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5.1 Failure Probability of Seizing Transmission Opportunity The failure probability of seizing transmission opportunity measures the efficiency of CH-CSMA/CA MAC in capturing the transmission opportunity offered by asynchronous CH rendezvous schemes for a rendezvous pair. As shown in Fig.4, failure probability β of seizing transmission opportunity raises with increase of SU density η per channel, as collision becomes severe. By employing EVCS, the performance gap between VCS and OVCS is narrowed about 67%, 61% and 59.5% on average for these four asynchronous CH rendezvous schemes when η = 10, 30, 50. Due to limited duration of each hop slot, failure probability of seizing transmission opportunity is very high when collision increases, which can be improved by extending the duration of hop slot. However, from expression of eETTR in formula (1), the parameters ETTR and EIRI will increase when the duration of hop slot is extended and the resulting channel access delay eETTR will also increase. The trade-off between failure probability of seizing transmission opportunity and channel access delay is an interesting problem which needs further study.

5.2 Channel Access Delay As we have known that the failure probability of seizing transmission opportunity increases with collision intensification, the channel access delay is expected to increase according to the expression in formula (1), which is verified by the simulation results when we change SU density η per channel from 10 to 50 by step 20 (shown in Fig.5). Compared with VCS mechanism, EVCS mechanism can effectively suppress the increase trend of channel access delay when

collision becomes severe and keep the channel access delay approximating to the optimal results under OVCS mechanism, which indicates the extra delay injected into backoff countingdown process under EVCS mechanism has limited impact on channel access delay for rendezvous pairs. In addition, we can see that the delay under VCS mechanism when η = 10 almost equals to that under EVCS mechanism when η = 30, which proves the effectiveness of EVCS mechanism on improvement of channel access delay. As JS and ETCH_ASYN are proposed to guarantee multiple times of rendezvous in a period of their designed CHSs, they can largely decrease ETTR and EIRI. As a result, their performances of channel access delay for rendezvous pairs outperform those of CRSEQ and DRSEQ.

5.3 Network Throughput As presented in Fig.6, almost all the network throughput of CH-CSMA/CA MAC integrating with these four asynchronous CH rendezvous schemes raises first and then levels off when enlarging system capacity by increasing channel number and SU density per channel. The reason is that when system capacity is enlarged, the network can accommodate more concurrent communication links, but the establishment delay of each communication link also increases as ETTR and EIRI both increase with increase of channel number. When the system capacity is enlarged to a critical point, the improvement of network throughput is exhausted by the resulting channel access delay. Note that the improvement of network throughput under EVCS mechanism when enlarging system capacity is larger than that under VCS mechanism, which means the CRNs under EVCS mechanism is more scalable than those under VCS mechanism. Compared with VCS mechanism, the network throughput is improved by 69.6%, 84.8% and 85.4% on average under EVCS mechanism

CH-CSMA/CA MAC protocol for Cognitive Radio Networks

CH-CSMA/CA MAC to further improve the performance of networking for asynchronous CH rendezvous based CRNs.

Figure 6 Network throughput (α = 0.3) CH-CSMA/CA (CRSEQ)

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The work in this paper was supported by the National Natural Science Foundation of China under grant No.61070203.

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for these four asynchronous CH rendezvous schemes when η = 10, 30, 50. More interestingly, when channel number is less than 6, the throughput of CRSEQ in the case η = 10 is superior to those in other two cases (i.e., η = 30, 50). The reason is that CRSEQ is designed under asymmetric model and it makes rendezvous channels appear unevenly in a period, which verifies the conclusion in (Liu et al., 2012a) that CRSEQ is inefficient under symmetric model.

6 Conclusion By employing existing asynchronous CH rendezvous schemes as the approach to the problem of control channel establishment in MAC design for SRMC CRNs, we propose CH-CSMA/CA MAC without requirement of high radio capability. In order to introduce 802.11 DCF to meet the slotted operation manner of existing asynchronous CH rendezvous schemes, we make some necessary modifications on it, such as expanded backoff frozen mechanism in backoff counter control for protecting packets from being truncated and PUs from interference. Moreover, we identify the problem of channel busy-time inconsistency which is unique to the CSMA/CA MAC designed for asynchronous CH rendezvous based CRNs, and the EVCS mechanism is proposed to alleviate the impact of channel busy-time inconsistency problem on networking performance. The effectiveness of EVCS mechanism of CH-CSMA/CA MAC for the networking performance improvement is verified through extensive simulation. In our future work, we plan to integrate our previous work (Liu et al., 2012b), which uses neighbor cooperation to reduce ETTR and EIRI efficiently, with the

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