Unit 4 MAC Protocols for Cognitive Radio Networks

1. Unit 4 MAC Protocols for Cognitive Radio Networks

2. Topics of cognitive radio: • Classical spectrum sensing • Measurement of radio power strength • False alarm ratio and detection ratio of energy detection • Cooperative spectrum sensing • False alarm ratio and detection ratio • Fusion rules and threshold setting • Indoor positioning • Triangulation positioning • Learning-Based positioning • MAC protocols for cognitive radio networks • CR resource scheduling • CR routing

3. CR MAC protocols • Spectrum access • Infrastructure-based CR networks (CRN) • Random access protocols • Time-Slotted protocols • Hybrid protocols • Ad hoc CR networks • Random access protocols • Time-Slotted protocols • Hybrid protocols • Sensing coordination • CR channel scheduling • CR routing • Cross-Layer design

4. Random access protocols (contention-based) • No need for network time synchronization • Carrier sense multiple access with collision avoidance (CSMA/CA) • Time-Slotted protocols (coordination-based) • Need network-wide time synchronizations • Time is divided into slots for both the control channel and data transmission • Hybrid protocols (Dynamic spectrum access (DSA) driven) • Control signaling generally occurs over synchronized time slots • Data transmission may use random access schemes • RTS-CTS handshakes Institute of Communications Engineering, ECE, NCTU

5. C. Cormio and K. R. Chowdhury “A survey on MAC protocols for cognitive radio networks,” Ad Hoc Networks 7 (2009) Institute of Communications Engineering, ECE, NCTU

6. MAC protocols for infrastructure-based CRN A Wi-Fi like CSMA/CAprotocol [16] Channel access with RTS-CTS handshake SU has a longer carriersensing time s Coexistence among the PUs and CR SUs Both CR SUs and PUsestablish single-hopconnection to theirbase-stations (BSs) spectrum sensing Institute of Communications Engineering, ECE, NCTU

7. A time-slotted protocol (IEEE 802.22) • A TDMA channel access scheme • At the start of each superframe, there is a superframe control header (SCH) to inform of the current available channels • Extensive support for spectrum sensing • Spectrum recovery Institute of Communications Engineering, ECE, NCTU

8. The frame structure within each superframe The frame control header (FCH) contains the sizes the DS- and US- MAP fields The DS/US MAPs give the scheduling information The urgent coexistence situation (UCS) notification informs of the presence of incumbent licensees that are just detected Information exchanges among CR networks in the self-coexistence interval using a contention-based scheme Institute of Communications Engineering, ECE, NCTU

9. Spectrum sensing support Fast sensing: done at the rate of 1 ms/channel Fine sensing: performed on-demand with a much longer duration to increase QoS by decreasing the false alarm ratio Spectrum recovery Backup channels are used to restore communications in case a channel needs to be vacated after PU appearance Institute of Communications Engineering, ECE, NCTU

10. A DSA-driven protocol [28] The data transfer occurs in pre-determined time slots Control signaling uses random access scheme A cluster-based MAC Dynamic spectrum access (DSA) algorithm Clustering algorithm: SUs are grouped in clusters Negotiation mechanism for SUs Institute of Communications Engineering, ECE, NCTU

11. Issues for the realization of CRN Control information exchange in CRN Common control channel Pros: Network synchronization and broadcasting Cons: Unlikely to have a global common control channel Split phase Pros: No need for common control channel (CCC)s Cons: Dividing time frames into control and data phases Frequency hopping sequency Pros: Transmission are more reliable Cons: Require a tight synchronization Institute of Communications Engineering, ECE, NCTU

12. Institute of Communications Engineering, ECE, NCTU

13. Spectrum sensing optimization Band occupancy prediction Band occupancy scheduling Sensing scheduling in wideband scenario Joint sensing and resource optimization Power control and rate optimization Coexistence of multiple CRNs Cartography-Enabled route optimization Institute of Communications Engineering, ECE, NCTU

14. MAC protocols for Ad hoc CRN • A Wi-Fi like CSMA/CA protocol [20] • Distributed channel assignment • A dedicated out-of-band common control channel (CCC) • Each mobile host maintain two data structures • Current usage list: record the addresses, data channels as well as the expected time of use of its neighbors • Free channel list (FCL) • FCL is matched at both the sender and receiver ends using the RTS-CTS handshake • No specific support for spectrum sensing (may be O.K.) • May use the split-phase method to avoid using a dedicated CCC Institute of Communications Engineering, ECE, NCTU

15. Hardware constrained CSMA/CA MAC (HC-MAC) [11] Could have a dedicated common control channel, or use a single channel only Hardware constraints are divided into two classes Sensing constraints: consider the tradeoff between the sensing time and the accuracy Transmission constraints: related to bandwidth range and the maximum allowable number of channels To determine how many channels to be sensed, a stopping rule is determined for successive channel sensing Consider the tradeoff between the available bandwidth and the cost of sensing, in particular if the channel is found to be occupied or unavailable for use Choose a stopping rule to maximize the reward for channel searching Institute of Communications Engineering, ECE, NCTU

16. The MAC protocol is constituted by three operation phases of Contention: The C-RTS and C-CTS packets are sent over the CCC for gaining access to the channels The transmission pair that wins the contention exchange S-RTC and S-CTS packets for each channel that is sensed Sensing: A decision is made at the end of each sensing run on whether to initiate sensing on a new channel Transmission: After the channels are decided by the node pair, the data transmission takes place on the multiple granted channels The T-RTS and T-CTS packets are exchanged on the CCC to signal the end of this transfer and release the channels for other users Institute of Communications Engineering, ECE, NCTU

17. A time-slotted cognitive MAC (C-MAC) [4] A rendezvous channel (RC) Node coordination, PU detection Multiple channel resource reservation A backup channel (BC) Use to immediately provide a choice of alternative spectrum bands in case of the appearance of a PU Time is framed. Each frame consists of A beacon period (BP) (see the figure in the next page) Not simultaneously sent over all the specific bands A data transmission period (DTP) Upon power-on, each CR user scan all the available spectrums If it hears a beacon, then it may choose to join that specific band Set the global RC to the band specified in the beason Institute of Communications Engineering, ECE, NCTU

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19. Distributed beaconing Each BP is further time-slotted so that individual CR users issue there beacons without interference Re-broadcast the received beacon information to help inform its neighbors Inter-Channel coordination CR users periodically tune to the RC and transmit their beacons Resynchronization Update neighborhood topology Beacon information contains New data spectrum requests Announce spectrum changes by the CR users Coexistence: Non-overlapping quiet period (QP) for each spectrum bands Institute of Communications Engineering, ECE, NCTU

20. References C. Cormio, K. R. Chowdhury, “A survey on MAC protocols for cognitive radio networks,” Ad Hoc Network, vol. 7, 2009, pp. 1315-1329 A. D. Domenico, E. C. Strinati, and M.-G. D. Benedetto, “A survey on MAC strategies for cognitive radio networks,” IEEE Commu. Surveys & Tutorials, Vol. 14, No. 1, First Quarter, 2012, pp. 21-44 Institute of Communications Engineering, ECE, NCTU

21. Classification of common control channel (CCC) design

22. Overlay CCC • CCC is permanently or temporarily allocated to the CRN. • The CCC spectrum is currently not used by PUs. • May need to vacate the CCCs when PUs come back. • Underlay CCC • Same band used by PUs can be allocated to the CRN. • Control messages are transmitted in low power over a large bandwidth such that the control messages appear to PUs as noise (spread spectrum). • Looks like a dedicated CCC.

23. In-band CCC • The CCCs allocated to data channels. • Susceptible to PU activity, which varies from region to region. • CCC coverage is local. • High CCC establishment overhead. • Suitable to military or emergency networks. • Out-of-band CCC • The CCCs allocated in dedicated spectrum such as unlicensed bands or licensed spectrum. • Coverage is generally considered global, while local is possible (depends on the allocated band).

24. Sequenced-based CCC • Control channels are allocated according to a radon or predetermined channel hopping sequence. • Goal of this design is to diversify the control channel allocation over spectrum and time spaces in order to minimize the impact of PU activity. • Different CR users may use different hopping sequences, different neighboring pairs may communicate on different control channels. • A.k.a multiple rendezvous control channel (MRCC). • Key element is the construction of hopping sequences.

25. Group-based CCC • Grouping CR users in a neighborhood to use a common control channel. • Group formation before CCC selection v.s. CCC selection before group formation • Still may incur control channel starvation. • How to efficiently respond to PU activity is also a design issue. • Another challenge is the inter-group communication. • Two broad categories • Neighboring coordination • Clustering

26. Dedicated control channel • Control channel is predetermined in licensed or unlicensed bands. • An attractive solution due to • Usually unaffected by PU activity and considered always “available”. • Available network-wide with global coverage • Would incur both saturation and security problems. • Possible allocation • Guard bands • Unlicensed bands (access coordination and interference avoidance)

27. Ultra wideband CCC • Using spread spectrum technique. • Due to the limitation on UWB transmission power, the transmission range is limited. • Experimental studies show that UWB radios can achieve a range of 100 meters.

28. CCC design challenges • Control channel saturation • The CCC capacity cannot accommodate the control traffic from a large number of users. • More likely to occur on a dedicated CCC. • Still would happen to rendezvous control channel rendezvous convergence. • Rendezvous convergence indicates the rendezvous of a large number of neighboring users on the same channel by using sequenced-based CCC design.

29. Solutions • Limit the control traffic on the CCC. • E.g., sensing data quantization and dynamic sensing period (feasible). • Adjust the bandwidth ratio of the CCC over the data bands. • Not always feasible. • Allow slow migration of the CCC band on the traffic load. • Moving the CCC to a better channel in terms of channel quality and bandwidth efficiency (feasible). • Dynamic channelization

30. Robustness to PU activity • Robustness means “maintaining control communications when PUs appear in the allocated CCC”. • Solutions • Channel evacuation protocol • Broadcast warning messages, which is sent as a CDMA signal by using a predefined spreading code, when detecting PUs. • Sequence-based hopping CCC • Need time synchronization. • Difficulty for control message broadcast.

31. CCC coverage • Prefer large CCC coverage to do control message broadcast. • However, it’s not always possible and can be quite a challenge. • For sequence-based CCC design: CCC coverage is usually limited to a node pair. • For group-based CCC design: CCC coverage varies with the group size.

32. Control channel security • CCC is the easy target for the single point of failure. • Easy to disable any reception of valid control messages by injecting a strong interference signal to the CCC. • Traditionally spread spectrum techniques are utilized to mitigate the jamming attacks. • Not easy to deal with compromised users. • Two solutions • Dynamic CCC allocation • CCC key distribution

33. Sequence-based rendezvous • Blind random rendezvous • Aim at minimize the maximum/expected time-to-rendezvous. • Work well even when users are not synchronized to each other. • Each user selects a permutation of the N channels to construct its sequence. Luiz A. DaSilva, and Igor Guerreiro, “Sequence-based rendezvous for dynamic spectrum access,” IEEE DySPAN 2008, pp. 1-7.

34. The selected permutation appears (N+1) times: N times appear contiguously and one appears interspersed. • An illustrative example: 5 potential channels • Selected permutation: (3, 2, 5, 1, 4) • Generated sequence • 3, 3, 2, 5, 1, 4, 2, 3, 2, 5, 1, 4, 5, 3, 2, 5, 1, 4, 1, 3, 2, 5, 1, 4, 4, 3, 2, 5, 1, 4 • In matrix form:

35. Avoiding PUs • CR users sense the channels in the selected sequences. • Remove those channels, on which PUs are detected, from the sequences. • CR users visit channels in the order of the modified sequences. • Reset the PU discovery process at some point to account for PUs’ eventually vacating channels.

36. Expected time to rendezvous • Blind random rendezvous • Prioritize channels with same sequence family

37. Efficient recovery control channel (ERCC) design in cognitive radio ad hoc networks • Neighbor discovery • CCL (common channel list) updates • Efficient PU activity recovery Brandon F. Lo, Ian F. Akyildiz, and Abdullah M. Al-Dhelaan, “Efficient recovery control channel design in cognitive radio ad hoc networks,” IEEE Trans. On Vehicular Technology, Vol. 59, No. 9, Nov. 2010, pp. 4513-4526.

38. Neighboring discovery • For each CR user: • Perform local observations to obtain a list of available channels in decreasing order of channel quality (named preference channel list, PCL). • Initially CCL is PCL. • Construct a channel hopping sequence. • Perform channel hopping to discover neighbors through handshaking. • Update CCL through weight assignment (weight is the number of reachable neighbors). • Finally perform CCC assignment.

39. Channel hopping sequence • From obtained PCL, calculate each channel’s selection probability. • Therefore, channels with higher preference in the CCL appear more often in the channel hopping sequence.

40. Handshaking procedure • The SU (each SU performs this procedure independently) first broadcasts a beacon (carrying SU ID and CCL) with random backoff, and listens to the channel for any beacon broadcast. • If this SU receives a beacon from a neighbor, it replies an ACK. • Fix channel dwell time. • Update neighbor list as well as the associated control channel when needed. The associated control channel may be updated for meeting more neighbors or better channel quality. • Each SU individually determines the CCC of each link, based on its CCL and the neighbor’s CCL. No further control message exchange.

41. CCL update • CCL update with local sensing information • Local sensing  updated PCL  weight assignment if needed (for new available channel)  CCL update  beacon broadcast to inform its neighbors. • CCL update with neighbor’s information

42. Efficient PU activity recovery • New CCC allocation from the CCL • Choose the best channel in CCL. • Neighboring list update for lost neighbors • Through exchanged CCLs to update neighbors. • Control radio adaptation • Update the “must tune” channel list (i.e., all selected CCCs to all neighbors).

43. HC-MAC: A Hardware-Constrained Cognitive MAC for Efficient Spectrum Management

44. Two hardware constraints of a cognitive radio • Sensing constraint: a cognitive radio is capable to sense limited bandwidth of spectrum during a certain amount of time. • Transmission constraint: the spectrum which can be utilized by a single secondary node for its transmission is limited by hardware constraints.

45. The two limitations raise the problem of how to optimize the sensing decision for each sensing slot. • An simple example: each channel provides the same data rate B; the sensing time for a single channel is t and the maximum transmission time is T. • Decision A: achievable data rate is BT/(T+2t).

46. Decision B: achievable data rate is 2BT/(T+3t). • Decision C: achievable data rate is BT/(T+3t).

47. Optimal stopping of spectrum sensing • Two defined objects in stopping rule • A sequence of random variables X1, X2, …, XN, whose joint distribution is assumed to be known (channel sensing results). • A sequence of real-valued reward functions, y0, y1(x1), y2(x1, x2),.., yN(x1, x2, x3,... xN) (reward in terms of achievable data rate). • Let Xn denote the 0-1 (occupied-idle) state of the nth channel probed and the probability Pr(Xn=1)=p is assumed to be equal for every channel. • Let the maximum number of adjacent channels a single secondary user can simultaneously use be W. • Let the maximum number of spectrum fragments it can aggregate is F.

48. The number of fragments, for a band of spectrum with adjacent channels {i, i+1, …, j} is denoted as Frag{i, j}. • Let bn be the maximum number of usable channels within n adjacent channels (starting from 1), subject to the above constraints (W, F), namely

49. The function yn is (let c=T/t) • Assume each available channel presents a unit of data rate, then yn is actually the total effective data rate during the time interval T+nt after making the stopping and transmission decision. • Assume the maximum number of channels a user can probe before making a stopping decision is at most K.

50. Denote • Then where p and q are the probabilities of Xk=1 and Xk=0.