Unit 4 MAC Protocols for Cognitive Radio Networks

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Unit 4 MAC Protocols for Cognitive Radio

Networks

Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu

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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


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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


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 4


C. Cormio and K. R. Chowdhury “A survey on MAC protocols for cognitive radio networks,” Ad Hoc Networks 7 (2009)



MAC protocols for infrastructure-based CRN A Wi-Fi like CSMA/CA

protocol [16] Channel access with

RTS-CTS handshake SU has a longer carrier

sensing time s Coexistence among the

PUs and CR SUs Both CR SUs and PUs

establish single-hopconnection to theirbase-stations (BSs)

Institute of Communications Engineering, ECE, NCTU

spectrum sensing

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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

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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



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



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



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





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




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

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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



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



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





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 bandsInstitute of Communications Engineering,

ECE, NCTU 19


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



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Classification of common control channel (CCC) design


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.

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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).

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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.

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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

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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)

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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.

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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.

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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

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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.

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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.

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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

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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.

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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:

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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.

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Expected time to rendezvous Blind random rendezvous

Prioritize channels with same sequence family

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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.

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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.

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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.

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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.

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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

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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).

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

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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.

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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).

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

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

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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.

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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

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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.

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Denote

Then

where p and q are the probabilities of Xk=1 and Xk=0.

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Protocol overview of HC-MAC Time frame is separated to 3 parts: contention, sensing,

transmission. Three types of RTS/CTS frames

C-RTS/C-CTS: contention and spectrum reservation in contention part.

S-RTS/S-CTS: exchange channel availability information between sender and receiver in each sensing slot.

T-RTS/T-CTS: notify the neighboring nodes the completion of the transmission.

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Protocol operations If one node wants to transmit, it first sends a C-RTS on ch0

after random backoff. The intended receiver replies a C-CTS message on ch0. Any other CR users hearing either the C-RTS or C-CTS

message will defer their operation and wait for the notification message on ch0.

After reserving the sensing period, the transmission pair conducts sensing in each channel and exchange S-RTS and S-CTS on ch0 if the channel is available for both sides.

When a stop agreement is made between the pair, data transmission is conducted in the selected channels.

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When the transmission finished, they will switch back to the control channel again and exchange T-RTS/T-CTS.

This T-RTS/T-CTS exchange ends other neighbors’ deferment and the neighboring node participates in another round of contention.

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Protocol design of contention phase No need for the global synchronization. Any node entering the network first listens to control

channel for a time interval td=tpK+T.

Any node receives C-RTS/C-CTS defer and wait for the T-RTS/T-CTS.

Adopt cw to alleviate collision.

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Max sensing time Max transmission time


Protocol design of sensing phase Sensing phase consists of one or multiple sensing slots, each

of which includes the actual spectrum sensing (ts) and negotiation (te) between the pair.

A sensing stop or continuing decision is made at the end of each spectrum sensing slot.

The decision is made by both side simultaneously and does not need any further negotiation, upon both sides have the same probability of channel availability. Solution: piggybacking the estimated probability in RTS/CTS

exchanges in contention and sensing stages. Set an estimation window EW. The final decision uses the average of these two (one is from the

sender, and the other is from the receiver).

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Protocol design of transmission phase Utilize a set of available channels to transmit packets. Maximum transmission time is T. After finishing the transmission, the pair exchanges T-

RTS/T-CTS to announce the completion of transmission. T-RTS/T-CTS ends the deferring of the neighboring nodes

and starts the next round of contention.

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A simple example

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State transition diagram of HC-MAC

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Sensing exposed terminal problem Interference as well as sensing inaccuracy from two hop

away nodes on a secondary pair who wins in the contention period.

Solutions Force all secondary nodes quiet during a certain time

interval. Feasible for infrastructure-based CRN (e.g., IEEE 802.22); not

available in ad hoc CRN. In HC-MAC, a transmission pair reserve multiple channels

for a certain period of time for its sensing and transmission. Inefficient spectrum utilization since the available channels

not used by this pair are not utilized by neighboring pairs.

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Modification: equip one more radio. This radio is dedicated for the control message exchanges. The sensing results and access decisions can then be exposed to neighbors in real time on the control channel.

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Documents

Unit 4 MAC Protocols for Cognitive Radio Networks