1 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
Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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
Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
C. Cormio and K. R. Chowdhury “A survey on MAC protocols for cognitive radio networks,” Ad Hoc Networks 7 (2009)
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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)
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spectrum sensing
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Institute of Communications Engineering, ECE, NCTU
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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,
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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Classification of common control channel (CCC) design
Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
Expected time to rendezvous Blind random rendezvous
Prioritize channels with same sequence family
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
Decision B: achievable data rate is 2BT/(T+3t).
Decision C: achievable data rate is BT/(T+3t).
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
Denote
Then
where p and q are the probabilities of Xk=1 and Xk=0.
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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
Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
A simple example
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
State transition diagram of HC-MAC
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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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Unit 4: MAC for CRN Hsi-Lu Chao, Sau-Hsuan Wu
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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