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

Unit 4
MAC Protocols for Cognitive Radio
Networks
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Unit 4: MAC for CRN
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Hsi-Lu Chao, Sau-Hsuan Wu
Topics of cognitive radio:
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Classical spectrum sensing
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Cooperative spectrum sensing
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False alarm ratio and detection ratio
Fusion rules and threshold setting
Indoor positioning
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Measurement of radio power strength
False alarm ratio and detection ratio of energy detection
Triangulation positioning
Learning-Based positioning
MAC protocols for cognitive radio networks
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CR resource scheduling
CR routing
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CR MAC protocols
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Spectrum access
 Infrastructure-based CR networks (CRN)
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Ad hoc CR networks
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Random access protocols
Time-Slotted protocols
Hybrid protocols
Random access protocols
Time-Slotted protocols
Hybrid protocols
Sensing coordination
CR channel scheduling
CR routing
Cross-Layer design
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Random access protocols (contention-based)
 No
need for network time synchronization
 Carrier sense multiple access with collision avoidance
(CSMA/CA)
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Time-Slotted protocols (coordination-based)
 Need
network-wide time synchronizations
 Time is divided into slots for both the control channel and
data transmission
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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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C. Cormio and K. R. Chowdhury “A survey on MAC protocols for cognitive radio networks,” Ad Hoc Networks 7 (2009)
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MAC protocols for infrastructure-based CRN
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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-hop
connection to their
base-stations (BSs)
spectrum sensing
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Hsi-Lu Chao, Sau-Hsuan Wu
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
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Hsi-Lu Chao, Sau-Hsuan Wu
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 selfcoexistence interval using a contention-based scheme
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 Spectrum
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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
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Hsi-Lu Chao, Sau-Hsuan Wu
recovery
Backup channels are used to restore communications in case a
channel needs to be vacated after PU appearance
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A DSA-driven protocol [28]
 The
data transfer occurs in pre-determined time slots
 Control signaling uses random access scheme
 A cluster-based MAC
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Dynamic spectrum access (DSA) algorithm
Clustering algorithm: SUs are grouped in clusters
Negotiation
mechanism for SUs
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Issues for the realization of CRN
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Control information exchange in CRN
 Common
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Pros: Network synchronization and broadcasting
Cons: Unlikely to have a global common control channel
 Split
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control channel
phase
Pros: No need for common control channel (CCC)s
Cons: Dividing time frames into control and data phases
 Frequency
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hopping sequency
Pros: Transmission are more reliable
Cons: Require a tight synchronization
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Spectrum sensing optimization
 Band
occupancy prediction
 Band occupancy scheduling
 Sensing scheduling in wideband scenario
 Joint sensing and resource optimization
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Power control and rate optimization
Coexistence of multiple CRNs
Cartography-Enabled route optimization
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MAC protocols for Ad hoc CRN
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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
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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]
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Could have a dedicated common control channel, or use a
single channel only
Hardware constraints are divided into two classes
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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
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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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The MAC protocol is constituted by three operation phases of
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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 SRTC 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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A time-slotted cognitive MAC (C-MAC) [4]
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A rendezvous channel (RC)
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A backup channel (BC)
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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
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Node coordination, PU detection
Multiple channel resource reservation
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
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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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Distributed beaconing
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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
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CR users periodically tune to the RC and transmit their beacons
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Resynchronization
Update neighborhood topology
Beacon information contains
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New data spectrum requests
Announce spectrum changes by the CR users
Coexistence:
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Non-overlapping quiet period (QP) for each spectrum bands
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References
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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
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Overlay CCC
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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
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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
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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
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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
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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
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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
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Neighboring coordination
Clustering
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Dedicated control channel
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Control channel is predetermined in licensed or unlicensed
bands.
An attractive solution due to
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Usually unaffected by PU activity and considered always
“available”.
Available network-wide with global coverage
Would incur both saturation and security problems.
Possible allocation
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Guard bands
Unlicensed bands (access coordination and interference
avoidance)
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Ultra wideband CCC
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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
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Control channel saturation
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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
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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
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Robustness means “maintaining control communications
when PUs appear in the allocated CCC”.
Solutions
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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
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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
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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.
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Not easy to deal with compromised users.
Two solutions
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Dynamic CCC allocation
CCC key distribution
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Sequence-based rendezvous
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Blind random rendezvous
Aim at minimize the maximum/expected time-torendezvous.
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
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Selected permutation: (3, 2, 5, 1, 4)
Generated sequence
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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
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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
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Blind random rendezvous
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Prioritize channels with same sequence family
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Efficient recovery control channel (ERCC) design in
cognitive radio ad hoc networks
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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. 45134526.
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Neighboring discovery
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For each CR user:
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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
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From obtained PCL, calculate each channel’s selection
probability.
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Therefore, channels with higher preference in the CCL appear
more often in the channel hopping sequence.
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Handshaking procedure
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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
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CCL update with local sensing information
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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
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New CCC allocation from the CCL
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Neighboring list update for lost neighbors
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Choose the best channel in CCL.
Through exchanged CCLs to update neighbors.
Control radio adaptation
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Update the “must tune” channel list (i.e., all selected CCCs to all
neighbors).
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HC-MAC: A HardwareConstrained Cognitive MAC for
Efficient Spectrum Management
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Two hardware constraints of a cognitive radio
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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.
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Decision A: achievable data rate is BT/(T+2t).
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Decision B: achievable data rate is 2BT/(T+3t).
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Decision C: achievable data rate is BT/(T+3t).
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Optimal stopping of spectrum sensing
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Two defined objects in stopping rule
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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)
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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
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Then
Hsi-Lu Chao, Sau-Hsuan Wu
where p and q are the probabilities of Xk=1 and Xk=0.
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Protocol overview of HC-MAC
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Time frame is separated to 3 parts: contention, sensing,
transmission.
Three types of RTS/CTS frames
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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
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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
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No need for the global synchronization.
Any node entering the network first listens to control
channel for a time interval td=tpK+T.
Max sensing time
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Max transmission time
Any node receives C-RTS/C-CTS defer and wait for the TRTS/T-CTS.
Adopt cw to alleviate collision.
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Protocol design of sensing phase
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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.
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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
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Utilize a set of available channels to transmit packets.
Maximum transmission time is T.
After finishing the transmission, the pair exchanges TRTS/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
58
Unit 4: MAC for CRN

Hsi-Lu Chao, Sau-Hsuan Wu
State transition diagram of HC-MAC
59
Unit 4: MAC for CRN

Sensing exposed terminal problem


Hsi-Lu Chao, Sau-Hsuan Wu
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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