The MAC Design for Cognitive Radios

1
The MAC Design for Cognitive Radios

• Lizhi Charlie Zhong

2
Outline

• Design Requirements for the CR system
• Design of a Cognitive MAC
• High throughput MAC design

3
Current Situation

• Bandwidth is allocated at given time, space,
  frequencies for a given functionality.
• Allocated spectrum is not fully utilized for many
  reasons (historical, technical or political).
• A possible solution cognitive radio
• No/little functionality restriction large
  application space, integrated service etc.
• Hop between time, space and frequencies for
• Wide bandwidth high data rate, low power etc.
• Good spectrum long range, low cost/free, less
  interference etc.

4
CR Design Requirements
CR cognitive radio

• Primary (common to all CR applications)
• Minimum interferences to the primary users
• Secondary (application specific)
• High data rate e.g. Gigabit/s wireless network
• Or low cost e.g. 3/m flat rate calling plan
• Or low power e.g. battery life of months
• Or long range e.g. wide area 802.11

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The Design of a Gbps Wireless Network
Gigabit/s

• Cognitive operation to capture the available
  spectrum
• High-throughput operation to make the best out of
  the captured spectrum

The rest of this presentation will focus on this
application.
6
Outline

• Design Requirements for the CR system
• Design of a Cognitive MAC
• High throughput MAC design

7
Functional Breakdown
MAC
Channel assignment
Non time critical task
Interference management
Sensing
PHY
Time critical task
Time critical tasks need to be done at the lowest
level!
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Comparison between the CR and the Unlicensed Band
CR
802.11

• Only one channel
• Back off if the channel is busy
• Sense other signals of the same type/network.
• Interferences in the unlicensed band (e.g.
  cordless phone).
• Unlicensed users have equal priorities.

• A user can use multiple channels at the same
  time Channel not known in advance.
• May switch to other channels if a channel is
  busy.
• Sense primary users.
• Interferences from other cognitive radios.
• A link can be interrupted at any time.

The MAC design is therefore different.
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The MAC Design Problem
Perform channel assignment based on information
from the PHY according to a set of policies
associated with the secondary system design
requirements.

• Policies
• Enough aggregated bandwidth
• Usage diversity (to improve the QoS)
• Assign the channels in the same group to a user

MAC

• Information from the PHY
• A set of virtual channels
• For each channel
• - If it is used by the primary users
• - If the CR operation on it is aborted due to
• the appearance of a primary user
• - Which group it belongs to
• - Which usage set it belongs to

PHY
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What the MAC needs from the PHY (1/3)

• The PHY defines what a virtual channel is
  frequency/
• sub-carrier, time slot, code or space/spatial
  channel.
• 2. It reports the activities of the PU in each
  channel.

PU primary user
used
available
PU
PU
aborted
CR
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What the MAC needs from the PHY (2/3)
3. The PHY sets its preferences by defining the
group e.g. Channels that can be easily combined
by the RF front end are in one group.
User bandwidth
Group A
Group B
a user will only use the channels in the same
Group
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What the MAC needs from the PHY (3/3)
4. The PHY defines the usage set. Usage set all
channels in the set will be gone at the same time.
Group A
Usage set 1
Usage set N
Usage set 2
Diversity across different usage sets
And code information across the channels (the
information will be intact even if some channels
are down).
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The Algorithm

• Case 1 Initial channel assignment
• Locate the group with enough available bandwidth
• Spread the channels across different usage sets.
• Case 2 CR operation is aborted in any channel
• Find the available spectrum in the current group.
• Spread the channels across the remaining usage
  sets.

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Discussions

• If not enough bandwidth, temporarily suffer from
  reduced data rate
• Improved spectral efficiency (reduced bandwidth
  requirement) reduces the chance this happens.
• High throughput MAC design
• How to deal with non-contiguous wideband?
• Super Wideband OFDM (80MHz, up to 1GHz)
• Use a control channel or scan a wide band
• Network architecture AP or not?
• Collisions with other CRs
• Proposal Standardization on the signature of a
  cognitive radio.

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The TV Band Example

• High data rate (but not Gbps) is still possible.
• Only one group the entire TV band.
• Usage Set is 6 MHz wide.
• The MAC algorithm remains the same.
• A control channel may not be required.

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Outline

• Design Requirements for the CR system
• Design of a Cognitive MAC
• High throughput MAC design

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The IEEE802.11n

• Should provide up to 300Mbps (or 100Mbps at the
  MAC interface) in the 20MHz bandwidth of 802.11a
• PHY MIMOLDPC .
• MAC throughput increase
• Many ideas proposed
• Only the ones with small changes will go
  through

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High Throughput MAC for CR

• Goal Gbps with only 100 MHz bandwidth

From 802.11n to CR

• All of the proposed ideas for throughput increase
  can be applied in the CR setting (surprise!).
• Design freedom does not even have to be 802.11
  like.

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Features of this MAC

• Reduced overhead
• High channel utilization
• Opportunistic in nature
• A nice mix of power control, multi-hop, ad-hoc
  operation and multi-channel access.
• Short range
• Multi-user access

20
References

• R.W. Brodersen, A. Wolisz, D. Cabric, S.M. Mishra
  and D. Willkomm, CORVUS A cognitive radio
  approach for usage of virtual unlicensed
  spectrum, white paper.
• J. Notor, Cognitive radio operation in the TV
  band.
• J. Sydor, 5 GHz cognitive radio an approach to
  rural community broadband access, CRC MILTON
  project white paper.
• D. Cabric, Research opportunities in cognitive
  radios, BWRC winter retreat, January 2004.
• A. Wolisz, Cognitive radio system
  architecture/MAC considerations, Presentation at
  ST, March 2004.

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Conclusions

• Cognitive radios make Gbps wireless network a
  possibility.
• The MAC is responsible for the adaptive channel
  assignment based on the activities of the primary
  users.
• It also needs to be optimized for throughput.