1. Avoiding Adjacent Channel Interference with Multi-Radio Mesh Points
November 2005
Avoiding Adjacent Channel Interference with Multi-Radio Mesh Points
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Avaya Labs

2. Avoiding Adjacent Channel Interference with Multi-Radio Mesh Points
Mathilde Benveniste, Avaya Labs Research

3. Multi-radio mesh points
November 2005
Multi-radio mesh points
A mesh point can use multiple radios to increase node throughput
Multi-radio MPs are needed near the portal of large meshes (e.g. enterprise, campus, hotspot, hot zone, municipal mesh …)
Traffic increases geometrically with the number of hops
Example:
Binary-tree mesh with root at portal connects 30 MAPs with traffic to/from the DS
Traffic through the portal is 30 times BSS traffic
If portal has single 11a radio, nodes are limited to 54/30 Mbps = 1.8 Mbps average rate per BSS
To support 11 Mbps per BSS, a 330 Mbps radio would be needed at the portal
Load =x
Load =3x
Load =7x
Load =15x
Load =30x
Portal
MAP
Avaya Labs

4. Multi-channel MPs not a substitute for multi-radio MPs
November 2005
Multi-channel MPs not a substitute for multi-radio MPs
A multi-channel MP may choose a traffic channel(s) from among several channels
A multi-channel MP with a single traffic radio has maximum data rate of a single radio (e.g. 11a radio rate  54 Mbps)
To support 11Mbps per BSS in previous example, a 330 Mbps radio would be needed at the portal
Multi-channel MPs do not meet the need for multi-radio MPs
Avaya Labs

5. Routing is not a substitute for multi-radio MPs
November 2005
Routing is not a substitute for multi-radio MPs
Special routing can balance traffic across mesh to reduce node rate requirements by taking advantage of statistical variation of loads
The total load through the portal in the earlier example is still the sum of the loads offered at the mesh APs
Optimized routing does not eliminate the need for multi-radio MPs
Avaya Labs

6. Challenge with multi-radio mesh points
November 2005
Challenge with multi-radio mesh points
Avaya Labs

7. Adjacent Channel Interference
November 2005
Adjacent Channel Interference
Multi-radio MPs can suffer adjacent channel interference (ACI) while transmitting and receiving simultaneously on adjacent channels [4]
Adjacent channel interference is caused by energy on channels that are adjacent in the RF spectrum
ACI is serious when the receiver is physically close a transmitter on an adjacent channel
A transmitter and receiver in the same box [MP] represents the worst case
The number of available non-adjacent channels is small
The available unlicensed RF spectrum has several segments of adjacent channels: one in 2.4 GHz range and 3 (or 4) in the 5 GHz range
Channels from different segments would not cause one another ACI
Avoiding ACI by using only channels from different segments lowers efficiency of channel use
Low channel re-use – only a few channels can be used by multi-radio MPs in the mesh
Avaya Labs

8. 1 Summary description of CCC features
November 2005
Proposed Solution
Multi-radio MPs can avoid ACI by consistently putting mesh traffic in one direction as follows:
Use the common control channel approach
All mesh traffic associated with a given MP is either all ‘in’ or ‘out’ at any point in time (thus adjacent channels can be used for mesh traffic)
Acknowledgments are sent on a different, non-adjacent, channel (e.g. the control channel) so that they do not collide with mesh traffic received on adjacent channels
a group ACK on the control channel can be used for TXOPs
MRTS/MCTS (instead of RTS/CTS) are sent on the control channel, which must not cause ACI to traffic channels
the control channel can be selected in a different RF segment from that of the mesh traffic channel
Avaya Labs

9. Spectrum partitioning
November 2005
Spectrum partitioning
Select two RF spectrum regions, I and II, such that a pair of channels one from each region do not cause ACI to one another (channels within each segment may be adjacent to one another)
Example: Region I is the GHz band, and Region II comprises other 5 GHz bands
Segment contains control channel; multi-radio MPs shall not use channels from this segment
Consists of channels for mesh traffic transmissions by all MPs
Single-radio nodes + CC
Multi-radio nodes I II
RF spectrum
Avaya Labs

10. Conclusions Multi-radio MPs are needed in large wireless mesh networks
November 2005
Conclusions
Multi-radio MPs are needed in large wireless mesh networks
Adjacent channel interference, which would severely limit capacity (and ease of implementation), can be avoided with a simple MMAC change
The proposed MMAC is distributed
It enables use of most channels (adjacent channels included)
Multi-radio MPs can thus avoid adjacent channel interference without loss of mesh capacity or the need for new hardware or new PHY protocol
Avaya Labs

11. November 2005
References
[1] R. Draves, J. Padhye, and B. Zill, “Routing in multi-radio, multi-hop wireless mesh networks,” MobiCom’04.
[2] J. Robinson, K. Papagiannaki, C. Diot, N. Gao, and L. Krishnamurthy, “Experimenting with a multi-radio mesh networking testbed,” WiNMee 2005.
[3] J. H. Winters, “Smart antennas and their application to wireless ad hoc networks,” PIMRC’05.
[4] J. H. Winters, S. A. Mujtaba, and M. Benveniste, “Adjacent channel interference and its impact on the Mesh MAC”, IEEE r0, 2005
Avaya Labs