1. Improving Network Throughput Through Spatial Diversity in Wireless Mesh Networks
Hyuk Lim, Chaegwon Lim, Jennifer C. Hou
Department of Computer Science
University of Illinois at Urbana Champaign

2. Control Knobs for Mitigating Interference
To mitigate interference and maximize the network capacity, there are several control knobs:
Transmit power  topology control
Carrier sense threshold  trade-off between spatial reuse and interference level
Scheduling concurrent transmissions for interference-free connections
Channel diversity  use of non-overlapping channels

3. Interference
Flows that are routed along different paths within the interference range compete for the channel bandwidth, resulting in inter-flow interference.
Consecutive packets in a single flow may be spread over the route and interfere with each other, resulting in intra-flow interference.
transmission range Y Z A B C D
interference range
Queue at A

4. Exploring Spatial Diversity Through Scheduling
What if we schedule packet transmissions as follows Y Z A B C D
Case 1
Case 2

5. Exploring Spatial Diversity Through Scheduling
Issues to be considered
How do we find sets of nodes that result in the least inter flow interference only with the use of location information
How do we schedule concurrent transmissions for packets that belong to interference-free connections
Interleave packet transmissions for interference-free connections.

6. Determining Interference-free Sets of Nodes
Option 1: Use geographic locations of next-hop nodes
Can be readily obtained by GPS
Misleading because the distance between two nodes may not be a good index of interference (e.g., the interference may not be significant if there is an obstacle between them).
Option 2: Use received signal strength
More representative in determining the level of interference
Can be readily obtained through the sensory functions implemented in most IEEE interface cards.

7. Determining Interference-free Sets of Nodes
We focus on transporting downstream traffic at gateway nodes
Gateway nodes are responsible for transporting a large amount of downstream traffic
Instrument nodes that can communicate with the GN directly or through a relay node to perform RSS measurements.
RSS measurements are performed within two hops of the GN. Can be extended to h hops from the GN.
Tradeoff between control overhead/complexity and accuracy in inferring interference.
Construct, based on RSS measurements, a virtual coordinate system in which the distance between two nodes represents the level of interference

8. RSS Measurement
Through exchange of hello packets, a GN n gathers RSS measurement
between itself and a node m that can directly communicate with it.
between a neighbor node of m’s and m.
Node n constructs S=[sij], where sij is (-RSS) measurement made in dBm, 1<= i, j <= p, and p is the number of node n’s one-hop neighbors

9. Virtual Coordinate System
The jth column of S represents node j’s coordinates in a p-dimension.
These coordinates are correlated with each other  it is difficult to identify components that play an important role.
PCA comes to rescue.
PCA transforms a data set that consists of a large number of correlated variables to a new set of uncorrelated principal components.

10. Principal Component Analysis

11. Singular Value Decomposition
Obtain the SVD of S
The columns of the pxp matrix U=[u1,…., up] are the principal components and the orthogonal basis of the new subspace.
By using the first q columns of U, Uq, we project the p-dimensional space into a q-dimensional space:

12. Determining Coordinates for Two-Hop Neighbors

13. Corner Case Two neighbor nodes of the GN are outside each
other’s transmission ranges

14. Use of TXOP to Coordinate Transmission
TXOP: Transmission opportunity defined in IEEE e

15. Coordinating Transmission Order
The GN looks up a candidate frame from the queue in the LLC.
T= the set of neighbor nodes to which frames were sent after the GN grasped the medium.
The GN looks up to N frames in the LLC in order to locate a frame f such that routing(f) and i do not interfere, for every i in T.
After the GN uses the medium, it sets a contention window that is larger than what is originally specified in IEEE DCF, in order to give more opportunities to its neighbor nodes.

16. LLC Layer Implementation for GN

17. MAC Layer Implementation for GN

18. Transmit_with_backoff(p,retx)