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 jhou@cs.uiuc.edu http://lion.cs.uiuc.edu

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

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

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

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.

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 802.11 interface cards.

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

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

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.

Principal Component Analysis

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:

Determining Coordinates for Two-Hop Neighbors

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

Use of TXOP to Coordinate Transmission TXOP: Transmission opportunity defined in IEEE 802.11e

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 802.11 DCF, in order to give more opportunities to its neighbor nodes.

LLC Layer Implementation for GN

MAC Layer Implementation for GN

Transmit_with_backoff(p,retx)