JPL's Wireless Communication Reference Website

Chapter: Data Networks
Section: Random access

Spatial Collision Resolution

Spatial Collision Resolution (SCR) is a method that can be used in cellular packet data networks to allow multiple portable terminals to access a communications infrastructure. It combines frequency reuse (in space or area) with multiple access. The goal is to develop new radio access techniques that are better suitable to bursty multi-media traffic.


In high-capacity spectrum-efficient packet-switched networks, adjacent base stations preferably compete for non-disjoint (i.e., interfering) spectrum resources, rather than using a fixed frequency reuse pattern. Spectrum efficiency is optimal if the full system bandwidth can be used in all cells. However, continuing interference between transmissions from adjacent base station may severely affect the performance of such networks. This suggests that efficient, coordinated resolution of collisions between packet retransmissions in adjacent cells is necessary to guarantee efficient performance of wireless data and multi-media networks. SCR mitigates the effect of continued collisions in packet-switched radio data networks with dense frequency reuse:

Basic Idea

All base stations share the same transmit channel, which has frames of three time slots. The areas covered by each base station are assigned a sequence number {1, 2 or 3} according to a map-coloring scheme which ensures that adjacent areas always have a different number. In normal operation, a base station can transmit in any time slot regardless of its number. If base stations in adjacent areas happen to transmit simultaneously, most signals may nonetheless 'capture' their intended receiver. With some probability however, interference erases some messages involved in this `collision'. In the latter case, the base station will retransmit the lost message in the slot of the next frame with the corresponding number. During this retransmission, all adjacent base stations are silenced to prevent another collision. This coordination can be performed by protocols using the fixed backbone infrastructure, connecting all base stations.


Cellular packet data network with C = 1 under normal Contiguous Frequency Assignment (CFA) operation, i.e., if no destructive collision occurs. Any base station can transmit in any time slot, accepting the risk of excessive interference from cochannel transmissions in nearby cells.

Spatial Collision Resolution (SCR) in cellular packet data network. Base stations only transmit in the time slots with the corresponding sequence number. During retransmissions, all adjacent base stations must refrain from transmitting.

JPL's Wireless Communication Reference Website (c) Rolando Diesta and Jean-Paul M.G. Linnartz, 1993, 1995.