Frequency reuse and channel assignment strategies

Frequency reuse

Frequency reuse is the use of the same radio channels in different cells that are far enough apart that their co-channel interference stays within limits. The total channels S are divided among N cells of a cluster, and the cluster is repeated M times over the service area, so capacity C = M × S grows with the number of clusters without extra spectrum.

        ___
    ___/ 2 \___
   / 7 \___/ 3 \      One N = 7 cluster: each cell
   \___/ 1 \___/      uses a different channel set.
   / 6 \___/ 4 \      The cluster is repeated, so the
   \___/ 5 \___/      next cell "1" is D = 4.58R away.
       \___/

Channel assignment strategies

1. Fixed channel assignment (FCA)

   • Each cell is given a fixed set of channels in advance.
   • A call is blocked if all channels of that cell are busy.
   • Borrowing strategy: a cell may borrow a channel from a neighbouring cell if none of its own is free; the MSC ensures the borrowed channel does not cause interference.
   • Simple and needs little control, but cannot adapt to changing traffic.

2. Dynamic channel assignment (DCA)

   • Channels are not permanently assigned; on each call request the MSC allocates a channel from the common pool using an algorithm that considers future blocking, reuse distance, channel usage and cost.
   • Uses channels efficiently and lowers blocking, but needs real-time data collection and heavy MSC computation.

3. Hybrid channel assignment – some channels fixed in each cell, the rest in a common pool assigned dynamically.