Cell Capacity Boost: Frequency Reuse and Spatial Isolation

This article explores techniques for boosting cell capacity, focusing on frequency reuse and spatial isolation. We’ll compare the benefits and drawbacks of each approach. There are primarily two methods to increase cell capacity:

• Frequency Reuse
• Spatial Isolation

Frequency Reuse

Figure 1: Frequency Reuse in GSM Radio Frequency Planning

As illustrated in Figure 1, frequency reuse involves using the same frequencies in different cells within a network. This allows for more efficient use of the available spectrum.

However, this method can lead to inter-cell interference if the transmitted signal power is too high. Think of it like two people trying to talk at the same time; if they’re close enough and loud enough, they’ll interfere with each other. You can explore different frequency reuse patterns used in GSM network deployment.

In satellite systems, frequency reuse can effectively double the satellite’s bandwidth and capacity. Typically, satellite bandwidth is composed of pairs of 12 transponders. The same frequency channel can be used in one transponder and another. Using this approach, two transponders transmit on the same frequency.

Unfortunately, this can result in both intra-cell and inter-cell interference. This is where spatial isolation comes in.

Spatial Isolation

Figure 2: Spatial Isolation

Spatial isolation employs narrow beamwidth antennas. These antennas focus on specific, narrow regions, thereby increasing cell capacity. The reason for this capacity increase is because different earth stations can use the same frequencies without causing interference, as the antennas are highly directional.

These antennas are often referred to as spot beam antennas or smart antennas. Figure 2 illustrates how the same antenna can provide cellular service at the same frequency to different mobile users by using focused beams.

In satellite systems, this can be achieved using beamforming smart antenna design. Extra sets of satellite transponders are also employed. Furthermore, special antenna techniques are used to segregate the inputs and outputs. For example, a satellite can typically use up to 4 sets of 12 transponders each, all using the same RF frequencies. With frequency reuse techniques combined with spot beam antennas, a total of 48 satellite transponders can potentially be utilized.