Concept of Frequency Reuse

The mobile communication network has evolved from macro-cell system to the micro-cell system, as shown in the above figure. Under macro-cell system, the traffic is low, frequency resource is relatively rich. So only one BTS is needed with high transmit power to cover a larger area. Within this area, channel groups {f1, f2, f3... fn} are used, frequency reuse is unnecessary. When user and traffic increase, the original channels (f1, f2...fn) are no longer enough, the only way is to reuse frequency. In network wireless design, the transmit power of BTS is lowered to reduce the coverage of each BTS. The area originally covered by one BTS is now covered by several BTS , while the entire network figure resembles a honeycomb. Frequency reuse means that the channel used by one cell is reused in another cell after certain distance. If total channel numbers are fixed, it can increase the channel number in unit area. Frequency reuse is conditioned, since the same frequency carrier is used repeatedly in different cells, big or small co-channel and adjacent frequency interferences are unavoidable,  depending on the distance and the intensity of signals. To ensure that the carrier-to-interference ratio in all design service areas of the system complies with the requirement, different frequency reuse technologies should be adopted according to different conditions in frequency planning.

Example of Frequency Reuse

lFrequency resource is limited. If there is 8MHz frequency resource, 8 MHz = 40 channels × 8 timeslots = 320

p Max. 320  users  can access the network at the same  time.

lIf every frequency is reused N times

pMax. 320×N uses can access the network at the same time.

It is known that GSM system is a cellular system and the total channel resource is limited. In this case, the limited channel resources have to be reused in different cells to expand system capacity. Suppose that a GSM network operator has only 8MHz band, 40 frequency carriers or 320 channels. Even all channels are used for user communication, only 320 users are allowed to communicate at the same time. If each frequency carrier is reused for n times, then 320×n users can communicate at the same time.

Reuse Density

lThe spectrum utilization ratio can be expressed by frequency reuse density (freuse), which reveals the tightness of the frequency reuse and can be expressed by the following equation

l

p

pNARFCN is the total number of the available channel numbers,

pNTRX is the number of TRXs configured for the cell.

Question

(1) Frequency bandwidth is 12MHZ, if frequency reuse density is 4x3,each cell has how many TRX?

(2) Frequency bandwidth is 6MHZ, if frequency reuse density is 2x3,each cell has how many TRX?