- Supposing there are 300 cells
- Bandwidth: 8 MHz (40 frequency)
“0” BCCH layer: re-use =14, (14 frq.)
“+” Normal TCH layer: re-use =10, (20 frq.)
” ^-“ Aggressive TCH layer: re-use = 6, (6 frq.)
If there are 40 frequency carriers, main site mode is s3/3/3 when using regular reuse. With Multi-layer reuse pattern, site mode is divided into 3 layers ( 4 layers actually, but two layers have the same number of carriers) as shown in the figure above. The site mode can be s4/4/4. BCCH frequency carrier reuse density is 14 (loose reuse) in each cell.
The longer co-channel distance in the network ensures that the frequency carrier interference complies with the requirement. There are two frequency carriers with reuse density 10 in each cell, the co-channel reuse distance is shorter than BCCH frequency carrier and certain interferences exist, thus it is very hard to maintain good communication quality. The remaining frequency carrier has a reuse density 6 and short co-channel reuse distance, severe interferences can make these frequency carriers unavailable for communication at all.
To solve this problem, baseband frequency hopping is adopted in the network. The voice of same channel is transmitted by different frequency carriers, so the frequency carrier with severe interferences (reuse density is 6) only affects the fragmentary timeslot of a communication. With the error correction and detection function of the system, the entire communication quality can be guaranteed and the system is able to work normally.
The main reason that Multi-layer reuse pattern technology can realize close frequency reuse layer by layer to add TRX is: though the interference on specific frequency carrier is increased in the cell, the frequency carriers with slight interference and the frequency carriers with severe interference are mixed together by using frequency hopping technology, the same information flow is transmitted through different frequency carriers, and the interferences are averaged.
The bit error rate is very high when transmitting from the frequency carrier with more interferences, but it only lasts a short time, the Viterbi decoder still can demodulate correctly. For better performance of frequency hopping, at least three carriers are needed for baseband frequency hopping. Usually, BCCH is not involved in frequency hopping.
It’s obvious that layered close frequency reuse poses certain requirement on site mode, the minimum configuration of the site mode should be s4/4/4. In practical application, the average reuse density is about 7.5~8 at least (varying with network condition, environment, traffic and distribution, etc.). Taking 8 as an example, there must be more than 32 total frequency resources available.