Mobile radio carriers suffer from frequency-selective interferences, for example, fading due to the multi path propagation phenomena. As the carrier signal attenuates with distance, frequency selective interference can have an increasingly significant affect on the signal quality.
Frequency hopping (FH) employs a constantly changing transmission frequency on the radio carrier. Therefore the effects of frequency selective interference will be reduced by producing an averaging effect over the interference caused on each frequency employed within the FH sequence. This results in an overall improvement in S/N ratio.
Type of Frequency Hopping
There are two types of FH
- Base Band Hopping (BBH)
BBH can be employed if there are several TRXs. The call is switched to each TRX in accordance with the assigned hopping sequence.
- Synthesizer Frequency Hopping (SFH)
In SFH single TRX can be used. The call is switch in single TRX within its TCHs.
BBH happen as follow figure;
Each TRX has a dedicated BCCH. As an example, for each TRX has BCCH of 992, 994, and 996. This method is hardy used, because each cell has more BCCH and effect on frequency re use.
SFH happen as following figure;
Each TRX has set of frequencies. Hopping is happening on same TRX among channels. Most of time, SFH is used.
The BCCH carrier is not hopping. Only TCHs are hopping.
Hopping Sequence Number (HSN)
The pattern of hopping frequencies is sequential or random. For a GSM network, can be assigned one cyclic hopping pattern or any one of 63 random cyclic hopping patterns. Each those patterns can be defined by a Hopping Sequence Number (HSN) and the range of HSN is 0 - 63.
F1 F2 F3 F4 F1 F2 F3 F4 – Sequential
F1 F4 F3 F2 F1 F4 F3 F2 –Random
Mobile Allocation Index Offset (MAIO)
TCH carriers must start at different point in sequence to avoid co-channel (C/I) interference. Hopping sequence for each TRX must be different or have a mobile allocation index offset (MAIO).
As a example, the BTS consisted with three TRXs and they all have same HSN of 39. But the MAIO for each TRX is 0, 2 and 6. Following figure show HSN as 61 and MAIO as 0. The screen shot is taken by TEMS tool.
