Technique Frequency Hopping in GSM Network

Previous post used to introduce to Frequency Hopping in GSM Network ready and this post continues to talked about technique of Frequency Hopping.

What kind of Frequency Hopping?

1. Baseband Frequency Hopping

In baseband frequency hopping (BFH), each TRX is assigned a fixed frequency. After a burst is processed by the baseband processing unit, the BSC determines the frequency for transmitting the burst according to the HSN. Then, the burst is routed to the transmitter of the proper frequency through the bus. The timeslots on a TRX can use different MA lists, but the frequencies in an MA list must be a subset of fixed frequencies assigned to all hopping TRXs.

Working principle of BFH

In BFH, all the bursts of a single call may be transmitted through multiple TRXs, and each TRX uses a fixed frequency to transmit bursts. FH is implemented by routing a burst to the transmitter of a proper frequency through the bus. For example, a call uses the hopping sequences f0, f1, and f2. During transmission, the bursts using f0, f1, and f2 are routed from the baseband processing units to the transmitters of TRX 1, TRX 2, and TRX 3, respectively.

In GSM systems, the frequencies of the BCCH and extended BCCH must remain unchanged, that is, the BCCH and extended BCCH cannot participate in FH. In BFH, other channels on the BCCH TRX except the BCCH can participate in FH to obtain FH gains. If other channels on the BCCH TRX participate in FH, the BCCH TRX participates in FH. If other channels on the BCCH TRX do not participate in FH, the BCCH TRX does not participate in FH.

When the BCCH TRX participates in FH, the MA lists for timeslots on the BCCH and extended BCCH are different from those for other timeslots. The MA lists for the BCCH and extended BCCH timeslots do not contain frequencies assigned to the BCCH TRX. The MA lists for other timeslots can contain frequencies assigned to the BCCH TRX.

According to the implementation method of BFH, the maximum number of frequencies in an MA list equals the number of hopping TRXs. That is, a larger number of hopping TRXs leads to a larger number of frequencies available for FH and better effect of interference suppression If the BCCH TRX participates in FH, the number of available frequencies for FH increases, improving system performance.

In BFH, all the bursts of one call are transmitted through multiple TRXs. If any TRX becomes faulty, FH fails and call drops may occur.

The combiner used for BFH produces low signal attenuation, providing a large coverage area. The number of frequencies available for FH, however, depends on the number of TRXs, and accordingly, the FH gain is small. In consideration of this, BFH is applicable to areas using loose frequency reuse patterns.

2. RF Frequency Hopping

In RF FH, each TRX is not assigned a fixed frequency, and different timeslots on a single TRX and different TRXs can use different MA lists. The number of frequencies in different MA lists vary. The number of frequencies in an MA list does not depend on that of hopping TRXs and can exceed that of hopping TRXs. During transmission, the TRX determines the frequency used to transmit a burst according to the HSN. Then, the burst is transmitted on the selected frequency.In below figure, f0 to fn indicate all frequencies in an MA list. (GBFD-113701 Frequency Hopping (RF hopping, baseband hopping))

Working principle of RF FH

In comparison to BFH, more frequencies can be used in RF FH for greater FH gains. Generally, RF FH better improves the performance and anti-interference capability of the BSC than BFH.
In general, the hybrid combiner is used for RF FH and produces high signal attenuation. Therefore, the coverage area is small. RF FH is applicable to cells using tight frequency reuse patterns. In these cells, co-channel and adjacent-channel interferences are strong. Therefore, more frequencies are used in an MA list, to obtain great FH gains and improve the antiattenuation capability of the BSC. If the number of frequencies participating in RF FH is greater than four, the system performance is improved significantly.

The BCCH and extended frequencies must remain unchanged. In RF FH, the transmit frequency of each TRX changes. Therefore, the TRX carrying the BCCH and extended BCCH timeslots cannot participate in RF FH.

3. Synthesized Frequency Hopping

In synthesized frequency hopping (SFH), some TRXs adopt RF FH and others adopt BFH within the same cell. One TRX, however, can use either RF FH or BFH.

For BFH, only the frequency assigned to a TRX participates in FH. For RF FH, a single TRX can use multiple frequencies. Generally, the number of hopping frequencies is greater than the number of TRXs, helping obtain sufficient FH gains. If the number of TRXs in a cell is small but the frequencies are sufficient, RF FH can be adopted to improve the system performance. If the number of TRXs in a cell is large but the frequencies are insufficient, BFH can be adopted to improve the system performance.

In general, SFH is applicable to co-BCCH cells. In the overlaid subcell of a co-BCCH cell, a tight frequency reuse pattern is adopted, and therefore the co-channel and adjacent-channel interference is strong. The overlaid subcell generally works on the 1800/1900 MHz band, and the frequencies are sufficient. The underlaid subcell of a co-BCCH cell generally works on the 900/850 MHz band, a loose frequency reuse pattern is adopted, and the frequencies are insufficient. In general, the BCCH and extended BCCH are configured in the underlaid subcell. To adapt to these characteristics of the co-BCCH cell, SFH can be adopted. In the overlaid subcell, the TRXs use RF FH, and more frequencies are used to improve the antiinterference capability of the system. In the underlaid subcell, the TRXs use BFH to improve the system interference, requiring no additional frequency.

Also read Overview frequency Hopping in GSM Network