6 Frequency hopping capability

05.013GPPGeneral descriptionPhysical layer on the radio pathTS

The frequency hopping capability is optionally used by the network operator on all or part of its network. The main advantage of this feature is to provide diversity on one transmission link (especially to increase the efficiency of coding and interleaving for slowly moving mobile stations) and also to average the quality on all the communications through interferers diversity. It is implemented on all mobile stations.

The principle of slow frequency hopping is that every mobile transmits its time slots according to a sequence of frequencies that it derives from an algorithm. The frequency hopping occurs between time slots and, therefore, a mobile station transmits (or receives) on a fixed frequency during one time slot ( 577 µs) and then must hop before the time slot on the next TDMA frame. Due to the time needed for monitoring other base stations the time allowed for hopping is approximately 1 ms, according to the receiver implementation. The receive and transmit frequencies are always duplex frequencies.

The frequency hopping sequences are orthogonal inside one cell (i.e. no collisions occur between communications of the same cell), and independent from one cell to an homologue cell (i.e. using the same set of RF channels, or cell allocation). The hopping sequence is derived by the mobile from parameters broadcast at the channel assignment, namely, the mobile allocation (set of frequencies on which to hop), the hopping sequence number of the cell (which allows different sequences on homologue cells) and the index offset (to distinguish the different mobiles of the cell using the same mobile allocation). The non‑hopping case is included in the algorithm as a special case. The different parameters needed and the algorithm are specified in 3GPP TS 05.02.

In case of multi band operation frequency hopping channels in different bands of operation, e.g. between channels in GSM and DCS, is not supported. Frequency hopping within each of the bands supported shall be implemented in the mobile station.

It must be noted that the basic physical channel supporting the BCCH does not hop.

For COMPACT, frequency hopping is not permitted on CPBCCH or CPCCCH for a specific amount of blocks. On other frequency hopping channels, a reduced mobile allocation is used on the corresponding blocks.

In CTS, the frequency hopping capability shall be used. The frequency hopping sequences are independently chosen by each CTS-FP. The hopping sequence is derived by the CTS-MS from parameters transmitted during the attachment procedure. The different parameters needed and the algorithm are specified in 3GPP TS 05.02. It must be noted that the basic physical channels supporting the CTSBCH and some other particular channels do not hop (see GMS 05.02).

Figure 3: Channel organization in the 51‑frame multiframe