Inter-frequency handovers are needed to pass data from one radio frequency in one cell to another radio frequency in another cell. This is more important for inter-operability between 3GPP FDD and second-generation systems such as GSM or IS-95.

In order to complete inter-frequency handovers, an efficient method is needed for making measurements on other frequencies while still having the connection running on the current frequency. In 3GPP (FDD) this is achieved by using compressed mode, which effectively interrupts the signal for a short time as demonstrated in the following figure.

Although a transmission gap effectively reduces the amount of transmission time available, the quality of service must not be affected. This is achieved by increasing the volume of data transmitted in the remaining time; the data is compressed. This is done by following methods and it depends on the link direction.

  • Higher layer scheduling
  • Reducing the spreading factor by 2
  • Puncturing

In downlink, all the three methods are applicable; however in uplink, the puncturing method is not used.

Frames containing no transmission gaps are sent with the same slot format and the same power as in the non-compressed mode. A different slot format usually with a higher number of pilot bits is used in the compressed ranges.

The uplink transmit power can be increased (CM power offset mode) automatically or manually by defining a power offset.

Transmission Gaps

In compressed mode, slots from Nfirst to Nlast are not used for transmission of data. This is referred as a transmission gap length (TGL). The transmission gap has a maximum length of 14 slots. Since at least eight active slots must be sent per frame, gaps comprising seven slots and more have to be distributed over two neighboring frames, as shown in the following figure.

Frame Structures

Uplink

In compressed mode, DPCCH slot formats with TFCI fields are changed. There are two possible compressed slot formats for each normal slot format. The selection between them is dependent on the number of slots that are transmitted in each frame in compressed mode. The following figure represents the frame structure in uplink compressed mode transmission.

Downlink

There are two different types of frame structures defined for downlink compressed mode. Type A maximizes the transmission gap length and type B is optimized for power control.

For type A, the pilot field of the last slot in the transmission gap is transmitted. Transmission is turned off for the rest of the transmission gap, as shown in the following figure.

For type B, the TPC field of the first slot in the transmission gap and the pilot field of the last slot in the transmission gap are transmitted. Transmission is turned off for the rest of the transmission gap.

Transmission Gap Pattern

The transmitted signal consists of two patterns that are sent alternately and each pattern comprises two transmission gaps, as shown in the following figure.

The preceding diagram includes all the parameters necessary to define the transmission gaps in the signal and are characterized as follows:

  • TGSN (Transmission Gap Starting slot Number): Slot number of the first missing (idle) slot in the transmission gap.
  • TGL1 (Transmission Gap Length 1): Duration of the first transmission gap. This value is expressed in number of slots.
  • TGL2 (Transmission Gap Length 2): Duration of the second transmission gap. This value is expressed in number of slots.
  • TGD (Transmission Gap start Distance): Duration between the two transmission gaps within a transmission gap pattern. This value is expressed in number of slots.
  • TGPL1 (Transmission Gap Pattern Length): Duration of the first transmission gap pattern. This value is expressed in number of frames.
  • TGPL2 (Transmission Gap Pattern Length): Duration of the second transmission gap pattern. This value is expressed in number of frames.