Compressed mode in UMTS is an important concept that enables functionality such as inter-frequency and inter-system/inter-RAT (Radio Access Technology) handovers. Compressed mode is the mode in which the transmitter (base band) on Node B and/or User Equipment (UE) creates gaps in its transmission in Downlink and/or Uplink in order for the UE receiver to tune its receive frequency to the non-used desired frequency and perform measurements.
Not all UEs require the compressed mode to perform measurements. Some advanced terminals come with dual RF receiver capability in which they will be able to receive simultaneously on two carriers. Such mobiles would be able to make inter frequency measurements even without compressed mode configurations. However, when the UE capability does not allow it to make measurements on the non-used frequency (ie, other than the currently used frequency on which the UE has camped on), it can utilize the transmission gaps in the compressed frames to make measurements on the desired frequency.
UEs that require compressed mode for inter frequency measurements shall support one transmission gap sequence for each measurement purpose in FDD. More than one transmission gap pattern can be active at a time if the UE supports several measurement purposes. However, higher layers ensure that gaps from different transmission gap patterns do not overlap. The following measurement purposes are applicable:
- FDD
- TDD
- GSM Carrier RSSI Measurement
- Initial BSIC Identification
- BSIC re-confirmation
- E-UTRA
There are two ways by which a transmission gap can be achieved at the transmitter:
- Compressed Mode by Higher Layer Scheduling (HLS)
- Compressed Mode by Spreading Factor (SF) Reduction
Before we delve deeper, in subsequent posts, on to how a transmission gap is created in the SF reduction method, it is useful to understand various compressed mode related parameters and how they are related to the physical layer transmission timings. This is the objective of the current post.
The following diagram depicts the relation of Transmission Gap Patterns with respect to Connection Frame Numbers (CFN)
The Transmission Gap Pattern is the pattern consisting of one or two transmission gaps. This pattern is repeated ‘TGPRC’ (Transmission Gap Pattern Repetition Count) number of times. The transmission gap pattern is of length TGPL1. In the above picture the TGPL1 is equal to two frames.
Below figure depicts the details of the Transmission Gap Pattern which is mentioned above.
TGSN (Transmission Gap Starting Slot Number) signify the slot number in which the first gap (TGAP1) of the transmission gap pattern starts. The first transmission gap is of length TGL1 (Transmission Gap Length1). TGD (Transmission Gap start Distance) signify the number of slots from the beginning of the first transmission gap at which second transmission gap (TGAP2) starts. The second transmission gap is of length TGL2 (Transmission Gap Length2). As mentioned earlier, TGPL1 (Transmission Gap Pattern Length) signifies the length in frames of the full transmission gap pattern.
In forth coming posts we would discuss details on how compressed mode affects various procedures, how higher layers configure and control compressed mode, various types of measurements that are carried out in compressed mode.
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