WCDMA Compressed Mode for Measurements Explained

WCDMA compressed mode is the mode where gaps are published by the WCDMA network for neighbor cell measurements. This page also covers compressed mode methods used in WCDMA.

In GSM systems, there are dedicated idle frames available, which are used for neighbor cell measurements required for cell re-selection. The same is depicted in various GSM frames used for signaling (51 frame multiframe) and traffic (26 frame multiframe) as well as GPRS frame structure (52 frame multiframe).

In the case of WCDMA systems, such idle frames are not available, and the frame structure is continuous and not bursty. Hence, there is no dedicated place in the frame structure to perform measurements.

There is a concept called compressed mode, in which the network provides gaps for the Mobile/UE to perform measurements. During these gaps, no conversation/packet data flow happens, and the gaps are solely used for neighbor cell measurements.

When a mobile is not active, it performs measurements in discontinuous transmission (DTx) and discontinuous reception (DRx) mode.

There are various applications using WCDMA service, which include voice, data, etc. Voice applications require a constant bit rate; hence, the bit rate is increased on both sides of the gap to maintain the constant bit rate.

Compressed mode allocates gaps in transmission either in UL (Uplink) or DL (Downlink) or both. During this gap time period, the UE switches to the alternate carrier frequency of neighbor cells, performs measurements, switches back to the serving cell frequency, and continues with normal operation.

WCDMA Compressed Mode Methods

• Puncturing
• Spreading factor reduction
• Higher layer Signaling

Puncturing

By way of the Puncturing method, part of the data from transmission is removed/deleted, and a gap is introduced, which is used for measurement purposes.

Spreading Factor Reduction

By way of the Spreading factor reduction method, the data rate in UL/DL transmission is changed by changing the length of the channelization code. This is used to create a gap by doubling the rate, and hence the UE/NodeB will complete transmission/reception in half the time than the normal data rate.

Higher Layer Signaling

By way of the higher-order signaling method, the transmission gap is generated by restricting the allowed transmission rate for some time; hence, a gap is created for measurement purposes.