WCDMA compressed mode used for measurements

This tutorial section on WCDMA (UMTS) basics covers following sub topics:
UMTS Tutorial  UMTS channels  measurements  PHY layer  TDD vs FDD PHY  RRC States  Security  Cell search  Bands  UARFCN  Frame 

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

GSM/GPRS idle frames
In GSM system there are dedicated idle frames available, which are used for neighbour cell measurements required for cell re-selection. The same is depicted in various GSM frames used for signalling(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 frame structure is continuous and not bursty. Hence there is no place dedicated in the frame structure to perform measurements. There is a concept called compressed mode, in which network provides gaps for Mobile/UE to perform measurements. During these gaps no conversation/packet data flow happens and the gaps are solely used for neighbour cell measurements.

WCDMA compressed mode
When mobile is not active, it performs measurements in discontinuous transmission(DTx) amd discontinuous reception(DRx) mode. There are various applications using WCDMA service,which include voice,data etc. Voice application requires constant bit rate, hence bit rate is increased on both the sides of gap to maintain the constant bit rate.

Compressed mode allocates gap in transmission either in UL(Uplink) or DL(Downlink) or both. During this gap time period, UE switches to the alternate carrier frequency of neighbour cells,perform measurements,switch back to the serving cell frequency and continue with normal operation which it was doing.

WCDMA Compressed Mode Methods

•  Puncturing
•  Spreading factor reduction
•  Higher layer Signalling

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

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

By way of higher order signalling method, the transmission gap is generated by restricting allowed transmission rate for sometime; hence gap is created for measurement purpose.


UMTS Network Architecture
UMTS Protocol Stack
UMTS Logical, Transport and physical channels
UMTS Spreading,scrambling and modulation
WCDMA Compressed mode

RF and Wireless Terminologies