UMTS Interview Questions with Answers(150 Questions)
What is Compressed mode and why it is required ?
Compressed mode, also known as the Slotted Mode, is needed when making measurements on another frequency (inter-frequency) or on a different radio technology (inter-RAT). In the Compressed Mode the transmission and reception are stopped for a short time and the measurements are performed on other frequency or RAT in that time. After the time is over the transmission and reception resumes. To make sure that the data is not lost, the data is compressed in the frame making empty space where measurements can be performed.
Compressed mode is not necessary. If the UE has a second receiver it can make measurements on that receiver while continuing with the transmission/reception on the first receiver. This does not happen in practise as the cost would go up. The UE capabilities define whether a UE requires compressed mode in order to monitor cells on other FDD frequencies and on other modes and radio access technologies. UE capabilities indicate the need for compressed mode separately for the uplink and downlink and for each mode, radio access technology and frequency band.
A UE shall support compressed mode for all cases for which the UE indicates that compressed mode is required. A UE does not need to support compressed mode for cases for which the UE indicates that compressed mode is not required. For these cases, the UE shall support an alternative means of making the measurements. The UE shall support one single measurement purpose for one transmission gap pattern sequence. The measurement purpose of the transmission gap pattern sequence is signalled by higher layers.
The figure above gives an idea of how the frame is compressed for performing measurements. In compressed frames, TGL slots from Nfirst to Nlast are not used for transmission of data. As illustrated in figure, the instantaneous transmit power is increased in the compressed frame in order to keep the quality (BER, FER, etc.) unaffected by the reduced processing gain. The amount of power increase depends on the transmission time reduction method. What frames are compressed, are decided by the network. When in compressed mode, compressed frames can occur periodically, as illustrated in figure, or requested on demand. The rate and type of compressed frames is variable and depends on the environment and the measurement requirements.
Last edited by telecominfo; 12-20-2011 at 03:21 PM.
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