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UMTS (Transport Format) - 18

Transport Format [Under UMTS]
» Phy layer services - 25.302 «

One of the major goals of UMTS is to support various kinds of services. Code division access allows differentiation between transmissions with codes, but along with it, comes challenges like "power control". Straightforward way is to allocate separate codes for channels, users etc. But the challenges/limitations of CDMA will not allow us to do that. We have to use "codes" (and so the radio resources) efficiently. One of the best options would be sharing of codes among channels and users as much as possible. In UMTS, channels are shared among users (with RNTI used for identification). Also, number of (dedicated or similar) transport channels are "multiplexed" (onto one or more physical channels). Interface between MAC and PHY has been designed with this in mind. This brings us to a concept of Transport Format (TF).

Every transport channel has certain characteristics and the values of these characteristics form Transport Format for the particular channel.

The transport (or data exchange between MAC and PHY) works in units, called Transport Blocks (TB). Set of TBs is called Transport Block Set (TBS), with the condition that all TBs of TBS will have same size. TB size or TBS size is simply size of TB or TBS in bits respectively.

The time within which TB need to be transmitted (or exchanged between MAC and PHY) is called Transmission Time Interval (TTI). As UMTS uses frames of 10 ms, TTI can have discrete values like 10, 20, 40, 80 ms.

In addition to above characteristics, there are other characteristics like channel coding (turbo/covolutional), CRC size etc.

UMTS has specified range of values (with granularity) for each characteristic of particular type of transport channel (e.g. BCH has TB size of 246 bits, whereas RACH has TB size till 5000 bits with granularity of 1 bit).

A transport format for a channel will have values for each of the above characteristics. Set of TFs is called Transport Format Set (TFS). TB size and TBS size can be changed by transmitting entity for every TTI; these characteristics are said to be Dynamic part of TF. Rest of the characteristics are said to be Semi-static part of TF. The idea of dynamically changing TB/TBS size every TTI is to adapt the transmission rate to the source data (being given to MAC) rate. If Semi-static part is to be changed, it require RRC signaling exchange leading to "reconfiguration" of channel.

All TFs in TFS will have same Semi-static part and different Dynamic part. For for a transport channel, depending on source data rate, MAC can choose one of the TFs from TFS. But there is another important point here: multiple transport channels can be multiplexed onto one physical channel like PCH and FACH onto S-CCPCH, multiple DCHs onto DPCH.

In case of multiplexing of transport channels, *not* all TFs of one transport channel are "compatible" with all TFs of another transport channel (with which it is to be multiplexed). Of course, some combinations are possible; a possible combination of TFs is called as Transport Format Combination (TFC). Set of such TFCs is called Transport Format Combination Set (TFCS).

So, for multiplexed channels, MAC has to choose one of the TFCs from TFCS depending on the source data rates for the multiplexed channels.

Let us take an example where we have three DCHs with following transport configuration (if you recall from earlier article, transport configuration is informed by network during RRC connection establishment). These are to be multiplexed onto DPCH.

Transport channel
TFS TB size (bits) TBS size (bits)
TTI (ms) Coding, CRC etc.
DCH1
TF10 20 20
TF11 40 80
TF12 160 160
TF13 320 320
10 Conv coding, CRC 8 bits, ...
DCH2
TF20 40 40
TF21 40 80
10 Conv coding, CRC 12 bits, ...
DCH3
TF30 320 320
TF31 320 1280
20 Conv coding, CRC 24 bits, ...

TFC DCH1 DCH2 DCH3
TFC1 TF10 TF20 TF30
TFC2 TF11 TF20 TF30
TFC3 TF11 TF21 TF30
TFC4 TF12 TF20 TF30

Here is graphical representation of data exchange that may take place between MAC and PHY.

So transmitting MAC chooses appropriate TF for a transport channel and gives it to PHY. Receiving PHY/MAC need to know the TF that has been chosen for the particular TTI. Communication of TF related information is done by PHY layer. This brings us to a concept of CCTrCh (Coded Composite TRansport CHannel). We will look at CCTrCh in next article.

References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.

Copyright © Samir Amberkar 2010§ §

RRC connection establishment « UMTS Index » CCTrCh