W-CDMA - Radio Bear Realization

The UTRAN standard does not mandate how to realize radio bearers, i.e. what user plane protocol modes should be selected by RRC for a specific bearer service request. Below some typical realizations are outlined.

Conversational bearers may use PDCP if they originate from the Internet and have headers that should be compressed. For voice services originating from PSTN, PDCP is typically not used. The delay requirements are typically too stringent to allow for retransmission of erroneous data. Therefore unacknowledged RLC is used if segmentation of higher layer data units is required, and otherwise transparent RLC is used, which is the typical case for voice bearer services. The delay requirements typically require that the dedicated MAC mode is used. Streaming bearers with very stringent delay requirements may be realized in the same way as conversational bearers. However, for bearers with looser delay requirements, it is possible to use acknowledged RLC and common/shared MAC, which may yield higher capacity. PDCP usage is not affected by this.

Interactive and Background bearers typically originate from the Internet and hence PDCP is used. The delay requirements allow for acknowledged RLC and common/shared MAC. High priority Interactive bearers could use the dedicated MAC mode.

• One Physical Control Channel and one or more physical data channels form a single Code Composite Transport Channel (CCTrCh). There can be more than one CCTrCh on a given connection but only one physical layer control channel is transmitted in such a case.
• Frame Structure of Transport Channels: The UTRA channels use the 10ms radio frame structure. The longer period used in the system frame period. The System Frame Number (SFN) is a 12 –bit number used by several procedures that span more than a single frame. Physical layer procedures, such as the paging procedure or random access procedure, are examples of procedures that need a longer period than 10ms for correct definition.
• Scrambling code: It is needed to separate terminals or base stations from each other. It would not matter if the actual spreading were done with identical code for several transmitters. The chip rate As the chip rate is already achieved in the spreading by the channelisation codes, the symbol rate is not affected by the scrambling.