MAPPING OF LOGICAL CHANNELS ONTO PHYSICAL CHANNELS

Logical channels are transmitted on physical channels. The method of placing logical channels on physical channels is called mapping. While most logical channels take only one time slot to transmit, some take more. If so, the logical channel information is carried in the same physical channel time slot on

consecutive TDMA frames. Because logical channels are short, several logical channels can share the same physical channel, making the use of time slots

more efficient. DCCH on TS-1 or TS-2? Over years TS-1 had been used for DCCH and even today most of the operators are using TS-1. However some of the operators are using Extended Cells with diameter longer than 35km(up to 120km) as described in chapter 3 under heading TIMING ADVANCE. These extended cells require 2 time slots per channel otherwise the adjacent TS will experience interference. This applies also to BCH and DCH, hence Ericsson recommend TS-2 for DCH(note: this is only a recommendation, even though TS—1 is used today all over the world). This would allow TS- 0/-1 for BCH and TS-2/3 for BCH in extended cells. In this book however TS-1 will be shown in examples and figures. The radio channel and the time slots for DCH are defined in the BSC data. The figure below shows the carrier frequencies for a sample cell, including an additional allocation of a time slot for DCCH in C1

(due to a high call set-up load in the cell).

Mapping of control and traffic logical channels to

physical channels

CARRIER 0, TIME SLOT 0

Time slot 0 of the first carrier frequency in a cell is always reserved for signaling purposes. In this way, when an MS is determining whether a carrier frequency is a BCCH carrier, it knows where to look. On the downlink, BCH and CCCH information is transmitted. The only logical channel on the uplink is RACH. By having the uplink free for RACH only, a mobile subscriber can initiate a call at any time.

CARRIER 0, TIME SLOT 1

Generally, time slot 1 of the first carrier frequency in a cell is reserved for signaling purposes. The only exceptions to this is cells with high or low traffic load. As can be seen in Figure, if there is a high traffic load in a cell, it is possible to assign a second (or more) physical channel for the purpose of call set-up (using DCCH). This may be any physical channel other than time slots 0 and 1 on carrier frequency 0. Similarly, if there is a low traffic load in a cell, it is possible to use physical channel 0 on carrier frequency 0 for all signaling information: BCH, CCCH and DCCH. By doing so, physical channel 1 can be spared for traffic. Eight SDCCH's and 4 SACCH's can all share the same physical

channel. This means that 8 calls can be set-up simultaneously on one physical channel.

CARRIER 0, TIME SLOT 2-7 AND ALL TIME SLOTS ON OTHER

CARRIERS IN THE SAME CELL

All time slots in a cell other than those assigned for signaling information are used for traffic, i.e. speech or data. Logical channel TCH is used. In addition, at regular intervals during a call, an MS transmits to the BTS measurements it has made about signal strength and quality. Logical channel SACCH is used for this, replacing one TCH time slot at a time.

SAMPLE TRAFFIC CASE: CALL TO AN MS

The following traffic case describes a call to an MS and highlights the use of some logical channels during the call.

Call to an MS

1. The MSC/VLR knows which LA the MS is located in. A paging message is sent to the BSC's controlling the LA. 

2. The BSC’s distribute the paging message to the BTS's in the desired LA. The BTS's transmit the message over the air interface using PCH.

3. When the MS detects a PCH identifying itself, it sends a request for a signaling channel using RACH.

4. The BSC uses AGCH to inform the MS of the signaling channel (SDCCH and SACCH) to use.

5. SDCCH and SACCH are used for call set-up. A TCH is allocated and the SDCCH is released.

6. The MS and BTS switch to the identified TCH frequency and time slot. The MS generates ring tone. If the subscriber answers, the connection is established. During the call, signals can be sent and received by the MS using SACCH.