Explain about TDMA channels

NA-TDMA Channels:

In NA-TDMA, there are no common channels such as those used in GSM. The digital call set-up uses the 21 set-up channels which are shared with the analog system.

Supervision of the Digital Voice Channel: 

The supervision channels in NADC are similar to those in GSM:

(i) Fast Associated Control Channel FACCH is a blank and burst channel equivalent to a signaling channel for the transmission of control and supervision messages between the base station and the mobile station. It consists of 260 bits. Mostly FACCH is used for handoff messages.

(ii)Slot Associated Control Channel SACCH is a signaling channel including twelve code bits present in every time slot transmitted over the traffic channel whether these contain voice or FACCH information.

Mobile-Assisted Handoffs (MAHO): 

The mobile station performs signal quality measurements on two types of channels:

1. Measures the RSSI (received signal strength indicator) and the BER (bit error rate) information of the current forward traffic channel during a call.

2. Measures the RSSI of any RF channel which is identified from the measurement order message from the base station.

MAHO consists of three messages:

1. Start measurement order:

Measurement order message—sent from the base station to the mobile station.

Measurement order acknowledge message—sent from the mobile station to the base

station.

2. Stop measurement order:

Stop measurement order—sent from the base station to the mobile station.

Mobile acknowledges—sent from the mobile station to the base station.

3. Channel quality message (mobile to base only)

The mobile transmits the signal quality information over either the SACCH or FACCH.

In the case of discontinuous transmission (DTX):

(a) Whenever the mobile is in the DTX high state, the mobile transmits channel quality information over the SACCH

(b) When the mobile is in the DTX low state, the mobile transmits the channel quality information over the FACCH

Handoff Action: When a handoff order is received, the mobile station is at DTX high state and stays at that state. If the mobile station is at DTX low state it must enter the DTX high state and wait for 200 ms before taking the handoff action. Handoff to a digital traffic channel is described as follows:

1. Turn on signaling tone for 50 ms, turn off signaling tone, turn off transmitter which was operating on the old frequency.

2. Adjust power, tune to new channel, set stored DVCCs to the DVCC field of the received message.

3. Set the transmitter and receiver to digital mode, set the transmit and receive rate based on the message-type field.

4. Set time slot based on the message-type field.

5. Set the time alignment offset to the value based on the TA field.

6. Once the transmitter is synchronized, enter the conversation task of the digital traffic channel.