SynchronizationBy:

Loren K. Schwappach

Prepared for:

Dr. Santiago

CTU, EE463, Communications 1

October 2010

First some quick notes about TDM:

• As the number of independent message sources increases the intervals allotted to each source by the commutator is reduced, all sources must be accommodated into a time interval equal to the sampling interval.

• Since pulses too short can impair transmit it is necessary in practice to reduce the number. In a North American Public Switched Telephone Network (PSTN) T1 System, TDM is used to transmit 24 Channels.

Synchronization:

• In a PCM system with TDM it is essential that timing operations at the receiver (exempting time lost in transmission and regenerative repeating) closely follow the operations of the transmitter, this amounts to requiring a local clock at the receive operating in synchronization with the transmitter.

• Used in Time Division Multiplexing to lock the commutator and decommutator and keep the transmitter and receiver in phase.

Synchronization:

• One method of synchronization is to set aside a code word at the end of a frame (transmitted every other pulse) and send it via the Commutator. Thus a clock is created at half the frame rate and can be used by the receiver to establish synchronization.

• Re-synchronization is the procedure of examining code elements one by one by setting the clock back by one code element until the synchronizing pulse is detected, this process often includes the use of buffers.

LPF

LPF

LPF

Pulse Modulator

Communication Channel

Pulse Demodulator

LPF

LPF

LPF

TDM system block diagramBy Loren K. Schwappach

Low-pass (anti-aliasing)

filters

Commutator DeCommutator

Clock pulses Clock pulses

Synchronized

Low-pass (reconstruction)

filtersMessageInputs

1

2

N

1

2

N

MessageOutputs

Anti-aliasing filter: Removes non-essential frequencies.

Commutator: Uses electronic switching to (1) take a narrow sample of each of the N input messages at a rate fs (slightly higher than 2W), (2) sequentially interleave these N samples inside the sampling interval.

Pulse Modulator/Demodulator: Transforms the multiplexed signal into a suitable form. Often requires equalization to ensure satisfactory operation.

Decommutator: Performs opposite operation of commutator. Operates in synchronization.

M1Sync

Element1

M2Sync

Element0

MNSync

Element1

M1Sync

Element0

M2Sync

Element1

MNSync

Element0

Example of Synchronization:

Questions:

References:

Haykin, S., “Analog and Digital Communications 2nd Edition” John Wiley & Sons, Haboken, NJ, 2007.