TD-SCDMA, TDD and FDD

After my posting on TD-SCDMA so many people asked me about what TD-SCDMA is. I am surprised that so many people are not aware of TD-SCDMA. So here is a quick posting on that.

TDD and FDD Mode of Operation

Basically most of the UMTS networks in operation are Frequency Division Duplex (FDD) based. There is also another variant called the Time Division Duplex or TDD. In reality there is more than one variant of TDD, so the normal 5MHz bandwidth TDD is called Wideband TDD of WTDD. There is also another name for WTDD to confuse people, called the High Chip Rate TDD (HCR-TDD). There is another variant of TDD as would have guessed known as the Narrowband TDD (NTDD). NTDD is also known as Low Chip Rate TDD (LCR-TDD) and most popularly its known as TD-SCDMA or Time Division Synchronous CDMA.

"Synchronous" implies that uplink signals are synchronized at the base station receiver, achieved by continuous timing adjustments. This reduces the interference between users of the same timeslot using different codes by improving the orthogonality between the codes, therefore increasing system capacity, at the cost of some hardware complexity in achieving uplink synchronization.

The normal bandwidth of FDD or TDD mode of operation is 5 MHz. This gives a chip rate of 3.84 Mcps (Mega chips per second). The corresponding figure for TD-SCDMA is 1.66 Mhz and 1.28 Mcps.

Assymetric operation in TDD mode

The advantage of TDD over FDD are:

• Does not require paired spectrum because FDD uses different frequencies for UL and DL whereas TDD uses the same frequency hence its more easy to deploy
• Channel charachteristics is the same in both directions due to same band
• You can dynamically change the UL and the DL bandwidth allocation depending on the traffic.
  

The dis-advantage of TDD over FDD are:

• Switching between transmission directions requires time, and the switching transients must be controlled. To avoid corrupted transmission, the uplink and downlink transmissions require a common means of agreeing on transmission direction and allowed time to transmit. Corruption of transmission is avoided by allocating a guard period which allows uncorrupted propagation to counter the propagation delay. Discontinuous transmission may also cause audible interference to audio equipment that does not comply with electromagnetic susceptibility requirements.
• Base stations need to be synchronised with respect to the uplink and downlink transmission times. If neighbouring base stations use different uplink and downlink assignments and share the same channel, then interference may occur between cells. This can increase the complexity of the system and the cost.
• Also it does not support soft/softer handovers

Timing Synchronisation between different terminals

By the way, in Release 7 a new TDD mode of operation with 10 MHz bandwidth (7.86 Mcps) has been added. Unfortunately I dont know much about it.

You can read more about TD-SCDMA in whitepaper 'TD-SCDMA: the Solution for TDD bands'

You can find more information on TD-SCDMA at: http://www.td-forum.org/en/