Most of the 3G UMTS networks use a frequency division duplex scheme where uplink and downlink transmit simultaneously and use different frequencies. However time division duplex, TDD where uplink and down link use the same frequency but transmit at different times has some distinct advantages in some circumstances.
It is particularly useful where the amount of data required in uplink and downlink is different as it is possible to adjust the length of time allocated to uplink and downlink accordingly.
As a result UMTS TDD was used for a number of mobile internet connections, although it did not experience hugely widespread use.
TDD - time division duplex
A communications system requires that communication is possible in both directions: to and from the base station to the remote station. There are a number of ways in which this can be achieved. The most obvious is to transmit on one frequency and receive on another. The frequency difference between the two transmissions being such that the two signals do not interfere. This is known as frequency division duplex (FDD) and it is one of the most commonly used schemes, and it is used by most cellular schemes.
It is also possible to use a single frequency and rather than using different frequency allocations, use different time allocations. If the transmission times are split into slots, then transmissions in one direction take place in one time slot, and those in the other direction take place in another. It is this scheme that is known as time division duplex, TDD, and it is used for UMTS-TDD.
In order for radio communications systems to be able to communicate in both directions it is necessary to have what is termed a duplex scheme. A duplex scheme provides a way of organizing the transmitter and receiver so that they can transmit and receive. There are several methods that can be adopted. For applications including wireless and cellular telecommunications, where it is required that the transmitter and receiver are able to operate simultaneously, two schemes are in use. One known as FDD or frequency division duplex uses two channels, one for transmit and the other for receiver. Another scheme known as TDD, time division duplex uses one frequency, but allocates different time slots for transmission and reception.
When using a TDD system, there are a number of characteristics that are pertinent for TDD systems. These characteristics need to be accommodated when developing or using TDD systems.
- Utilisation of unpaired bands: Typically there is more traffic in the downlink (network to the mobile) than in the uplink (mobile to network). Accordingly the operator is able to allocate more time to the downlink transmission than the uplink. This is not possible with the paired spectrum required for FDD systems where it is not possible to re-allocate the use of the different bands. As a result of this, it is possible to make very efficient use of the available spectrum.
- Discontinuous transmission: In any TDD system it is necessary to switch between transit and receive. This takes a certain amount of time. Not only does it take time for the mobile and the base station to change between transmit and receive in terms of ramping up or down the power, along with the settling of any transients. In addition to this the time is required between transmit and receive to accommodate the transmission time between the mobile and the base station. As a result a guard band is required.
- Uplink / downlink interference: As both uplink and downlink share the same channel there can be interference between the two transmission directions. To overcome this, base stations are synchronised to ensure that they do not transmit when an adjacent base station is receiving, otherwise the better siting and possible higher power level will cause interference.
- Equivalent conditions for uplink and downlink: As both uplink and downlink use the same channel, they are subject to the same propagation conditions. With FDD systems using different frequencies for the uplink and downlink there are significant differences. By using the same frequency fading conditions can be counteracted more effectively.
UMTS TDD / FDD comparison
While UMTS TDD and UMTS FDD are both specified in the same standard and share very many properties, there are naturally some differences.
|Parameter||UMTS TDD||UMTS FDD|
|Multiple access method||TDMA, CDMA||CDMA|
|Channel spacing||5 MHz||5 MHz|
|Carrier chip rate||3.84 Mcps||3.84 Mcps|
|Time slot structure||15 / 14 slots / frame||15 slots / frame|
|Frame length (ms)||10||10|
|Multirate concept||Multicode, multislot and OVSF||Multicode, and OVSF|
|Burst types||(1) traffic bursts|
(2) random access burst
(3) synchronisation burst
|Detection||Coherent based on midamble||Coherent based on pilot symbols|
|Dedicated channel power control||Uplink: open loop 100 Hz or 200 Hz rate|
Downlink: closed loop max 800 Hz rate
|Fast closed loop 1500 Hz rate|
|Spreading factors||1 .. 16||4 .. 512|
 for TD-SCDMA the channel spacing is 1.6 MHz
 OVSF = Orthogonal variable Spreading Factor
UMTS TDD within 3GPP
All the standards for UMTS 3G systems have been defined under the auspices of 3GPP - the third generation partnership project. The standards not only define the FDD systems, but also the TDD system.
In these specifications, it was the original intent of UMTS that the TDD spectrum would be used to provide high data rates in selected areas forming what could be termed 3G hot zones.
UMTS TDD details
UMTS TDD uses many of the same basic parameters as UMTS FDD. The same 5 MHz channel bandwidths are used. UMTS TDD also uses direct sequence spread spectrum and different users and what can be termed "logical channels" are separated using different spreading codes. Only when the receiver uses the same code in the correlation process, is the data recovered. In W-CDMA all other logical channels using different spreading codes appear as noise on the channel and ultimately limit the capacity of the system. In UMTS TDD, a scheme known as multi user detection (MUD) is employed in the receiver and improves the removal of the interfering codes, allowing higher data rates and capacity.
In addition to the separation of users by using different logical channels as a result of the different spreading codes, further separation between users may be provided by allocating different time slots. There are 15 time slots in UMTS TDD. Of these, three are used for overhead such as signalling, etc and this leaves twelve time slots for user traffic. In each timeslot there can be 16 codes. Capacity is allocated to users on demand, using a two dimensional matrix of timeslots and codes.
In order for UMTS TDD to achieve the best overall performance, the transport format, i.e. the modulation and forward error correction can be altered for each user. The schemes are chosen by the network, and will depend on the signal characteristics in both directions. Higher order forms of modulation enable higher data speeds to be accommodated, but they are less resilient to noise and interference, and this means that the higher data rate modulation schemes are only used when signal strengths are high. Additionally the levels of forward error correction can be changed. When errors are likely, i.e. when signal strengths are low or interference levels are high, Similarly higher levels of forward error correction are needed under low require additional data to be sent and this slows the payload transfer rate. Thus it is possible to achieve much higher data transfer rates when signals are strong and interference levels are low.
Spectrum allocations for UMTS TDD
Standard allocations of radio spectrum have been made for 3G telecommunications systems in most countries around the globe. In Europe and many other areas spectrum has been allocated for UMTS FDD between 1920MHz to 1980MHz and 2110MHz to 2170MHz. For UMTS TDD spectrum is primarily located between 1900MHz and 1920MHz and between 2010MHz and 2025MHz. In addition to this there are some other allocations around 3 GHz.
UMTS TDD performance
UMTS TDD is able to support high peak data rates. Release 5 of the UMTS standard provides HSDPA (high-speed downlink packet access). The scheme allows the use of a higher order modulation scheme called 16-QAM (16 point quadrature amplitude modulation), which enables peak rates of 10 Mbps per sector in commercial deployments. The next release increases the modulation to 64-QAM, and introduces intercell interference cancellation (called Generalized MUD) and MIMO (multiple in, multiple out). In combination, these increase the peak rate to 31 Mbps per sector.
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