UMTS TDD (Universal mobile telecommunications system - time division duplex) is a growing standard. Although UMTS TDD is not as widely deployed as the more popular UMTS FDD which is being deployed for the 3G mobile phone systems, UMTS TDD is nevertheless being used and providing a viable service for many applications. In particular it is being used to provide mobile broadband data services, and other applications may include its use in providing mobile TV applications.
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.
One of the major advantages of TDD systems such as UMTS TDD is that it is possible to vary the capacity in either direction. By altering the proportion of time allocated for transmission in each direction (downlink and uplink) it is possible to enable it to match the traffic load in each direction.
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.
UMTS TDD within 3GPP
Al 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
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.
Future
UMTS TDD, while not as widely deployed as UMTS FDD nevertheless offers significant advantages for a number of applications. While currently being used for mobile broadband, it appears as if it could serve to provide mobile TV, and other data in a filed where new methods of transport are being sought.
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.
One of the major advantages of TDD systems such as UMTS TDD is that it is possible to vary the capacity in either direction. By altering the proportion of time allocated for transmission in each direction (downlink and uplink) it is possible to enable it to match the traffic load in each direction.
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.
UMTS TDD within 3GPP
Al 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
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.
Future
UMTS TDD, while not as widely deployed as UMTS FDD nevertheless offers significant advantages for a number of applications. While currently being used for mobile broadband, it appears as if it could serve to provide mobile TV, and other data in a filed where new methods of transport are being sought.
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