UMTS TDD
- a summary or tutorial about the basics of UMTS TDD, the time division
duplex cellular technology that forms part of the UMTS 3G telecommunications
system
UMTS TDD (Universal mobile telecommunications system - time
division duplex) is a growing cellular technology. 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 widely 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. In this way, UMTS is a growing cellular
technology which will be far more widely used in the years to come
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
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
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, this cellular technology is also
being considered for use with providing mobile TV services, and in addition to
this there are many other data applications for which this cellular technology
could be used in a field where new methods of transport are always being sought.
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