the air interface, frequencies, spreading and power control used
within UMTS or Wideband CDMA, WCDMA, cellular telecommunications system
Physical layer within UMTS / WCDMA is totally different to
that employed by GSM. It employs a spread spectrum transmission in the form of
CDMA rather than the TDMA transmissions used for GSM. Additionally it currently
uses different frequencies to those allocated for GSM.
Frequencies
There are currently six bands that are specified for use for UMTS / WCDMA
although operation on other frequencies is not precluded. However much of the
focus for UMTS is currently on frequency allocations around 2 GHz. At the World
Administrative radio Conference in 1992, the bands 1885 2025 and 2110 2200
MHz were set aside for use on a world wide basis by administrations wishing to
implement International Mobile Telecommunications-2000 (IMT-2000). The aim was
that allocating spectrum on a world wide basis would facilitate easy roaming for
UMTS / WCDMA users.
Within these bands the portions have been reserved for
different uses:
- 1920-1980 and 2110-2170 MHz Frequency Division Duplex (FDD, W-CDMA)
Paired uplink and downlink, channel spacing is 5 MHz and raster is 200 kHz.
An Operator needs 3 4 channels (2x15 MHz or 2x20 MHz) to be able to build
a high-speed, high-capacity network.
- 1900-1920 and 2010-2025 MHz Time Division Duplex (TDD, TD/CDMA)
Unpaired, channel spacing is 5 MHz and raster is 200 kHz. Transmit and
receive transmissions are not separated in frequency.
- 1980-2010 and 2170-2200 MHz Satellite uplink and downlink.
Carrier frequencies are designated by a UTRA Absolute Radio
Frequency Channel Number (UARFCN). This can be calculated from:
UARFCN = 5 x (frequency in MHz)
UMTS uses wideband CDMA as the radio transport mechanism. The
channels are spaced by 5 MHz. The modulation that is used is different on the
uplink and downlink. The downlink uses quadrature phase shift keying (QPSK) for
all transport channels. However the uplink uses two separate channels so that
the cycling of the transmitter on and off does not cause interference on the
audio lines, a problem that was experienced on GSM. The dual channels (dual
channel phase shift keying) are achieved by applying the coded user data to the
I or In-phase input to the DQPSK modulator, and control data which has been
encoded using a different code to the Q or quadrature input to the modulator.
Spreading
The data to be transmitted is encoded using a spreading code particular to a
given user. In this way only the desired recipient is able to correlate and
decode the signal, all other signals appearing as noise. This allows the
physical RF channel to be used by several users simultaneously.
The data of a CDMA signal is multiplied with a chip or
spreading code to increase the bandwidth of the signal. For WCDMA, each physical
channel is spread with a unique and variable spreading sequence. The overall
degree of spreading varies to enable the final signal to fill the required
channel bandwidth. As the input data rate may vary from one application to the
next, so the degree of spreading needs to be varied accordingly.
For the downlink the transmitted symbol rate is 3.84 M
symbols per second. As the form of modulation used is QPSK this enables two bits
of information to be transmitted for every symbol, thereby enabling a maximum
data rate of twice the symbol rate or 7.68 Mbps. Therefore if the actual rate of
the data to be transmitted is 15 kbps then a spreading factor of 512 is required
to bring the signal up to the required chip rate for transmission in the
required bandwidth. If the data to be carried has a higher data rate then a
lower spreading rate is required to balance this out. It is worth remembering
that altering the chip rate does alter the processing gain of the overall system
and this needs to be accommodated in the signal processing as well. Higher
spreading factors are more easily correlated by the receiver and therefore a
lower transmit power can be used for the same symbol error rate.
The codes required to spread the signal must be orthogonal if
they are to enable multiple users and channels to operate without mutual
interference. The codes used in W-CDMA are Orthogonal Variable Spreading Factor
(OVSF) codes, and they must remain synchronous to operate. As it is not possible
to retain exact synchronisation for this, a second set of scrambling codes is
used to ensure that interference does not result. This scrambling code is a
pseudo random number (PN) code. Thus there are two stages of spreading. The
first using the OSVF code and the second using a scrambling PN code. These codes
are used to provide different levels of separation. The OVSF spreading codes are
used to identify the user services in the uplink and user channels in the
downlink whereas the PN code is used to identify the individual node B or UE.
On the uplink there is a choice of millions of different PN
codes. These are processed to include a masked individual code to identify the
UE. As a result there are more than sufficient codes to accommodate the number
of different UEs likely to access a network. For the downlink a short code is
used. There are a total of 512 different codes that can be used, one of which
will be assigned to each node B.
Synchronisation
The level of synchronisation required for the WCDMA system to operate is
provided from the Primary Synchronisation Channel (P-SCH) and the Secondary
Synchronisation Channel (S-SCH). These channels are treated in a different
manner to the normal channels and as a result they are not spread using the
OVSFs and PN codes. Instead they are spread using synchronisation codes. There
are two types that are used. The first is called the primary code and is used on
the P-SCH, and the second is named a secondary code and is used on the S-SCH.
The primary code is the same for all cells and is a 256 chip
sequence that is transmitted during the first 256 chips of each time slot. This
allows the UE to synchronise with the base station for the time slot.
Once the UE has gained time slot synchronisation it only
knows the start and stop of the time slot, but it does not know information
about the particular time slot, or the frame. This is gained using the secondary
synchronisation codes.
There is a total of sixteen different secondary
synchronisation codes. One code is sent at the beginning of the time slot, i.e.
the first 256 chips. It consists of 15 synchronisation codes and there are 64
different scrambling code groups. When received, the UE is able to determine
before which synchronisation code the overall frame begins. In this way the UE
is able to gain complete synchronisation.
The scrambling codes in the S-SCH also enable the UE to
identify which scrambling code is being used and hence it can identify the base
station. The scrambling codes are divided into 64 code groups, each having eight
codes. This means that after achieving frame synchronisation, the UE only has a
choice of one in eight codes and it can therefore try to decode the CPICH
channel. Once it has achieved this it is able to read the BCH information and
achieve better timing and it is able to monitor the P-CCPCH.
Power Control
As with any CDMA system it is essential that the base station receives all the
UEs at approximately the same power level. If not, the UEs that are further away
will be lower in strength than those closer to the node B and they will not be
heard. This effect is often referred to as the near-far effect. To overcome this
the node B instructs those stations closer in, to reduce their transmitted
power, and those further away to increase theirs. In this way all stations will
be received at approximately the same strength.
It is also important for node Bs to control their power
levels effectively. As the signals transmitted by the different node Bs are not
orthogonal to one another it is possible that signals from different ones will
interfere. Accordingly their power is also kept to the minimum required by the
UEs being served.
To achieve the power control there are two techniques that
are employed: open loop; and closed loop.
Open loop techniques are used during the initial access
before communication between the UE and node B has been fully established. It
simply operates by making a measurement of the received signal strength and
thereby estimating the transmitter power required. As the transmit and receive
frequencies are different, the path losses in either direction will be different
and therefore this method cannot be any more than a good estimate.
Once the UE has accessed the system and is in communication
with the node B, closed loop techniques are used. A measurement of the signal
strength is taken in each time slot. As a result of this a power control bit is
sent requesting the power to be stepped up or down. This process is undertaken
on both the up and downlinks. The fact that only one bit is assigned to power
control means that the power will be continually changing. Once it has reached
approximately the right level then it would step up and then down by one level.
In practice the position of the mobile would change, or the path would change as
a result of other movements and this would cause the signal level to move, so
the continual change is not a problem.
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