The Evolution and Future of
Mobile Communication Systems
Written by David G Ainscough
Copyright 2001 D.G.Ainscough
4.1 WCDMA
4.2 Carrier Spacing and Deployment Scenarios ..................................5
4.3 Logical Channels ...........................................................................6
4.4 Control Channels ...........................................................................6
4.5 Uplink Physical Channels ..............................................................7
4.6 Downlink Physical Channels..........................................................8
4.7 Packet Data ...................................................................................9
4.8 Handovers ...................................................................................11
4.8.1 Soft Handovers ..................................................................... 11
Chapter Summary and Key points ..................................................... 13
Chapter 4 : WCDMA (Wideband CDMA)
4.1 WCDMA
WCDMA of 3G potentially will offer the user significantly increased data
throughput rates.
Speeds of up to 2mbit/s and higher are being
predicted but it will be sometime before these are achieved.
New
techniques such as voice over IP are being developed for use on the
Transport Infrastructure and will bring significant advantages to the
operator especially in terms of data compression techniques used in
such transmission modes as ATM. Increased data throughput means
greater capacity for a given amount of frequency spectrum and thus
lower costs for both Infrastructure and spectrum licences. Potentially the
benefits of WCDMA to the user and operator are massive with greater
data rates for the user and reduced costs for the operator.
4.2 Carrier Spacing and Deployment Scenarios
The carrier spacing has a range of 200 kHz and can differ from 4.2 to 5.4
MHz, the different carrier spacing can be used to obtain suitable adjacent
channel protections depending on the interference scenario. Figure 30
shows an example, where the bandwidth of 15 MHz with three cell
layers. Larger carrier spacing can be applied between operators, than
within one operators band in order to avoid inter-operator interference.
Interfrequency measurements and handovers are supported by WCDMA
to make use of three cell layers and carriers.
Power
Operator Band 15Mhz
3 Cell Layers
Another WCDMA
Operator
4.2 - 5.0 MHz
4.2 - 5.0 MHz
5.0 - 5.4 MHz
Another WCDMA
Operator
5.0 - 5.4 MHz
Frequency
Figure 1 Frequency utilization with WCDMA
(Adapted from Holma, H. et al 2000)
4.3 Logical Channels
WCDMA mainly follows the ITU Recommendation M.1035 in the
definitions of logical channels.
The following logical channels are
defined for WCDMA.
4.4 Control Channels
BCCH : Broadcast Control Channel carries system and cell specific
channels;
PCH : Paging Channel for messages to the mobiles in the paging area;
FACH : Forward Access Channel for messages from the base station to
the mobile in one cell.
In addition to the two Control Channels there are also two Dedicated
Channels;
DCCH : Dedicated Control Channel, this covers two channels : standalone dedicated control channel (SDCCH), and the Associated Control
Channel (ACCH);
DTCH : Dedicated Traffic Channel for point to point data transmission in
the uplink and downlink.
4.5 Uplink Physical Channels
There are two dedicated channels and one common channel for the
uplink. User data is transmitted on one of these channels, the Dedicated
Physical Data Channel (DPDCH), as is the control information.
The
Random Access Channel is a common access channel.
The Dedicated Physical Control Channel (DPCCH) is needed to transmit
pilot symbols for coherent reception, power signaling bits, and rate
information for rate detection. The two basic solutions for multiplexing
physical control and data channels are time multiplexing and code
multiplexing.
A combined IQ and code multiplexing solution (dual-
channel QPSK) is used in the WCDMA uplink and to avoid
Electromagnetic Compatibility (EMC) problems with Discontinuous
Transmission (DTX).
There is a major drawback of a time multiplexed control channel, that is
where there are EMC problems that arise when DTX is used for data
transmission.
Figure 31 shows an example of DTX during normal
speech transmission. During periods when there is silence there is no
need for information bits to be transmitted.
This results in pulsed
transmission of the of the control data that has to be transmitted.
Data Present
DPCCH : Pilot +power control
Data Absent
Absent Data
Data Present
DPPCH + Data
Figure 2 Pulsed Transmission when time multiplexed control channel
(Adapted from Holma, H. et al 2000)
The Random Access Burst is made up of two parts; a preamble which is
made up of 16c256 chip (1ms) and a data part but data parts length is
not a fixed length and this is what gives WCDMA its variable data rates.
The random access system is based on slotted ALOHA technique;
Figure 32 shows the structure of the random access burst.
Before the transmission happens the Mobile Station (MS) has to carry
out a number of tasks:
Achieve chip, slot, and frame synchronization to target BSS from the
SCH;
Retrieve information from the BCCH about the random access code
used in the target cell/sector;
Estimate the downlink path loss, which is used together with a signal
strength target to calculate the required transmit power of the random
access request.
MS ID
16 X 256 chips
Reg. Service
CRC
Variable length
User Packet
Data Part
Preamble Part
Figure 3 The structure of a Random Access Burst in a WCDMA System
4.6 Downlink Physical Channels
There are three physical channels in the downlink; the primary and
secondary Common Control Physical Channels (CCPCH) carry the
downlink for the common control logical channels (BCCH, PCH and
FACH); the SCH provides timing information and in WCDMA is used for
handover measurements by the Mobile Station (MS).
The dedicated channels (DPDCH and DPCCH) are multiplexed, however
this means that there is an EMC problem caused by discontinuous
transmission, but this is not considered a significant problem to the
downlink, for two reasons :
There are signals to several users transmitted in parallel and at the same
time.
Base Stations are not so close to other electrical equipment.
4.7 Packet Data
There are two very different types of packet data transmission within the
WCDMA system.
Short data packets can be added to the random
access bursts and this method is called Common Channel Packet
Transmission (CCPT).
CCPT is most commonly used for short
infrequent packets, where link maintenance needed for a dedicated
channel would lead to an unacceptable overhead. Another factor for the
use of CCPT cuts out the delay that can be associated with a dedicated
channel.
It should also be noted that for common channel packet
transmission only open loop power control is in operation (Open Loop
Power control measures the interference conditions from the channel,
and adjusts the transmission power adoringly to meet the desired frame
error rate (Ojanpera, T etal. 1998)). CCPT should therefore be limited to
short packets that only use a limited capacity. This is shown in Figure
33.
RACH
Burst
User
Packet
Time between
Packets
RACH
Burst
User
Packet
Common Channel without fast power control
Figure 4 Packet Transmission on the Common Channel
For larger or more frequent packets transmitted on a dedicated channel,
a large single packet is transmitted using a single packet system, where
a dedicated channel is released immediately after the packet has been
transmitted. In a multipacket system the dedicate channel is maintained
by transmitting power control and synchronization information between
subsequent packets.
4.8 Handovers
The WCDMA systems base stations do not need to be synchronized,
and hence, there is no need for an external source of synchronization
such as GPS.
Asynchronous base stations have to be considered,
especially when designing soft handover algorithms and when
implementing position location services.
Before a soft handover happens, the MS measures observed timing
differences of the downlink SCHs from the two base stations. The timing
of a new downlink soft handover connection is adjusted with a resolution
of one symbol. For example, dedicated downlink signals from the two
base stations that are synchronized with an accuracy of one symbol.
This enables the mobile RAKE receiver to collect the micro diversity
energy from the two base stations. From this the timing adjustments of
dedicated downlink channels can be carried out with a resolution of one
symbol without losing orthogonality of downlink codes.
4.8.1 Soft Handovers
Soft handovers with WCDMA are very different to the handovers used in
the GSM system and this basic principle stems from the US system
CDMA. Basically the MS is connected to more than one BTS (Base
Station Transceiver) at a time.
The reason for this is to reduce
interference into other cells (BTSs).
It also has the advantage of
improving performance through macro diversity.
Figure 34 illustrates a soft handover principle. The uplink signal fro the
MS is received by both BTSs, which, after demodulation and combining,
pass the signal forward to the combining point, typically this is the BSC.
From a downlink point of view the same information is transmitted via
both BTSs, and the MS receives the information from both of the BTSs
as a separate multipath signals and can therefore combine them.
BSC
BTS
BTS
Figure 5 Principle of soft handover with two base station transceivers (BTS)
Infrequent handovers are needed for utilizations of a hierarchical call
structures; macro, micro and indoor cells. Several carriers and interfrequency handovers may also be used for taking care of high capacity
needs in hot spots.
Infrequent Handovers will be needed also for
handovers to the 2G systems (See UMTS). There are two methods that
are being considered for WCDMA, (1) Dual Receiver and, (2) Slotted
Mode.
Chapter Summary and Key points
GPRS saw the realization of the maximum data rate that can be attained
using the GSM system.
So there was a need for Wideband CDMA
(WCDMA), this is an enhanced version of the mobile communication
system that is used in the United States. The main use for WCDMA in
the UK will be for mobile data communications, this is because the UK
already has an effective voice system in the GSM system.
It should be noted that WCDMA has several major drawbacks, the
biggest being that with WCDMA used a form of DTX, this DTX causes
EMC (Electromagnetic Compatibility) problems. The DTX is used in both
Voice and Data transmission, but the problem only occurs when the MS
is in Data transmission mode.
WCDMA is not envisaged to by applied in the UK for the next 3-4 years,
this will probably be around the same time that UMTS is made available,
however the potential benefits of WCDMA to the user and operator are
massive with greater data rates for the user and reduced costs for the
operator.
WCDMA
is
an
Telecommunications
integral
part
System).
of
UMTS
UMTS
is
(Universal
an
Mobile
integration
of
communications systems, and will hopefully provide a solid base for
mobile communications in the future.
