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WCDMA
BackgroundThere has been a tremendous growth in wireless
communication technology over the past decade.
The significant increase in subscribers and traffic, new
bandwidth consuming applications such as gaming,
music down loading and video streaming will place
new demands on capacity. The answer to the capacity
demand is the provision of new spectrum and the
development of a new technology – Wideband CDMA
or referred to as WCDMA.
WCDMA was developed in order to create a global
standard for real time multimedia services that
ensured international roaming. With the support of ITU
(International Telecommunication Union) a specific
spectrum was allocated – 2GHz for 3G telecom
systems. The work was later taken over by the 3GPP
(3rd Generation Partnership Project), which is now
the WCDMA specification body with delegates from all
over the world.
WCDMA a development from GSM and CDMA
Naturally there are a lot of differences between
WCDMA and GSM systems, but there are many similarities
as well.
The GSM Base Station Subsystem (BSS) and the
WCDMA Radio Access Network (RAN) are both
connected to the GSM core network for providing a
radio connection to the handset. Hence, the technologies
can share the same core network.
Furthermore, both GSM BSS and WCDMA RAN
systems are based on the principles of a cellular radio
system. The GSM Base Station Controller (BSC) corresponds
to the WCDMA Radio Network Controller
(RNC). The GSM Radio Base Station (RBS) corresponds
to the WCDMA RBS, and the A-interface of
GSM was the basis of the development of the Iu-interface
of WCDMA, which mainly differs in the inclusion
of the new services offered by WCDMA.
The significant differences, apart from the lack of
interface between the GSM BSCs and an insufficiently
specified GSM Abis-interface to provide multi-vendor
operability, are more of a systemic matter. The GSM
system uses TDMA (Time Division Multiple Access)
technology with a lot of radio functionality based on
managing the timeslots. The WCDMA system on the
other hand uses CDMA, as described below, which
means that both the hardware and the control functions
are different. Examples of WCDMA-specific functions
are fast power control and soft handover.
Code Division Multiple Access and WCDMA
Code Division Multiple Access (CDMA) is a multiple
access technology where the users are separated by
unique codes, which means that all users can use the
same frequency and transmit at the same time. With
the fast development in signal processing, it has
become feasible to use the technology for wireless
communication, also referred to as WCDMA and
CDMA2000.
In cdmaOne and CDMA2000, a 1.25 MHz wide radio
signal is multiplied by a spreading signal (which is a
pseudo-noise code sequence) with a higher rate than
the data rate of the message. The resultant signal
appears as seemingly random, but if the intended
recipient has the right code, this process is reversed
and the original signal is extracted. Use of unique
codes means that the same frequency is repeated in
all cells, which is commonly referred to as a frequency
re-use of 1.
WCDMA is a step further in the CDMA technology.
It uses a 5 MHz wide radio signal and a chip rate of
3.84 Mcps, which is about three times higher than the
chip rate of CDMA2000 (1.22 Mcps).
The main benefits of a wideband carrier with a higher
chiprate are:
• Support for higher bit rates
• Higher spectrum efficiency thanks to improved
trunking efficiency (i.e. a better statistical averaging)
• Higher QoS
Further, experience from second-generation systems
like GSM and cdmaOne has enabled improvements to
be incorporated in WCDMA. Focus has also been put
on ensuring that as much as possible of WCDMA
operators’ investments in GSM equipment can be reused.
Examples are the re-use and evolution of the
core network, the focus on co-siting and the support
of GSM handover. In order to use GSM handover the
subscribers need dual mode handsets.
Radio Network Functionality
For optimal operation of a complete wireless system
i.e. from handset to radio access network (RAN) several
functions are needed to control the radio network
and the many handsets using it. All functions described
in this section, except for Handover to GSM, are
essential and therefore necessary for a WCDMA system.
Power control
The power control regulates the transmit power of the
terminal and base station, which results in less interference
and allows more users on the same carrier.
Transmit power regulation thus provides more capacity
in the network.
With a frequency re-use of 1, it is very important to
have efficient power control in order to keep the interference
at a minimum. For each subscriber service the
aim is that the base station shall receive the same
power level from all handsets in the cell regardless of
distance from the base station. If the power level from
one handset is higher than needed, the quality will be
excessive, taking a disproportionate share of the
resources and generating unnecessary interference
with the other subscribers in the network. On the other
hand, if power levels are too low this will result in poor
quality. In order to keep the received power at a suitable
level, WCDMA has a fast power control that
updates power levels 1500 times every second. By
doing that the rapid change in the radio channel is
handled. To ensure good performance, power control
is implemented in both the up-link and the down-link,
which means that both the output powers of the handset
and the base station are frequently updated.
Power control also gives rise to a phenomenon
called “cell breathing”. This is the trade-off between
coverage and capacity, which means that the size of
the cell varies depending on the traffic load. When the
number of subscribers in the cell is low (low load),
good quality can be achieved even at a long distance
from the base station. On the other hand, when the
number of users in the cell is high, the large number of
subscribers generates a high interference level and
subscribers have to get closer to the base station to
achieve good quality.
Soft and softer handover
With soft handover functionality the handset can communicate
simultaneously with two or more cells in two
or more base stations. This flexibility in keeping the
connection open to more than one base station results
in fewer lost calls, which is very important to the operator.
To achieve good system performance with a frequency
re-use of 1 and power control, soft and softer
handover is required. Soft and softer handover enables
the handset to maintain the continuity and quality of
the connection while moving from one cell to another.
During soft or softer handover, the handset will
momentarily adjust its power to the base station that
requires the smallest amount of transmit power and
the preferred cell may change very rapidly.
The difference between soft and softer handover is
that during soft handover, the handset is connected to
multiple cells at different base stations, while during
softer handover, the handset is connected to multiple
cells at the same base station. A drawback with soft
handover is that it requires additional hardware
resources on the network side, as the handset has
multiple connections. In a well-designed radio network,
30–40 % of the users will be in soft or softer
handover.
Handover to GSM (inter-system handover)
When WCDMA was standardized a key aspect was to
ensure that existing investments could be re-used as
much as possible. One example is handover between
the new (WCDMA) network and the existing (GSM)
network, which can be triggered by coverage, capacity
or service requirements.
Handover from WCDMA to GSM, for coverage reasons,
is initially expected to be very important since
operators are expected to deploy WCDMA gradually
within their existing GSM network. When a subscriber
moves out of the WCDMA coverage area, a handover
to GSM has to be conducted in order to keep the connection.
Handover between GSM and WCDMA can
also have a positive effect on capacity through the
possibility of load sharing.
If for example the numbers of subscribers in the
GSM network is close to the capacity limit in one area,
handover of some subscribers to the WCDMA network
can be performed.
Another function that is related to inter-system
handover is the compressed mode. When performing
handover to GSM, measurements have to be made in
order to identify the GSM cell to which the handover
will be made. The compressed mode is used to create
the measurement periods for the handset to make the
required measurements. This is typically achieved by
transmitting all the information during the first 5 milliseconds
of the frame with the remaining 5 milliseconds
being used for measurements on the other systems.
Inter-frequency handover(intra-system handover)
The need for inter-frequency handover occurs in high
capacity areas where multiple 5 MHz WCDMA carriers
are deployed. Inter-frequency handover, which is
handover between WCDMA carriers on different frequencies,
has many similarities with GSM handover,
for example the compressed mode functionality.
Channel type switching
In WCDMA there are different types of channels that
can be used to carry data in order to maximize the
total traffic throughput. The two most basic ones are
common channels and dedicated channels. Channel
type switching functionality is used to move subscribers
between the common and the dedicated
channel, depending on how much information the subscriber
needs to transmit.
The dedicated channel is used when there is much
information to transmit, such as a voice conversation
or downloading a web page. It utilizes the radio
resources efficiently as it supports both power control
and soft handover.
The common channel, on the other hand, is less
spectrum efficient. One benefit is that the common
channel reduces delays as many subscribers can
share the same resource. Hence it is the preferred
channel for the transfer of very limited information.
Admission control
As there is a very clear trade-off between coverage
and capacity in WCDMA systems, the admission control
functionality is used to avoid system overload and
to provide the planned coverage. When a new subscriber
seeks access to the network, admission control
estimates the network load and based on the new
expected load, the subscriber is either admitted or
blocked out. By this the operator can maximize the
network usage within a set of network quality levels,
i.e. levels depending on what kind of service/information
the subscriber wants to use.
Congestion control
Even though an efficient admission control is used,
overload may still occur, which is mainly caused by
subscribers moving from one area to another area.
If overload occurs, four different actions can be taken.
First, congestion control is activated and reduces the
bit rate of non real-time applications, to resolve the
overload. Second, if the reduced bit rate activity is not
sufficient, the congestion control triggers the inter- or
intra-frequency handover, which moves some sub-
scribers to less loaded frequencies. Third, handover of
some subscribers to GSM and fourth action is to discontinue
connections, and thus protect the quality of
the remaining connections.
Synchronization
One of the basic requirements when WCDMA was
standardized was to avoid dependence on external
systems for accurate synchronization of base stations.
This has been achieved by a mechanism, where the
handset, when needed, measures the synchronization
offset between the cells and reports this to the network.
In addition, there is also an option to use an
external source, such as GPS, for synchronizing the
nodes, i.e. to always provide the best solution both
asynchronous and synchronous nodes are supported
Basic architecture concepts/ System overview
In this section some fundamental views of the
WCDMA Radio Access Network will be presented.
This includes the WCDMA RAN architecture itself, the
radio interface protocol architecture, the Radio Access
Bearer concept and the role of the transport network
in a WCDMA RAN.
Radio Access Network (RAN) Architecture
The main purpose of the WCDMA Radio Access
Network is to provide a connection between the handset
and the core network and to isolate all the radio
issues from the core network. The advantage is one
core network supporting multiple access technologies.
The WCDMA Radio Access Network consists of two
types of nodes:
Radio Base Station (Node B)
The Radio Base Station handles the radio transmission
and reception to/from the handset over the radio
interface (Uu). It is controlled from the Radio Network
Controller via the Iub interface. One Radio Base
Station can handle one or more cells.
Radio Network Controller (RNC)
The Radio Network Controller is the node that controls
all WCDMA Radio Access Network functions. It
connects the WCDMA Radio Access Network to the
core network via the Iu interface. There are two distinct
roles for the RNC, to serve and to control. The Serving
RNC has overall control of the handset that is connected
to WCDMA Radio Access Network. It controls
the connection on the Iu interface for the handset and
it terminates several protocols in the contact between
the handset and the WCDMA Radio Access Network.
The Controlling RNC has the overall control of a particular
set of cells, and their associated base stations.
When a handset must use resources in a cell not controlled
by its Serving RNC, the Serving RNC must ask
the Controlling RNC for those resources. This request
is made via the Iur interface, which connects the RNCs
with each other. In this case, the Controlling RNC is
also said to be a Drift RNC for this particular handset.
This kind of operation is primarily needed to be able to
provide soft handover throughout the network.
Radio Access Bearers
The main service offered by WCDMA RAN is the
Radio Access Bearer (RAB). To establish a call connection
between the handset and the base station a
RAB is needed. Its characteristics are different
depending on what kind of service/information to be
transported.
The RAB carries the subscriber data between the
handset and the core network. It is composed of one
or more Radio Access Bearers between the handset
and the Serving RNC, and one Iu bearer between the
Serving RNC and the core network.
3GPP has defined four different quality classes of
Radio Access Bearers:
• Conversational (used for e.g. voice telephony)
– low delay, strict ordering
• Streaming (used for e.g. watching a video clip)
– moderate delay, strict ordering
• Interactive (used for e.g. web surfing)
– moderate delay
• Background (used for e.g. file transfer)
– no delay requirement
Both the Conversational and Streaming RABs
require a certain reservation of resources in the network,
and are primarily meant for real-time services.
They differ mainly in that the Streaming RAB tolerates
a higher delay, appropriate for one-way real-time
services.
The Interactive and Background RABs are so called
‘best effort’, i.e. no resources are reserved and the
throughput depends on the load in the cell. The only
difference is that the Interactive RAB provides a priority
mechanism.
The RAB is characterized by certain Quality of
Service (QoS) parameters, such as bit rate and delay.
The core network will select a RAB with appropriate
QoS based on the service request from the subscriber,
and ask the RNC to provide such a RAB.
Transport in WCDMA Radio Access Network
The WCDMA Radio Access Network nodes communicate
with each other over a transport network. The
3GPP specification provides a very clear split between
radio related (WCDMA) functionality and the transport
technology, meaning that there is no particular bias to
any technology. The transport network is initially based
on ATM, but IP will soon be included as an option.
Radio Interface Overview
The protocol stack of the radio interface between
WCDMA Radio Access Network and the handset
consists of a number of protocol layers, each giving
a specific service to the next layer above. The main
purpose with each layer is as follows:
Layer 3: Signaling to control the connection to the
handset.
Layer 2: If there is time for it, to retransmit packets
which has been received in error.
Layer 1: To transmit and receive data over the radio,
including basic protection against bit errors.
The Physical Layer (Layer 1) offers Transport
Channels to the MAC layer. There are different types
of transport channels with different characteristics of
the transmission. Common transport channels can be
shared by multiple handsets (e.g. FACH, RACH,
DSCH, BCH, PCH). Dedicated transport channels
(DCH) are assigned to only one handset at a time.
The transmission functions of the physical layer
include channel coding and interleaving, multiplexing
of transport channels, mapping to physical channels,
spreading, modulation and power amplification, with
corresponding functions for reception.
A frequency and a code characterize a physical
channel. The specifications include two modes: the
FDD mode (Frequency Division Duplex) and the TDD
mode (Time Division Duplex). The FDD mode is the
mainstream mode that operators are now deploying in
WCDMA. The TDD mode may eventually be deployed
as well, as a complement to the FDD mode. This document
does not describe the TDD mode.
The Medium Access Control (MAC) protocol
(Layer 2) offers logical channels to the layers above.
The logical channels are distinguished by the different
type of information they carry, and thus include the
Dedicated Control Channel (DCCH), Common Control
Channel (CCCH), Dedicated Traffic Channel (DTCH),
Common Traffic Channel (CTCH), Broadcast Control
Channel (BCCH) and the Paging Control Channel
(PCCH). The MAC layer performs scheduling and map-
ping of logical channel data onto the transport channels
provided by the physical layer. Also, for common
transport channels, the MAC layer adds addressing
information to distinguish data flows intended for different
handsets. One major difference to GSM is the
possibility to dynamically switch one logical channel
(data flow) onto different transport channel types, e.g.
based on the activity of the subscriber. This is called
channel type switching.
The Radio Link Control (RLC) protocol (Layer 2)
operates in one of three modes: transparent, unacknowledged
or acknowledged mode. It performs
segmentation/re-assembly functions and, in acknowledged
mode, provides an assured mode delivery service
by use of retransmission. RLC provides a service
both for the RRC signaling (the Signaling Radio
Bearer) and for the user data transfer (the Radio
Access Bearer).
The Radio Resource Control (RRC) protocol
(Layer 3) provides control of the handset from the
RNC. It includes functions to control radio bearers,
physical channels, mapping of the different channel
types, handover, measurement and other mobility
procedures. Because of the flexibility of the WCDMA
radio interface, this is a fairly complex protocol.
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2004 © Vovida Research, All rights reserved. |
