WCDMA Basics
Unlike
GSM and GPRS, which rely on the use of the TDMA protocol, WCDMA – like
CDMA - allows all users to transmit at the same time and to share the
same RF carrier. Each mobile user’s call is uniquely differentiated
from other calls by a set of specialized codes added to the
transmission. WCDMA base stations differ from some of the other CDMA
systems in that they do not have to be in system-wide time
synchronization, nor do they depend on a Global Positioning System
(GPS) signal. Instead, they work by transmitting a sync signal along
with the downlink signal. A downlink or forward link is defined as the
RF signal transmitted from the base station to the subscriber mobile
phone. It consists of the RF channel, scrambling code (one per sector),
an orthogonal variable spreading factor (OVSF) channel for signaling
(one per call), and one or more OVSF channels for data (Figure 2). It
also contains the sync signals (P-SCH and S-SCH), which are independent
of OVSF and scrambling codes. The RF signal transmitted from the mobile
phone is referred to as the uplink or reverse channel.
Fig
.2. WCDMA channel structure
The WCDMA
downlink and uplink data streams run at a constant 3.84 Mcps, are
divided into time slots and grouped as frames. The frame is the basic
unit of data information that the system works with in the coding,
interleaving and transmitting processes. Data transmitted via a WCDMA
network – whether digitized voice or actual data – is spread using a
code which is running at a 3.84 Mbps code rate. Once the transmitted
data is received by the subscriber’s mobile receiver, its demodulator/correlator reapplies the code and recovers the original
data (Figure 3). The signal received by the mobile is a spread signal
together with noise, interference and messages on other code channels
in the same RF frequency slot. The interference may emanate from
multiple sources including other users in the same cell or from
neighboring cells.
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WCDMA has two basic modes of operation:
• Frequency Division Duplex (FDD) mode. Here separate frequencies are
used for uplink and downlink. FDD is currently being deployed and is
usually referred to as WCDMA.
• Time Division Duplex (TDD) mode. In this mode, the uplink and
downlink are carried in alternating bursts on a single frequency. Note
that this Application Note focuses on FDD systems only.
One
of the important features of a WCDMA system is its highly adaptive
radio interface. WCDMA is designed to allow many users to efficiently
share the same RF carrier by dynamically reassigning data rates. The
spreading factor (SF) may be updated as often as every 10 ms, which in
turn, permits the overall data capacity of the system to be used more
efficiently.
Some of the key things to remember about WCDMA are:
• In WCDMA, the RF signal from each base station sector is “scrambled”
by multiplying the data and voice channels by a unique pseudo-noise
code, known as the Scrambling Code. The Scrambling Code is mixed prior
to the output of a base station or the output of a subscriber’s mobile
unit. WCDMA base stations (Node B’s) use one of 512 Scrambling Codes to
uniquely identify each sector in the network.
• Adjacent base stations use the same RF frequency for spectral
efficiency. WCDMA employs a frequency reuse method in which the same
frequency is used at every site, with forward links separated from one
another by Scrambling Codes.
• WCDMA uses channelization codes, known as OVSF codes or Spreading
Codes, to uniquely identify a Dedicated Physical Channel (DPCH) user
channel. At the receiver, the received RF signal passes through the
correlator, that separates and identifies the code channels (pilot,
signaling or user data/voice) of each WCDMA channel it sees. Other
spreading code channels are used for the pilot (P-CPICH), signaling,
user voice or user data. Higher user data rates can be achieved by
shortening the spreading factor, thereby increasing the transmission
rate. Note that the synchronization channels, P-SCH and S-SCH, do not
go through the OVSF spreading process. The OVSF codes are orthogonal
codes used to separate traffic in a WCDMA signal. Any mobile phone that
receives a transmitted data sequence and attempts to demodulate it
using the “wrong” orthogonal code, would interpret the information as
noise. The noise, when integrated over time, will net to zero. As a
result, interfering signals not intended for a given mobile phone are
effectively eliminated by signal processing in the mobile phone’s
receiver. The OVSF codes can be reused by each base station and mobile
phone within the same location, since the scrambling codes identify the
transmitting device.
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WCDMA Versus GSM
GSM
was the first digital cellular system. It uses TDMA as its air
interface standard and Gaussian Modulated Shift Keying (GMSK) on the RF
air interface. GSM systems in Europe operate in 900 and 1800 MHz bands,
while in the United States they operate in the 800 MHz (cellular) and
1900 MHz Personal Communications Services (PCS) bands.
There
are many similarities between WCDMA and GSM systems including the fact
that both the GSM Base Station Subsystem (BSS) and the WCDMA Radio
Access Network (RAN) provide a radio connection to the handset via the
same GSM core network (Figure 4). Both are also based on the principles
of a cellular radio system. The GSM Base Station Controller (BSC)
corresponds to the WCDMA Radio Network Controller (RNC), while the GSM
Radio Base Station (RBS) corresponds to the WCDMA RBS.
The
significant differences between the two standards, apart from the lack
of an interface between the GSM BSCs and an insufficiently specified
GSM Abis-interface to provide multi-vendor operability, include the
following:
• GSM uses TDMA technology with a lot of radio
functionality based on managing the timeslots. WCDMA systems use CDMA
technology in which both the hardware and control functions are
different.
• GSM was created with voice as the primary application. WCDMA includes
support for voice, high-speed packet data and multimedia applications.
• The underlying WCDMA air interface is much more performance sensitive
and its operation shares many more similarities with its rival CDMA2000
than its predecessor GSM. To achieve link-level performance gains over
GSM's equalization and frequency hopping techniques, WCDMA uses rake
receiver technology for diversity gain.
• WCDMA employs a fast power control scheme — 1500 Hz on both the up
and downlink — to deal with CDMA's inherent near-far interference
issues. GSM, which features a hard capacity due to its fixed frequency
reuse scheme, employs a very slow (2 Hz) power control scheme.
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