UMTS Basics

As described briefly in Chapter 4, “Third Generation (3G) Overview,” UMTS
includes two of the air interface proposals submitted to the International
Telecommunications Union (ITU) as proposed solutions to meet the requirements
laid down for International Mobile Telephony 2000 (IMT-2000). These
both use Direct Sequence Wideband CDMA (DS-WCDMA). One solution uses
Frequency Division Duplex (FDD) and the other uses Time Division Duplex
(TDD). The FDD solution is likely to see the greatest deployment—particularly
in Europe and the Americas. The TDD solution is likely to see deployment
primarily in Asia. In this chapter, we focus mainly on the FDD option.
In the FDD option, paired 5-MHz carriers are used in the uplink and
downlink as follows: uplink—1920 MHz to 1980 MHz; downlink—2110
MHz to 2170 MHz.Thus, for the FDD mode of operation, a separation of 190
MHz is used between the uplink and downlink. Although 5 MHz is the nominal
carrier spacing, it is possible to have a carrier spacing of 4.4 MHz to 5
MHz in steps of 200 kHz. This enables spacing that might be needed to
avoid interference, particularly if the next 5-MHz block is allocated to
another carrier.

For the TDD option, a number of frequencies have been defined, including
1900 MHz to 1920 MHz and 2010 MHz to 2025 MHz. Of course, with
TDD, a given carrier is used in both the uplink and the downlink so that no
separation exists.

In any CDMA system, user data is spread to a far greater bandwidth
than the user rate by the application of a spreading code, which is a higherbandwidth,
pseudo-random sequence of bits, known as chips. The transmission from each user is spread by a different spreading code, and all users
transmit at the same frequency at the same time. At the receiving end, the
signal from one user is separated from that of other users by despreading
the set of received signals with the spreading code applicable to the user in
question. The result of the despreading operation is the retrieval of the user
data in question, plus some noise generated as a result of the transmissions
from other users.

The ratio of the spreading rate (the number of chips per second) to the
user data rate (the number of user data symbols per second) is known as
the spreading factor. The greater the spreading factor, the greater the ability
to extract a given user’s signal from that of all others. In other words, for
a given user data rate, the higher the chip rate, the more users can be supported.
Alternatively, for a set number of users, the higher the chip rate, the
higher the data rates that can be supported for each user. Thus, the spreading
rate is of major significance. Of course, one gets nothing for nothing—
the higher the chip rate, the greater the occupied spectrum. The chip rate
in WCDMA is 3.84
106 chips/second (3.84 Mcps), which leads to a carrier
bandwidth of between 4.4 MHz and 5 MHz.

From a network architecture perspective, UMTS borrows heavily from
the established network architecture of GSM. In fact, many of the network
elements used in GSM are reused (with some enhancements) in UMTS.
This commonality means that a given Mobile Switching Center (MSC),
Home Location Register (HLR), Serving GPRS Support Node (SGSN), or
Gateway GPRS Support Node (GGSN) can be upgraded to support UMTS
and GSM simultaneously.

The radio access, however, is significantly different from that of GSM,
GPRS, and EDGE. In UMTS, the RAN is known as the UMTS Terrestrial
Radio Access Network (UTRAN). The components that make up the
UTRAN are significantly different from the corresponding elements in the
GSM architecture. Therefore, the reuse of existing GSM base stations and
Base Station Controllers (BSCs) is limited.

For some vendors, GSM base stations were planned in advance to be
upgradable to support WCDMA as well as GSM. Thus, for some vendors, it
is possible to remove some number of GSM transceivers from a base station
and replace them with some number of UMTS transceivers. For other vendors,
a completely new base station is needed. A similar situation applies to
BSCs. For most vendors, the technology of a UMTS Radio Network Controller
(RNC) is so different from that of a GSM BSC that the BSC cannot
be upgraded to act simultaneously as a GSM BSC and a UMTS RNC. Cases
will occur, however, where a BSC can be upgraded to simultaneously support
both GSM and UMTS, but that situation is less common.