Second Generation (2G)

To better understand the issues with third-generation (3G) and the interim
2.5G radio and network access platforms, it is essential to know the fundamentals
of second-generation (2G) systems. This chapter will attempt to
cover a vast array of topics with reasonable depth and breath related to
some of the more prevalent 2G wireless mobility systems that have been
deployed.
Second-generation is the generalization used to describe the advent of
digital mobile communication for cellular mobile systems. When cellular
systems were being upgraded to 2G capabilities, the description at that
time was digital and there was little if any indication of 2G since voice was
the service to deliver, not data. Personal communication systems at the time
of their entrance were considered the next generation of communication
systems and boasted about new services that the subscriber would want
and could be readily provided by this new system or systems. However, Personal
Communication Services (PCS) took on the same look and feel as
those originating from the cellular bands.
Second-generation mobility involves a variety of technology platforms as
well as frequency bands. The issues regarding 2G deployment are as follows:

■ Capacity
■ Spectrum utilization
■ Infrastructure changes
■ Subscriber unit upgrades
■ Subscriber upgrade penetration rates

The fundamental binding issue with 2G is the utilization of digital radio
technology for transporting the information content.

It is important to note that while 2G systems utilized digital techniques
to enhance their capacity over analog, its primary service was voice communication.
At the time 2G systems were being deployed, 9.6 Kbps was
more than sufficient for existing data services, usually mobile fax. A separate
mobile data system was deployed in the United States called Cellular
Data Packet Data (CDPD), which was supposed to meet the mobile data
requirements. In essence, 2G systems were deployed to improve the voice
traffic throughput compared to an existing analog system.


Digital radio technology was deployed in cellular systems using different
modulation formats with the attempt to increase the quality and capacity
of the existing cellular systems. As a quick point of reference in an analog cellular system, the voice communication is digitized within the cell site
itself for transport over the fixed facilities to the MTSO. The voice representation
and information transfer utilized in Advanced Mobile Phone Service
(AMPS) cellular was analog and it is this part in the communication
link that digital transition is focusing on.

The digital effort is meant to take advantage of many features and techniques
that are not obtainable for analog cellular communication. Several
competing digital techniques are being deployed in the cellular arena. The
digital techniques for cellular communication fall into two primary categories:
AMPS and the TACS spectrum. For markets employing the TACS
spectrum allocation, the Global System for Mobile communications (GSM)
is the preferred digital modulation technique. However, for AMPS markets,
the choice is between Time Division Multiple Access (TDMA) and Code Division
Multiple Access (CDMA) radio access platforms. In addition to the
AMPS/TACS spectrum decision, the IDEN radio access platform is available
and it operates in the specialized mobile radio (SMR) band, which is
neither cellular or PCS. With the introduction of the PCS licenses, three
fundamental competing technologies exist, which are CDMA, GSM, and
TDMA. Which technology platform is best depends on the application
desired, and at present, each platform has its pros and cons, including if it
is a regulatory requirement to utilize one particular platform or not.
Table 3-1 represents some of the different technology platforms in the
cellular, SMR, and PCS bands.

PCS was described at the time the frequency bands were made available
as the next generation of wireless communications. PCS by default has similarities
and differences with its counterparts in the cellular band. The similarities
between PCS and cellular lie in the mobility of the user of the
service. The differences between PCS and cellular fall into the applications
and spectrum available for PCS operators to provide to the subscribers.
The PCS spectrum in the United States was made available through an
action process set up by the Federal Communications Commission (FCC).
The license breakdown is shown in Figure 3-1.

The geographic boundaries for PCS licenses are different that those
imposed on cellular operators in the United States. Specifically, PCS
licenses are defined as MTAs and BTAs. The MTA has several BTAs
within its geographic region. A total of 93 MTAs and 487 BTAs are defined
in the United States. Therefore, a total of 186 MTA licenses were awarded
for the construction of a PCS network, and each license has a total of 30
MHz of spectrum to utilize. In addition, a total of 1,948 BTA licenses were
awarded in the United States. Of the BTA licenses, the C band has 30 MHz of spectrum, while the D, E, and F blocks will only have 10 MHz
available.

Currently, PCS operators do not have a standard to utilize for picking a
technology platform for their networks. The choice of PCS standards is
daunting and each has its advantages and disadvantages. The current philosophy
in the United States is to let the market decide which standard or
standards is the best. This is significantly different than that used for cellular
where every operator has one set interface for the analog system from
which to operate.

represents various PCS systems that are used throughout the
world, particularly in the United States. The major standards utilized so far
for PCS are DCS-1900, IS-95, IS-661, and IS-136. DCS-1900 utilizes a GSM
format and is an upbanded DCS-1800 system. IS-95 is the CDMA standard
that is utilized by cellular operators, except it is upbanded to the PCS spectrum.
The IS-136 standard is an upbanded cellular TDMA system that is
used by cellular operators. IS-661 is a Time Division Duplex system offered
by Omnipoint Communications with the one notable exception that it was
supposed to be deployed in the New York market as part of the pioneer preference
license issued by the FCC.
Presently digital or digital modulation is now prevalent throughout the
entire wireless industry. Digital communication references any communication
that utilizes a modulation format that relies on sending the information
in any type of data format. More specifically, digital communication is where the sending location digitizes the voice communication and then
modulates it. At the receiver, the exact opposite is done.
Data is digital, but it needs to be converted into another medium in order
to transport it from point A to point B, and more specifically between the
base station and the host terminal. The data between the base station and
the host terminal is converted from a digital signal into RF energy. Its modulation
is a representation of the digital information that enables the
receiving device, base station, or host terminal to properly replicate the
data.
Digital radio technology is deployed in a cellular/PCS/SMR system primarily
to increase the quality and capacity of the wireless system over its
analog counterpart. The use of digital modulation techniques enables the
wireless system to transport more bit/Hz than would be possible with analog
signaling utilizing the same bandwidth. However, the service offering
for 2G is mainly a voice offering.
Figure 3-2 is a block diagram representation of the differences between
an analog and a digital radio. Reviewing the digital radio portion of the diagram,
the initial information content, usually voice, is input into the microphone
of the transmission section. The speech then is processed in a
vocoder, which converts the audio information into a data stream utilizing
a coding scheme to minimize the amount of data bits required to represent
the audio. The digitized data then goes to a channel coder that takes the
vocoder data and encodes the information even more, so it will be possible
for the receiver to reconstruct the desired message. The channel-coded
information is then modulated onto an RF carrier utilizing one of several
modulation formats covered previously in this chapter. The modulated RF
carrier is then amplified, passes through a filter, and is transmitted out an
antenna.
The receiver, at some distance away from the transmitter, receives the
modulated RF carrier though use of the antenna, which then passes the
information though a filter and into a preamp. The modulated RF carrier is
then down-converted in the digital demodulator section of the receiver to an
appropriate intermediate frequency. The demodulated information is then
sent to a channel decoder that performs the inverse of the channel coder in
the transmitter. The digital information is then sent to a vocoder for voice
information reconstruction. The vocoder converts the digital format into an
analog format, which is passed to an audio amplifier connected to a speaker
for the user at the other end of the communication path in order to listen to
the message sent. 53