First Generation (1G) Cellular Systems

3
First Generation (1G) Cellular
Systems
3.1 Introduction
As mentioned in Chapter 1, the first public mobile telephone system, known as Mobile
Telephone System (MTS), was introduced in 1946. Although it was considered a big tech-
nological breakthrough at that time, it suffered many limitations such as (a) the fact that
transceivers were very big and could be carried only by vehicles, (b) inefficient way of
spectrum usage and (c) manual call switching. IMTS was an improvement on MTS offering
more channels and automatic call switching.
However, the era of cellular telephony as we understand it today began with the introduc-
tion of the First Generation of cellular systems (1G systems). The major difference between
1G systems and MTS/IMTS was the use of the cellular concept in 1G, which brought about a
revolution in the area of mobile telephony. This revolution took a lot of people by surprise,
even AT&T who estimated that just 1 million cellular customers would exist by the end of the
century, instead of the many hundreds of millions that exist today.
The use of the cellular concept greatly improved spectrum usage, for the reasons
mentioned in the previous chapters. However, 1G systems are now considered technologi-
cally primitive. Nevertheless, this does not change the fact that a significant number of people
still use analog cellular phones and an analog cellular infrastructure is found throughout
North America and other parts of the world. The moral lesson from this fact is obvious
and has been seen in other areas of technology as well – the market does not entirely follow
technological developments. However, the reason why 1G systems are considered primitive
is due to the fact that they utilize analog signaling for user traffic. This leads to a number of
problems:
†
No use of encryption. The use of analog signaling does not permit efficient encryption
schemes. Therefore, 1G systems do not encrypt traffic. Thus, voice calls through a 1G
network are subject to easy eavesdropping. Another problem is the fact that, by listening to
control channels, users’ identification numbers can be ‘stolen’ and used to place illegal
calls, which are charged to the user.

†
Inferior call qualities. Analog traffic is easily degraded by interference, which results in
inferior call quality. Contrary to digital traffic, no coding or error correction is applied in
order to combat interference.
†
Spectrum inefficiency. In analog systems, each RF carrier is dedicated to a single user,
regardless of whether the user is active (speaking) or not (idle within the call). This is the
reason for the inefficient spectrum usage compared to later generations of cellular systems.
3.1.1 Analog Cellular Systems
A number of analog systems have been deployed worldwide [1]. These are briefly described
below.
3.1.1.1 United States
The first commercial analog system in the United States, known as Advanced Mobile Phone
System (AMPS), went operational in 1982 offering only voice transmission. AMPS has been
very successful and even today there are many millions of AMPS subscribers in the United
States. Furthermore, AMPS has also been deployed in Canada, Central and South America
and Australia. AMPS divides the frequency spectrum into several channels, each 30 kHz
wide. These channels are either speech or control channels. Speech channels utilize
Frequency Modulation (FM), while control channels can use Binary Frequency Shift Keying
(BFSK) at a rates of 10 kb/s. Both data messages and frequency tones are used for AMPS
control signaling. In order to combat co-channel interference, AMPS uses either (a) a typical
frequency reuse plan with a 12-group frequency cluster with omnidirectional antennas or (b)
a 7-group cluster with three sectors per cell. The operating frequency of AMPS consists of 2 £
25 ¼ 50 MHz, which are located in the 824–849 MHz and 869–894 MHz bands. In a certain
geographical region, two carriers (service providers) can coexist, with each carrier possessing
25 MHz of the spectrum (either the ‘A’ or ‘B’ band).
3.1.1.2 Europe
In European countries, several 1G systems similar to AMPS have been deployed. These
include:
†

Total Access Communications System (TACS) in the United Kingdom, Italy, Spain,
Austria and Ireland
†
Nordic Mobile Telephone (NMT) in several countries
†
C-450 in Germany and Portugal
†
Radiocom 2000 in France
†
Radio Telephone Mobile System (RTMS) in Italy.
The most popular systems are TACS and NMT, which together accounted for over 50% of
analog cellular subscribers in 1995. As in the case of AMPS, all of the above systems employ
FM for voice channels and Frequency Shift Keying (FSK) for control channels. Channels are
spaced apart and these spacings are as follows: 25 kHz (TACS, NMT-450, RTMS); 10 kHz
(C-450); and 12.5 kHz (NMT-900, Radiocom 2000). All these systems base handover deci-
sions on the power received at the Base Station (BS) by the mobile one, except for C-450,
which performs handovers based on measurements of round-trip delay.
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3.1.1.3 Japan
In Japan, a total of 56 MHz is allocated to analog cellular systems (860–885/915–940 MHz
and 843–846/898–901 MHz). The first Japanese analog cellular system was the Nippon
Telephone and Telegraph (NTT) system, which began operation in the Tokyo metropolitan
area in 1979. The system utilized 600 duplex channels (spaced 25 kHz apart), which were
realized via transmission in the 925–940 MHz (uplink) and 870–885 MHz (downlink) bands.
Voice channels were again analog and control channels were 300 bps. In 1988, this rate was
increased to 2.4 kbps and the number of channels increased to 2400 via use of frequency
interleaving (channel spacing of 6.25 kHz). This new improved system allowed backwards
compatibility, thus dual mode terminals were built that could access both the old and new
system. Currently, NTT DoCoMo provides nationwide coverage in the 870–885/925–940
MHz bands. In 1987, two new operators were introduced:

†
‘IDO’, which operates the NTT high capacity system discussed above, covering the
Kanto-Tokaido areas in the 860–863.5/915–918.5 MHz bands. IDO has also introduced
NTACS (a variant of the European TACS system) in the 843–846/898–901 MHz and
863.5–867/918.5–922 MHz bands.
†
DDI Cellular Group, which provides coverage outside the metropolitan areas using the
JTACS/NTACS systems (a variant of the European TACS system) in the 860–870/915–
925 MHz and 843–846/898–901 MHz bands.
IDO and DDI have agreed to provide nationwide service by allowing roaming between their
systems.
3.1.2 Scope of the Chapter
The remainder of the chapter examines AMPS and NMT, two representative 1G cellular
systems. Although they might seem primitive, they were very successful at the time of their
deployment and in some ways have found use as a basis for the development of several 2G
systems. An example of this is D-AMPS, which is a 2G system evolving from AMPS;
covered in Chapter 4.
3.2 Advanced Mobile Phone System (AMPS)
AMPS [1–3] is a representative 1G mobile wireless system developed by Bell Labs in the late
1970s and early 1980s. As mentioned above, it was designed to offer mobile telephone traffic
services via a number of 30 kHz channels between the Mobile Stations (MSs) and the BSs of
each cell. These 30 kHz channels are used to carry voice traffic. The latter is a 3 kHz signal
that is carried over the AMPS channels via analog transmission.
3.2.1 AMPS Frequency Allocations
The FCC made the first allocation of bandwidth for AMPS in the late 1970 in order to enable
the operation of test systems in the Chicago area. The allocated bandwidth was in the 800
MHz part of the spectrum for a number of reasons:
†
Limited spectrum was available at lower frequencies, which are primarily occupied either
First Generation (1G) Cellular Systems 97

by FM radio or television systems. Lower frequencies are sometimes used by other
systems, for example, maritime systems.
†
Despite the fact that frequencies above 800 MHz are not very densely used, allocation of
frequencies in this bands for AMPS was undesirable due to the fact that signals in those
bands (e.g. several GHz) are subject to severe attenuation either due to path loss or fading.
Such deterioration of signal qualities could not easily be handled at the time AMPS was
developed due to the fact that error correction techniques for an analog system like AMPS
were in their infancy.
†
The 800 MHz band was a relatively unused band since few systems utilized it.
3.2.2 AMPS Channels
The operating frequency of AMPS consists of 2 £ 25 ¼ 50 MHz, which are located in the
824–849 MHz and 869–894 MHz bands. In a certain geographical region two carriers
(service providers) can coexist, with each carrier possessing 25 MHz of the spectrum (either
the ‘A’ or ‘B’ band). The transmit and receive channels of each BS are separated by 45 MHz.
Both traffic channels for carrying analog voice signals and control channels exist. In a certain
geographical area, two operators can exist and a different set of channels is assigned to each
operator. The two channel sets, A and B, comprise channels from 1 to 333 and from 334 to
666, respectively. Channels from 313 to 333 and from 334 to 354 are the control channels of
bands ‘A’ and ‘B’, respectively. Thus, each operator has 312 voice channels and 21 control
channels at its disposal. Each control channel can be associated with a group of voice
channels, thus each set of voice channels (either of bands ‘A’ or ‘B’) can be split into groups
of 16 channels, each group controlled by a different control channel.
As mentioned above, traffic channels (TCs) are 30-kHz analog FM channels used to serve
voice traffic. The main traffic channels are the Forward Voice Channel (FVC) and the Reverse
Voice Channel (RVC) carrying voice traffic from the BS to the MS and from the MS to the
BS, respectively. The network assigns them to the MS upon establishment of termination of a
call.
Control channels (CCs) carry digital signaling and are used to coordinate medium access of

Mobile Stations (MSs). Specifically, each MS that is not involved in a call (idle MS) is locked
onto the strongest CC in order to receive control information. The CCs of AMPS are
summarized below:
†
The Forward Control Channel (FOCC). This is a dedicated continuous data stream that is
sent from the BS to the MS at 10 kbps. FOCC is a time division multiplexed channel
comprising three data streams: (a) streams A and B, which are identified via the least
significant bit of the MS’s Mobile Identity Number (described later), with bit 0 identifying
stream A and bit 1 identifying stream B and (b) the busy-idle stream, which is used to
indicate the status of the RECC (described below). The use of the busy-idle stream reduces
the possibilities of collisions on the RECC, as this might be used by more than one MSs.
The FOCC is also used by the BS to inform a MS which RVC to use for a newly
established call.
†
The Reverse Control Channel (RECC). This is a dedicated continuous data stream that is
sent from the MS to the BS at 10 kbps.
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AMPS used both data messages and frequency tones for control signaling. The Supervisory
Audio Tone (SAT) and the Signaling Tone (ST) are described below.
3.2.2.1 The Supervisory Audio Tone (SAT)
SAT is sent on the voice channels and is used in order to ensure link continuity and enable
MSs and BSs to possess information on the quality of the link that connects them. Both the BS
and the MS send this tone on the FVC and RCC, respectively, and the tone is added prior to
the modulation of the voice signal. When a MS is switched on or has roamed under the
coverage of a new BS, it tunes to the FOCC and reads a 2-bit field known as the SAT color
code (SCC). The value of the SCC informs the MS which SAT to expect. SAT codes are
shown in Figure 3.1. SAT determination is performed every 250 ms and the three defined
SATs are at the following frequencies: 5.97 kHz, 6 kHz and 6.03 kHz.
3.2.2.2 The Signaling Tone (ST)
The ST is used to send four signals:

†
The ‘request to send’ signal, which is used to allow the user to enter more data on the
keypad while engaged in an ongoing conversation, T;
†
The ‘alert’ signal, which, once the MS has been alerted, is continuously sent on the RVC
until the user of the MS answers the call;
†
The ‘disconnect’ signal, which is sent by the MS over the RVC in order to indicate call
termination;
†
The ‘handoff confirmation’ signal, which is sent by the MS in response to the network’s
request for handoff of this MS to another BS.
3.2.3 Network Operations
Prior to describing some basic network operations in AMPS, we describe the three identifier
numbers used in AMPS:
†
The Electronic Serial Number (ESN). The ESN is a 32-bit binary string that uniquely
identifies an AMPS MS. This number is set up by the MS manufacturer and is burned into
a Read Only Memory (ROM) in an effort to prevent unauthorized changes of this number.
The fact that this number is stored in a ROM means that the MS will become inoperable if
someone tries to rewrite the ESN. The format off an ESN is shown in Figure 3.2. It
comprises three fields: (a) part 1, comprising bits from 24 to 31; this 8-bit field is the
First Generation (1G) Cellular Systems 99
Figure 3.1 Mapping of SATS to SCC codes.
manufacturers code (MFR), which uniquely identifies each manufacturer; (b) part 2 which
comprises bits from 18 to 23 and has remained unusable; and (c) part 3, which comprises
bits 0–17, which are assigned by the manufacturer to the MS. These bits are essentially the
MSs serial number. When a manufacturer has produced so many MSs that 18 bits are no
longer able to provide additional serial numbers for its MSs, it can apply to the FCC for an
additional MFR. Thus, it can continue to produce MSs and MSs will be identified by a

different MFR/serial number combination.
†
The System Identification Numbers (SIDs). These are 15-bit binary strings that are assigned
to AMPS systems and uniquely identify each AMPS operator. SIDs are (a) transmitted by
BSs to indicate the AMPS network they belong to and (b) used by MSs to indicate either
the AMPS network they belong to (in cases of two collocated AMPS networks), or to
determine roaming situations.
†
The Mobile Identification Number (MIN). This is a 34-bit string that is derived from the
MSs 10-digit telephone number.
3.2.3.1 Initialization
Once an AMPS MS is powered up, a sequence of events takes place. This sequence is briefly
described below:
†
Event 1. The MS receives systems parameters in order to conFigure 3.itself to use one of
the two AMPS networks.
†
Event 2. The MS scans the 21 control channels of the selected AMPS network to receive
control messages. If a control channel with an acceptable quality is found, this is selected.
†
Event 3. The MS receives a message on the control channel containing system parameters.
†
Event 4. The message received in Event 3 provides the MS with information that is needed
in order to update information that was received in possible previous initializations.
Furthermore, the MS reads the SID of the AMPS network in this message, compares it
to the SID of the network it belongs to and when the MS is in the service area of another
network, the MS can prepare for roaming operations.
†
Event 5.The MS identifies itself to the network by sending its MIN, ESN and SIDS via the
RECC.

†
Event 6.The AMPS network examines the parameters transmitted by the MS in Event 5 in
order to determine whether this MS is a roaming one or not.
†
Event 7.The BS verifies initialization parameters by sending a control message to the MS.
†
Event 8.The MS enters idle state and waits for a call establishment request. During idle
mode, the MS must perform operations to (a) ensure synchronization with the BS, (b)
make the network aware of the MS’s location.
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Figure 3.2 Structure of the 32-bit ESN.