BAB 9

DIGITAL MOBILE TELEPHONY

A basic concept of a cellular system is to provide ever-increasing capacity by dividing cells into
smaller and smaller sizes to increase frequency reuse. Unfortunately, the cell division concept has proven to
be impractical in terms of finding suitable locations for base station antennas and for getting repeated
construction authorizations from gov• erning organizations. The explosive demand for mobile telephones
in the early 1990s within the United States and elsewhere in the world helped stimulate the development of
new systems to accommodate the demand. Two basic approaches have been pur• sued: expanding the
channel capacity of existing systems and allocating new fre• quency bands to cellular mobile phone
service. All of the new systems utilize digital transmission in lieu of the analog FDM transmission used by
analog cellular systems such as AMPS in North America, TACS in Great Britain, and NMT in
Scandinavia.
Commercially viable digital cellular systems are enabled by the availability of low• cost digital signal
processing technology to provide solutions to problems in several key areas. First, it is necessary to
compress a digital speech signal into a low enough bit rate that digital voice transmission does not impose a
spectrum penalty with respect to analog systems. As discussed in Chapter 3, speech compression algorithms
have ad• vanced to the point that digitization can, in some cases, provide greater spectrum ef• ficiency than
analog systems. Second, the application of digital transmission to a mobile environment requires a
sophisticated equalizer to overcome the effects of dy• namic multipath transmission impairments. Third,
the susceptibility of the speech compression algorithms to channel errors requires sophisticated error
correction and control measures. Lastly, the low-bit-rate voice coders and the digital system archi• tectures
introduce significant artificial delay into the voice channel, which imposes the need for echo cancelers for
acceptable voice quality.

9.1 NORTH AMERICAN DIGITAL CELLULAR

North American Digital Cellular (NADC), also known as US Digital Cellular (USDC) or Digital-
AMPS (D-AMPS), represents a digital upgrade from the previously de• ployed analog cellular system
referred to as Advanced Mobile Phone Service.
DIGITAL MOBILE TELEPHONY (AMPS). The 0-AMPS system is designed to be compatible
with AMPS. In fact, a D-AMPS system installation can coexist with an AMPS installation, thus allowing a
,.
graceful migration from an all-analog service to an all-digital service. An analog-to• digital migration is
supported by a dual-mode phone that can operate as an AMPS phone in one call and as a D-AMPS phone
on the next call. D-AMPS is standardized by EIA!TIA as Interim Standards IS-54 and IS-136.

9.1.1 D·AMPS Transmission Format

The most significant aspect of maintaining compatibility with AMPS is the need to adhere to the
AMPS FDM channel structure. This channel structure uses 30-kHz-wide channels in the range 824-894
MHz. Within each 30-kHz FDM channel IS-54 defines six digital channels operating in a time division
multiple-access (TOMA) mode of op• eration, as shown in Figure 9. I. Transmission from a base station to
the mobiles is ac• complished with a continuous TOM stream with six time slots. Transmission from
each of the mobiles occurs in data bursts that are timed to arrive at the base station in separate,
nonoverlapping time slots synchronized to the outgoing time slots. Associ• ated with each burst from a
mobile is a guard time to prevent overlap and provide a transmitter ramp-up preceding the data. The guard
time between time slots is mini• mized by adjusting the transmit time of the mobiles with control
messages from the base station. These adjustments are dynamic to accommodate mobility.
The TOMA digital transmission frame format within each 30-kHz channel con• tains six time
slots with a total of 1944 bits. The repetition rate of each frame is 25 frames per second, which leads to
an aggregate bit rate of 48.6 kbps in the 30 kHz of bandwidth. The modulation format is 7t/4 shifted,
differentially encoded, quadrature phase shift keying. This format is essentially 4-PSK modulation with
two four-point constellations offset from each other by 7t/4 radians. By alternating between constel• lations,
a symbol transition of at least 1ri4 radians is always assured-a property that helps in symbol clock
recovery.
Full-rate voice coding utilizes two time slots in each frame for the voice informa•
tion. Thus, the system capacity with full-rate voice coding is three times that of an
AMPS system since there are three TDM voice channels within each FDM channel.
9.1 NORTH AMERICAN DIGITAL CELLULAR

If half-rate voice coding gets implemented in the future, the capacity expansion will be sixfold.

9.1.2 D-AMPS Speech Coding

The speech-coding algorithm is vector sum excited linear predictive (VSELP) coding [l], which is
described in Chapter 3. The D-AMPS VSELP algorithm processes the speech waveform in segments
of 20 msec duration. Each speech segment is repre• sented by 159 bits. Since there are two VSELP
frames in each TOMA frame, the raw data rate of the voice is 2 x 159 x 25 = 7950 bps. To the raw bit
date is added 5050 bps ofredundancy encoding for error correction and detection to produce a
composite, aggregate data rate of 13 kbps for a voice channel. As shown in Figure 9.2, there are
260 data channel bits and 64 overhead bits in each time slot. Table 9.1 describes the basic use of
each data field within the time slots.

9.1.3 D-AMPS Control Channel

In addition to providing a threefold capacity expansion, the digital nature of D-AMPS provides other
advantages that are not possible or at least difficult to achieve in an ana• log system such as AMPS.
The first of these is use of the CDVCC channel to ensure that a base station maintains connections
with intended mobiles. AMPS utilizes a similar feature with supervisory audio tones (SATs). A
SAT is a tone at 5970, 6000, or 6030 Hz that is inserted and removed from the audio signal
specifically for detect• ing fades and ascertaining connection integrity. The availability of only
three tones and the complexity of inserting, detecting, and repeating these tones are significant
limitations of AMPS.
A more significant advance provided by the digital nature ofD-AMPS involves the use of the SACCH
channel embedded in each time slot. Because this channel is always present, it is quite useful for
communicating control and supervision information
while speech is actively in progress. Specific, advantageous uses of this control chan-
DIGITAL MOBILE TELEPHONY

TABLE 9.1 Data Field Functions of 0-AMPS Time Slots

CDV Coded digital verification color code. A unique code sent by a

CC base station and returned by each mobile for base station
DATA confirmation of connection integrity.
Application bearer channel bits (voice or data). Can also be
GR used for a fast associated control channel (FACC) when there
RSVD is no active application or a situation arises when application
SAC transmission needs to be usurped.
CH Guard time. Mobile transmitter is off.
SYNC Ramp time. Mobile transmitter ramps up to assigned power
level. Reserved (unused).
Slow associated control channel. A continuous channel used
to send control and supervisory information.
Synchronization channel. Used for synchronization, equalizer
training, and time slot identification.

nel involve authentication, additional connection integrity, transmit power control, channel quality
measurement reports, handoffs to a new cell, keypad depressions, and calling number identification. The
SACCH control channel is also used for timing ad• justments specific to the TDMA operation.
A particular example of the usefulness of the SACCR channel is its support of mo• bile assisted hand off
(MARO). If a mobile with an established connection moves from one cell to another, the process of
handing the mobile off is performed with much more control and reliability than is possible in AMPS. The
MARO process begins by the base station telling the mobile to make channel quality measurements on the
cur• rent channel and on candidate channels for a potential handoff. Channel quality meas• urements
involve received signal power levels and bit error rates (BERs). The TDMA nature of D-AMPS facilitates
measurements of candidate channels by tuning to the candidate frequency during an inactive time slot.
After each set of measurements the mobile sends the results to the base station (via SACCH) whereupon
the base station can determine if a handoff is justified.

9.1.4 D·AMPS Error Control

There are three mechanisms incorporated into D-AMPS for mitigating the effects of channels errors: error
correction, error detection, and interleaving. Error correction is implemented with a half-rate convolution
coder for the perceptually most significant bits of the voice. There are 77 such bits out of a frame size of
159 bits. The half-rate convolution coding process therefore adds 77 more bits to the channel. Of the 77
bits,
12 are particularly important. A 7-bit CRC check sum is added for these bits to deter• mine if any of these
12 bits are received in error. When a CRC error occurs, certain critical parameters from the previous
error-free frame are used to reconstruct speech to avoid use of aberrant values. If several CRC errors
are received in successive frames, the reconstructed speech is muted. The general term for these operations
is bad frame masking.
The third error control mechanism involves separating the data in a single speech frame, interleaving it
with data from adjacent speech frames, and transmitting it in two.

9.2 GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS

time slots. This process reduces the possibility that a burst of errors will circumvent the error correction
capabilities of the convolutional coding. A drawback to interleav• ing is the delay it adds to the channel,
which must be accounted for in echo cancelers.

9.2 GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS

Global System for Mobile Communications (GSM) [2] is a cellular mobile commu• nications system
developed in Europe and standardized by the European Telecommu• nication Standards Institute (ETSI).
GSM has subsequently been adopted worldwide as the international digital mobile standard. Initial work
on GSM standardization be• gan in 1982. The first field trial of a GSM system occurred in 1991.
The International Telecommunication Union (ITU) allocated frequency spectrum at 935-960 MHz for
the downlink (base station to mobile) and 890-915 MHz for the uplink (mobile to base station). Even
though some of this spectrum was being used by established analog systems, there was no attempt to be
compatible with the existing frequency plan. (The incumbent analog systems in the various European
countries were incompatible with each other and a main goal of GSM was to establish a common standard so
backward compatibility was not a consideration.)

9.2.1 GSM Channel Structure

Starting with an open-spectrum plan provided GSM system designers with more free• dom in system design
than was available to D-AMPS designers. The most significant difference in GSM with respect to D-
AMPS is the use of 200-kHz-wide digital RF channels as opposed to the 30-kHz-wide D-AMPS RF
channels. Each GSM RF chan• nel operates at 270.833 Mbps using GMSK modulation. As mentioned in
Chapter 6, GMSK modulation is fairly closely related to 4-PSK modulation as used in D-AMPS. GMSK
modulation does, however, require more bandwidth than tightly filtered 4-PSK,
as evidenced by an information density of 270.883/200 = 1.35 bps/Hz for GSM and
48.6/30 = 1.62 bps/Hz for D-AMPS. The GMSK modulation format of GSM provides
a constant-envelope RF signal that is more efficient for RF power generation than is
tightly filtered 4-PSK modulation used by 0-AMPS. This efficiency is most important for hand-held battery
life."
As shown in Figure 9.3, a GSM RF channel utilizes digital TOMA with eight full• rate voice channels, in
contrast to three full-rate voice channels supported by a single D-AMPS RF channel. The ability to
terminate more TOMA channels on a single transceiver provides a cost advantage for GSM base stations.
(The cost of a 270.833- kbps TOMA transceiver is no different than the cost of a 48.6-kbps TOMA
trans• ceiver.)
The GSM system carries eight full-rate voice channels in 200 kHz of bandwidth, which amounts to 25
kHz per voice channel, a spectrum efficiency identical to Euro-

•As discussed below, GSM uses less speech compression than does D-AMPS, which leads to a higher rate
digital channel for voice and in tum requires more transmit power.

pean analog FDM systems of the time. Thus, the introduction of the GSM system did not provide spectrum
efficiency improvements as D-AMPS did. A GSM system does, however, provide cellular system efficiencies in
that digital transmission, in general, and strong error correction, in particular, allow operation at lower signal-to-
noise ra• tios. Greater noise or interference tolerance leads to longer transmission distances and/or greater
amounts of frequency reuse.
The burst period of a GSM system is 120/26/8:: 15/26 ms. This burst period is de•
rived from a 120-ms superframe consisting of 26 TOMA frames and 8 bursts per TOMA frame. Twenty-four
frames of a 26-frame super frame are allocated to traffic (e.g., voice) transmission while one of the frames is
allocated to a SACCH control channel for each traffic channel. The last TOMA frame of a superframe is
reserved. A unique aspect of GSM, with respect to D-AMPS, is that a TOMA burst format is used in both
directions of transmission, as opposed to only on the uplink from the mo• bile to the base station. The format of
these bursts is shown in Figure 9 .4, where it can be seen that there are 148 bits of data and an idle guard time
corresponding to the pe• riod of 8.25 bits. The burst transmission rate of a traffic channel can now be determined as
156.25/15/26:: 270.833 kbps. The fields within the burst are identified in Table 9.2.
The use of multiple bursts in the downlink direction, as opposed to continuous
transmission, is advantageous in that it inherently allows turning off the base station transmitter during idle
channels, which in tum reduces the total amount of interference between cells in a widespread and congested
installation. An advantage of continuous transmission, as used in D-AMPS, is the relative ease of
implementation and greater performance of the digital receiver in the mobile.

9.2 GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS

TABLE 9.2 Data Field Functions of GSM Time Slot

Flag A single bit used to signify voice or FACCH content in an associated TCH field
Guard Idle period of 8.25 bits intervals timing margin between bursts
Tail 3 "O" bits for equalizer training
TCH Field for transporting bearer data or FACCH data
Train Field of fixed data pattern used to train equalizers and acquire a data clock for the entire burst

9.2.2 GSM Speech Coding

GSM uses regular pulse excited-linear predictive coding with a long-term predictor loop (RPE-LTP) [3].
The RPE-LTP algorithm is described in Chapter 3. Speech is di• vided into 20-msec samples, each of
which is encoded as 260 bits, giving a total bit rate of 13 kbps. This is the original, full-rate speech-coding
algorithm. An enhanced full-rate (EFR) speech-coding algorithm has been implemented by some
North American GSM 1900 operators. EFR is said to provide improved speech quality using the existing 13-
kbps bit rate.

9.2.3 GSM Channel Coding and Modulation

GSM utilizes error control mechanisms similar to D-AMPS. First of all, the 260 bits of a speech frame are
divided into three classes:

Class Ia, 50 bits-most sensitive to bit errors

Class lb, 132 bits-moderately sensitive to bit errors
Class II, 78 bits-least sensitive to bit errors

Class Ia bits have a 3-bit CRC added for error detection. If an error is detected, the frame is replaced by a
slightly attenuated version of the previous correctly received frame. The 50 Class la, 3 CRC, 132 Class lb,
and a 4-bit tail sequence (a total of 189 bits) are processed by a half-rate convolutional encoder for error
correction. The out• put of the convolutional encoder is added to the 78 Class II bits to produce an ag•
gregate speech frame of 456 bits. Thus, the redundantly encoded speech rate is
456/0.020 = 22.8 kbps.
To further protect against the burst errors, each sample is interleaved. The 456 bits of output by the
convolutional encoder are divided into eight blocks of 57 bits, and these blocks are spread across eight
consecutive time slot bursts. Since a time slot burst carries two 57-bit blocks, each burst contains traffic from
two different speech samples.

9.2.4 GSM Mobile Station

The GSM mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the
Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to
subscribed services irrespective of a spe• cific terminal. By inserting the SIM card into another GSM
terminal, the user is able.

DIGITAL MOBILE TELEPHONY

to receive calls at that terminal, make calls from that terminal, and receive other sub•
scribed services.
The mobile equipment is uniquely identified by the International Mobile Equip• ment Identity (IMEI). The
SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the
system, a secret key for authen• tication, and other information. The IMEi and the IMSI are independent, thereby
al• lowing personal mobility. The SIM card may be protected against unauthorized use by a password or
personal identity number.

9.2.5 GSM Frequency Hopping

The mobile station is inherently frequency agile, meaning it can move between a trans• mit, receive, and monitor
time slot within one TOMA frame, all of which are normally on different frequencies. GSM makes use of this
inherent frequency agility to imple• ment slow frequency hopping-the mobile and BTS transmit each TOMA
frame on a different carrier frequency. The frequency-hopping algorithm is broadcast on the broadcast control
channel. Since multipath fading is dependent on carrier frequency, slow frequency hopping helps alleviate the
problem. In addition, cochannel interfer• ence is more random than when fixed-frequency allocations exist.

9.2.6 GSM Short Message Service

Short Message Service (SMS) is an integrated bidirectional messaging service that al• lows GSM cellular
subscribers, and various PCS offerings, to send and receive data. Individual messages (with GSM) can be up to 160
bits in length. Because the SACCH is used for SMS data transmission, messages can be received or transmitted
during a voice call. Initial applications of SMS focused on alphanumeric paging services with fundamental
differences: SMS is bidirectional and message delivery is guaranteed. Subsequent applications served by SMS
are voice mail notification, e-mail delivery, stock quotes, and downloading/updating of SIM cards.

9.3 CODE DIVISION MULTIPLE·ACCESS CELLULAR

The two digital cellular systems discussed in the previous sections, 0-AMPS and GSM, utilize a combination
of frequency division multiplexing and time division mul• tiplexing" as a method of partitioning a block of
allocated frequency spectrum into in• dividual communication channels. This section discusses systems with a
fundamentally different approach to channel definition-code division multiple ac• cess (CDMA)-that belong
to a class referred to as spread spectrum communications systems [4]. The term spread spectrum refers to the fact
that transmission bandwidth

•within a particular GSM cell it is conceivable that a single FDM channel supporting eight IDMA channels is
sufficient for sufficiently low traffic situations. In this case, the particular cell utilizes only TDMA.
Nevertheless, the mobiles still support FDM operations so they can move to different cells and switch
frequencies.

9.3 CODE DIVISION MULTIPLE-ACCESS CELLULAR

used by an individual channel is much wider than the inherent bandwidth required by the message being
transmitted. Spread spectrum systems have traditionally been used in military applications where the increased
complexity of implementation is justified by two particular features. First, it is relatively difficult to detect the
presence of a spread spectrum signal because the signal energy is spread across a wide band and is often masked
by background noise. Second, it is more difficult to jam a spread spec• trum signal because the jamming signal
energy must be spread across a wide band as opposed to being focused into a relatively narrow band. The
performance of a spread spectrum receiver is comparable to the performance of receivers for traditional nar•
rowband signals as long as the spread spectrum receiver knows and can synchronize to the method being used to
spread the spectrum.
Two general categories of spread spectrum communications are frequency-hopping
systems and direct-sequence modulation systems. A frequency-hopping system is one in which transmission at
any particular instant is confined to a relatively narrow band of frequencies commensurate with the inherent
bandwidth of the message. Instead of staying within one particular band as a conventional communications
system does, a frequency-hopping system jumps between narrow bands of frequencies within a large block of
spectrum in some prescribed manner. As mentioned in the previous section, a GSM cellular system has the ability
to operate with frequency hopping (specifically, slow frequency hopping).

9.3.1 CDMA Channel Establishment

A contemporary cellular CDMA system, as developed by Qualcomm and stand• ardized by EIAffIA in
Interim Standard IS-95 [5], uses direct-sequence spectrum spreading. One particular method of
implementing a direct-sequence spectrum spreading system is shown in Figure 9.5. In this system the source
data is "exclusive ored" with a relatively long digital codeword. In essence, the "exclusive or" process replaces a
I of the source data with the given codeword while a O of the source data is replaced with the bitwise complement
of the codeword. If a codeword contains n bits, the occupied spectrum of the transmitted signal is n times as wide
as if the source data

DIGITAL MOBILE TELEPHONY

were directly transmitted. Signals of other CDMA channels occupy the same band of frequencies but do so with
different spectrum-spreading codes, which allows separa• tion of the signals in the signal processing circuitry of
a receiver.
The basic process of separating CDMA channels in a receiver involves correlating a received signal with each of
the various codewords (i.e., channels) assigned to the cell. The correlation process produces a correlation
measurement by subtracting the number of mismatches in a codeword from the number of matches. Table 9 .3
lists an example set of codewords with particularly useful correlation properties. As indicated in the table, a
codeword has seven matches with itself and no mismatches for a net cor• relation of +7. A source data value of O
produces no matches and seven mismatches for a net correlation of -7. The measurement weights for all other
codewords are either
+ 1 or -1, depending on the values of the source data. If all seven channels defined in Table 9.3 are active in a
single cell, the worst-case interference between the codes pro• duces an interference value of either +6 or -6. Thus,
the desired data can be recovered with a discrimination threshold of O for each channel. (The worst-case net
measure• ment values are+ I for a 1 and -1 for a 0.) Notice that same correlation properties exist for all channel
codes with respect to the other codes in the table.

Example baseband waveforms for the seven-channel CDMA system of Table 9.3 are shown in Figure 9.6. Notice
the channel O receiver measurement is positive, which implies a data value of 1. In this example interference from
the adjacent channels ac• tually enhanced the channel O measurement from an expected value of7 to 9.
Discriminating between nominal measurement values of +7 for a desired signal
and +6 for worst-case interference is obviously very tenuous, particularly because the individual channels will be
received with different power levels. It is important to note, however, that an interference value of +6 can arise
only if the data values of all interfering channels destructively coincide. On average, the composite
interference has an average value of 0. The length of a spectrum-spreading code in IS-95 is actually
64 bits long, which means that the example worst-case interference would be 63 in re• lation to a desired channel
value of 64. The chances of destructive interference from all 63 channels, or even a large number of channels, is
astronomically small.
aspectrum-spreading code of dasired channel: 1110010
9.3 CODE DIVISION MULTIPLE-ACCESS CELLULAR

Figure 9.6 Example seven-channel CDMA encoding and decoding.

Example 9.1. Determine the probability of maximum interference of a 64-channel CDMA system
with 64-bit spreading codes. Also determine the effective signal-to-interference power ratio of the
same CDMA system. Assume all channels operate at the same effective power level at the receiver and that
all channel codes have a cross correlation of ±1 bit.

Solution. The probability of 63 destructive interferers is merely the probability of occurrence of 63

equally likely binary events: prob(max interference)= (0.5)63 =
1 x 10-19• The value of a desired receive signal is the autocorrelation of a codeword with itself and can
therefore be represented as a value of 64. The interference level is the sum of 63 binary random numbers
with equally likely values of ±1. Even though a single interferer does not produce a Gaussian probability
distribution, the sum of a large number of independent random variables approaches a Gaussian
distribution (central limit theorem). The mean and variance of an individual interferer are easily
determined to be O and 1, respectively. The mean and variance of a sum of 63 such variables are O and
63, respectively. The signal-to-interference ratio is now determined as

As presented in Chapter 6, the 18-dB SIR result of Example 9 .1 is quite sufficient to support an acceptable
error rate if the effective power level of all channels can be
448 DIGITAL MOBILE TELEPHONY

maintained to be equal. In actual practice, a CDMA deployment does not use all pos• sible codes in a single
cell, just as an FDM system does not use all frequencies in one cell. Thus, the amount of interference in a
cell is limited by the number of codes as• signed to the cell and a lesser amount of interference from
adjacent cells (assuming the adjacent cells do not use identical, synchronous spectrum-spreading codes).
The effect of varying power levels in the interfering channels is covered in some problems at the end of the
chapter.

9.3.2 CDMA Multipath Tolerance

A primary advantage of a CDMA transmission system is its robustness in the presence of multipath
conditions. The basic reason for multipath tolerance can be appreciated by examining the example codes
given in Table 9.3. Notice that each code is a cyclic shift of all other codes in the table. Because the
selected codes have low correlation with each other, a delayed version of any particular code has the same
low correlation with an undelayed version of itself. Thus, the effect of a multipath delay of more than one
spread spectrum bit (referred to as a chip) is no more than the effect of the inter• ference from another
CDMA channel, even if the delayed version is at the same power level as the primary signal.
The effect of a multipath condition on a D-AMPS channel may be much more dele• terious. Because a D-
AMPS system operates at a relatively narrow band of frequen• cies, it is possible that a complete fade
occurs for a particular channel at a particular physical location. If the user moves through the physical
location, the effect of the fade is a momentary dropout. If the user dwells at the location, the connection can
be lost unless a handoff occurs (to a new frequency and/or an adjacent cell). The slow fre• quency-
hopping feature of GSM ameliorates the effect of a complete multipath fade to a greater degree, but not as
effectively as a CDMA system. In GSM there may be momentary dropouts, but they are not long enough to
cause a dropped connection and do not require a handoff, even if a user dwells in a physical location where one
par• ticular frequency is totally lost. In essence, the frequency-hopping process of GSM is equivalent to repeated
and automatic handoffs to different frequencies.
From a somewhat philosophical point of view, the basic intent of a CDMA system
is to equalize the performance of all channels in the system. In a FDM!fDMA system it is likely that some
channels operate with very high performance while others operate at very low performance or cannot be used at
all. The existence of high-performance channels does not compensate for the existence of low-performance
channels. Thus, a system that equalizes the performance of all channels has a greater total capacity. The
primary reason for variable channel performance in FDM!TDMA is multipath fading. An FDMffDMA
transmitter typically operates with a certain amount of ex• cess power referred to as fade margin. The excess
power is not much of a problem with adjacent-channel interference because it is relatively easy to isolate FDM
channels with frequency guard bands and TDM channels with time guard bands. Cochannel in• terference from
one cell to another is the crux of the problem. If a particular channel

9.3 CODE DIVISION MULTIPLE-ACCESS \

is operating at an excess power level, that channel cannot be reused except at a rela•
tively larger distance.

9.3.3 CDMA Power Control

Effective transmit power control of the mobile units in a CDMA system is both a re• quirement and a benefit. It
is a requirement because a mobile transmitter that is close to a base station receiver will obliterate other mobiles
in the same cell that are farther away. This is referred to as the near-far problem of CDMA. If the transmit power
lev• els of all mobiles in a CDMA system are controlled to be no higher than absolutely necessary, the channels
can be reused more often. Although power control is used in FDMffDMA systems, it is not possible to operate
with bare minimum power levels because the system cannot respond fast enough to adjust the power levels for fast
mul• tipath fades. A side benefit of using minimum power levels in a CDMA mobile is in• creased battery life.
The power levels of a mobile are controlled in two ways: open loop and closed loop. In the open-loop mode, a
mobile can determine its transmit power level by meas• uring its received power level under the assumption that
transmission losses are equal in both directions. This assumption is reasonable for a CDMA system but not for
a FDMffDMA system because the latter are much more susceptible to independent frequency-selective
fading (multipath). Closed-loop power control involves base sta• tion measures of mobile received power and
adjustments to the mobile power levels with the control channel. Power control in IS-95 CDMA is described in
references [6-8].
Table 9.4 lists the basic parameters of the IS-95 CDMA digital cellular system for
the 800-MHz cellular band. The speech compression algorithm uses QCELP (Qual• comm code excited linear
prediction). The first commercial installation of CDMA oc• curred in Hong Kong in 1995.

9.3.4 CDMA Soft Handoff

A unique feature of a CDMA system is the ability of a mobile to simultaneously re•

ceive from more than one source. Because each cell in a CDMA cellular network

TABLE 9.4 IS-95 CDMA Mobile Telephone System Parameters

450 DIGITAL MOBILE TELEPHONY

transmits in a common frequency, a common RF receiver inherently receives the spread spectrum signal from
all adjacent base stations. Signals from multiple base sta• tions can then be acquired with multiple baseband code
correlators or by time sharing a single correlator with the separate codes. In a soft handoff operation the same
voice signal is distributed to selected cells adjacent to a currently active cell. An active mo• bile can then compare
the quality of the signals and switch to the best one before dis• connecting from the established base station.
A "make-before-break" operation is not feasible in an FDM!IDMA system wherein adjacent cells utilize
separate frequencies that require separate RF receivers. FDM!IDMA systems inherently use hard handoffs, which
require disconnecting from one base station before connecting to a new one. Notice, however, that a soft handoff
operation in a CDMA system increases the background interference because of the multiple active signals for a
single connection.

9.4 PERSONAL COMMUNICATION SYSTEM

A personal communication system (PCS) is a cellular system operating in a band of frequencies at 1.9 GHz. The
original concept for PCS included multiple, new features and services beyond those offered by a basic cellular
system. Some of the envisioned features were single telephone number for multiple services (voice, data, fax) and
user mobility for home or office use and location determination. Although some commer• cial PCS offerings
provide some new user features, initial North American PCS sys• tems are basically cellular systems utilizing a
new band of frequencies.
When the FCC allocated the PCS frequencies for the United States, they did so without stipulating which type
of system should be deployed. Thus, any organization that bids for and receives a franchise for PCS spectrum is
free to choose whatever type of system it wants for providing service to the public. As a result, North American
PCS systems have been developed with three different transmission formats: D-AMPS, GSM, and
CDMA. The D-AMPS implementation follows EIA!TIA stand• ard IS-136, which is basically a revision of IS-
54 that incorporates digital control channels. (IS-54 defines the use of an analog control channel for
compatibility with AMPS.)

9.5 VOICE PRIVACY AND AUTHENTICATION

Ensuring privacy of conversations and preventing fraud are two critical aspects of cel• lular telephone systems that
are addressed more completely in the digital systems than in the original analog systems. An FDM analog system
is particularly vulnerable to casual eavesdropping because a relatively simple scanner can be used to identify an
active channel and tune into the conversation. If the scanner has two receivers, the eavesdropper can listen to
both sides of the conversation, assuming the eavesdropper is in a high enough location or close enough to the
active mobile that it can receive the uplink signal.
9.6 IRIDIUM

Digital signals are inherently more complicated to intercept because an eavesdrop• per may need to monitor the
connection establishment process and not just tune in af• ter a conversation has started. Explicit encryption
parameters for optional voice privacy is established during call establishment, or possibly during a conversation,
by exchanging privacy control messages in the control channel. An eavesdropper must receive the relevant
information sent in both directions before eavesdropping is pos• sible. Even without explicit encryption, an
eavesdropper must be located somewhere near the base station to properly receive both sides of a TOMA
conversation. At other locations within a cell the eavesdropper will encounter overlapping time slots that will
inflict bit errors into the eavesdropper's received data. A CDMA system produces a similar effect when the
phase of two spectrum-spreading codes from two separate mo• biles coincide.
Authentication of a valid mobile station is significantly strengthened in digital sys•
tems. A major problem in analog cellular systems is the process of cloning, in which disreputable persons
monitor call establishment handshake procedures to acquire valid mobile equipment electronic serial
numbers (ESNs) and program them into counterfeit mobile units that are then used to place calls. Part of the
improved fraud prevention involves maintaining a more up-to-date database of valid mobile ESNs. The
strongest part of the fraud prevention involves determining authorization codes based on past call history in both
the base station and the mobile. The authorization code determined by the mobile and sent to the base station
must coincide with the authorization code calculated by the base station before service is allowed. Additional
steps may require entry of access codes by the user or, in the case of GSM, the mag• netic data card called the
Subscriber Identification Module (SIM).

9.6 IRIDIUM

Iridium is a satellite-based system for telephone and two-way paging services. The satellite system is a Low
Earth Orbit Satellite (LEOS) system, which means that signal powers and antenna sizes can be reduced with
respect to conventional geostationary satellites. (The Iridium system orbits are less than 500 miles, versus the
23,000 miles for geostationary satellites.) In addition to enabling lower power transmission, a LEOS avoids
the propagation delay of a geostationary satellite, which is a minimum of 500 msec, A basic disadvantage of a
LEOS system is the continuous movement of the satellites with respect to ground locations. Because of this,
Iridium provides a large number of satellites (66) so continuous coverage from at least one satellite is always
available.
Iridium phones are dual-mode phones. A phone first makes an attempt to place a call over a terrestrial cellular
system" but will default to the satellite network if local cellular coverage is not available. When communicating
through the Iridium system, a user first gets connected to the nearest available satellite. From there the communi•
cation might return to the ground or be relayed through multiple satellites before re-'Multiple versions of
handhelds are available to operate on TDMA, GSM, or CDMA cellular networks.

DIGITAL MOBILE TELEPHONY

turning to the ground. The second ground link might be direct to another Iridium user or it may involve a base
station with interconnection to a public telephone network. System aspects of the Iridium system are provided in
Table 9.5.

9.7 TRUNKED RADIO

The term trunked radio generally refers to Private Mobile Radio (PMR) communica• tions networks. Originally,
users of PMR equipment were allocated specific operating frequencies dedicated to each user (or organization).
Such allocations were obviously inefficient in terms of bandwidth utilization when the users did not have
continuous need for radio communications. Significant improvements in efficiency are achieved when the
separate channels are placed in a group and shared by a larger group of users on an as-needed basis. When a user
of a trunked radio system needs service, the radio equipment accesses an idle channel that becomes temporarily
assigned to that user. As soon as the users of a particular channel cease to transmit, the channel becomes avail•
able for other users. Access to a communications channel utilizes a control channel and a centralized controller
for resource allocation.
A trunked radio system is not a cellular system but does utilize a central node re• ferred to as a repeater. The
repeater receives a signal on one frequency, shifts it to an• other frequency, and transmits the signal on the new
frequency. Thus, end users do not communicate directly with each other." Transmission through the repeater is
more ef• fective because the tower is located at a high point in the coverage area utilizing a rela• tively high
transmit power that facilitates communications between end users who are likely to not have an adequate path
between them.
In a general sense, trunked radio systems are PMR systems that provide radio com• munications directly between
users without use of the public telephone network. However, the distinction between a trunked radio system
and a cellular system has be-

*some trunked radio equipment does support a (special) two-way mode of operation in which the users
conununicate directly with each other. This mode of operation is generally reserved for communications in
outlying areas where the repeater cannot provide service
REFERENCES

come blurred with U.S. offerings from companies like Nextel and Geotek. These com• panies utilize technology
developed by Motorola that augments the use of spectrum previously used for private radio service referred to
as Specialized Mobile Radio (SMR). SMR applications typically involve fleet operations such as taxi cabs and
de• livery vehicles in need of dispatch services wherein multiple mobiles simultaneously hear transmissions on a
common channel. SMR radios utilize analog FM/FDM trans• mission with 25-kHz channels.
The new equipment, generically referred to as Enhanced Specialized Mobile Radio (ESMR), upgrades analog
SMR transmission to digital TOM transmission in the same manner that D-AMPS upgrades AMPS. ESMR
transmission protocols allow for six digital TDM channels in each 25-kHz channel. ESMR systems support
conventional dispatch applications and other similar specialized services such as private group call• ing.
Mostimportantly, for this chapter, an ESMR system can provide connections to the public telephone network-
referred to as an interconnect feature. With this fea• ture, an ESMR system can function as a cellular telephone
system.

9.8 CELLULAR DIGITAL PACKET DATA

Cellular Digital Packet Data (CDPD) [9] is a system designed to provide data services up to 19.2 kbps as an
overlay of an AMPS installation. The primary service concept is to offer fixed and mobile data services
independent of the need to upgrade an AMPS system to a D-AMPS system. CDPD uses AMPS channels that
are not being used for voice. Because it is a packet-oriented data service, multiple users share a single
30-kHz channel-a significant savings in spectrum usage as compared to the use
of connection-oriented voice channels for data. Principal applications for CDPD are mobile Internet access and
credit card verification.
Access to the shared channel is accomplished with Digital Sense Multiple Access with Collision Detection
(DSMA/CD) which is similar to CSMA/CD of other radio systems and Ethernet LANs. CDPD uses OMSK
modulation with RS(63,47) forward error correction.

REFERENCES

l "Vector Sum Excited Linear Prediction (VSELP) 7950 Bit Per Second Voice Coding
Algorithm," Technical Description, Motorola, Schaumburg, Illinois, Nov. 14, 1989.
2 M. Rahnema, "Overview of the GSM System and Protocol Architecture," IEEE Communications
Magazine, Apr. 1993, pp. 92-100.
3 P. Vary, K. Hellwig, C. Galland, M. Russo, J. Petit, and D. Massaloux, "Speech Codec for the European Mobile
Radio System," in IEEE GLOBECOM 1989, Nov. 1989, pp.
29.B.2.
4 R. Dixon, Spread Spectrum Systems with Commercial Applications, Wiley, New York,
1994.
5 "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband
Spread Spectrum Cellular System," EIA!TIAJIS-95, Washington, DC, July 1993.
6 A. Salamasi and K. S. Gilhousen, "On the System Design Aspects of Code Division Multiple Access
(CDMA) Applied to Digital Cellular and Personal Communications Networks," Proceeding ofthe Forty-First
IEE Vehicular Technology Conference, May
1991, pp. 57-62.
7 A. J. Viterbi and A. M. Viterbi, "Erlang Capacity of a Power Controlled CDMA System," IEEE
Journal on Selected Areas of Communications, Nov. 1993, pp.
882-890.
8 K. S. Gilhousen, I. M. Jacobs, R. Padovani, A. J. Viterbi, L.A. Weaver, and C. E.
Wheatley, "On the Capacity of a Cellular CDMA System," IEEE Transactions on
Vehicular Technology, May 1991, pp. 303-312.
9 A. K. Salkintzis, "Packet Data over Cellular Networks: The CDPD Approach," IEEE Communications
Magazine, June 1999, pp. 152-159.
PROBLEMS

9.1 What is the available bearer rate of a half-rate digital channel in a D-AMPS sys - tern?
9.2 What is the data rate of the slow associated control channel in a D-AMPS sys-
tem?
9.3 What is the available bearer rate a full-rate digital channel in a GSM system?
9.4 What is the data rate of the slow associated control channel in a GSM system?
9.5 Determine the receiver output measurements for channels 1 and 2 for the CDMA
example of Figure 9.6.
9.6 What is the effective signal-to-interference ratio of a single CDMA uplink chan• nel operating at a
distance that is twice as far from the base station as 62 other channels? Assume a code length of 64, cross
correlations of ±1, and all trans• mitters operate at identical power levels. (a) Assume all interferers are
active. (b) Assume half the interferers operate with a 25% data rate because of no voice activity.
9.7 In a CDMA system with a code length of 64 determine the signal-to-interference
ratio of a single uplink CDMA channel if there are 16 active interfering channels operating at an effective receive
power level that is 12 dB higher that the desired channel.