WILEY Cellular Mobile Radio Systems User Manual
1
Introduction
The potential of communicating with moving vehicles without the
use of wire was soon recognized following the invention of radio
equipment by the end of the nineteenth century and its development
in the beginning of the twentieth century [1.1, 1.2]. However, it is only
the availability of compact and relatively cheap radio equipment
which has led to the rapid expansion in the use of land mobile radio
systems. Land mobile radio systems are now becoming so popular,
for both business and domestic use, that the available frequency
bands are becoming saturated without meeting even a fraction of
the increasing demand. To give an example, the estimated number
of mobile radios in use in 1984 was about 540 000 with a growth rate of
about 10% per annum in the UK; estimates of growth rates up to 20%
have been made for other European countries [1.3]. Using these
figures leads to estimates of approximately 2.5 million UK users by
the end of this century. A growth rate of 20% annually would lead to
more than 13 million users worldwide by the year 2 000. Such a figure
is comparable to the 11 million business and 18 million residential
fixed telephone in use at present [1.3].
Solutions to spectral congestion in the land mobile radio environ-
ment can be envisaged in the following ways.
Introduction
(a) The Cellular Concept
In cellular systems, spectral efficiency is achieved by employing
spatial frequency re-use techniques on an interference-limited basis.
Frequency re-use refers to the use of radio channels on the same
carrier frequency to cover different areas which are separated from
one another by a sufficient distance so that co-channel interference is
not objectionable [1.4]. This is achieved by dividing the service area
into smaller `cells', ideally with no gaps or overlaps, each cell being
1
2 INTRODUCTION
served by its own base station and a set of channel frequencies. The
power transmitted by each station is controlled in such a way that the
local mobile stations in the cell are served while co-channel interference, in the cells using the same set of radio channel frequencies, is
kept acceptably minimal. An added characteristic feature of a cellular
system is its ability to adjust to the increasing traffic demands through
cell splitting. By further dividing a single cell into smaller cells, a set of
channel frequencies is re-used more often, leading to a higher spectral
efficiency. Examples of analogue cellular land mobile radio systems
are AMPS (Advanced Mobile Phone System) in the USA, TACS (Total
Access Cellular System) in the UK and NAMTS (Nippon Advanced
Mobile Telephone System) in Japan ± the latter was the first to become
commercially available in the Tokyo area in 1979.
Cellular systems can offer several hundred thousand users a better
service than that available for hundreds by conventional systems. It is
fair to conclude, therefore, that the adoption of a cellular system is
inevitable for any land mobile radio service to survive ever increasing
public demands, particularly considering the severe spectrum congestion which is already occurring within many of the allocated
frequency bands. It is not surprising then, that almost the only common feature amongst the various proposals for second-generation
cellular systems for the USA, Europe and Japan, is the use of the
cellular concept. It is generally agreed that a cellular system would
greatly improve the spectral efficiency of the mobile radio service.
(b) Moving to Higher Frequency Bands
The demand for mobile radio service has been such that servere
spectrum congestion is occurring within many of the allocated frequency bands. First generation analogue cellular mobile radio occur
below 1 GHz. Second generation digital celllular mobile radio also
occur below 1 GHz. Personal Communication Networks (PCN) are
rapidly moving into the next GHz band (1.7±1.9 GHz) and a Universal
Mobile Telecommunications System (UMTS) ± envisaged for the end
of the 1990s, will be using part of the 1.7±2.3 GHz band [1.5]. It is
obvious that there are plenty of spectra above 1 GHz which makes it a
natural move to go for higher frequency bands than those currently in
use. Frequencies up to the millimetric band (about 60 GHz) are being
investigated. In these regions, large amounts of spectrum are available
to accommodate wideband modulation systems and the radio wave
attenuation is significantly greater than the free-space loss which
helps to define a very high capacity cellular system [1.6, 1.7]. Nevertheless, it is necessary to conduct detailed propagation measurements
INTRODUCTION 3
in these frequency bands as well as to define system parameters
adequately. Indeed, it is necessary to solve all the problems which
can arise at these frequencies before implementation is economically
viable and technologically possible.
(c) Maximizing the Degree to which the Present Mobile Bands are
Utilized
Despite the proven success of first-generation cellular systems, which
are predominantly FM/FDMA based, it is strongly believed that more
spectrally efficient modulation and multiple access techniques are
needed to meet the increased demand for the service. This has
prompted considerable research into more spectrally efficient techniques and modes of information transmission. As a consequence, a
wide variety of modulation and multiple access techniques are offered
as a solution. Amongst the modulation techniques suggested are
wideband and narrowband digital techniques (TDMA and FDMA
based), spread spectrum and ACSSB, alongwith conventional FM
analogue systems. Voice channel spacings vary from 5 kHz for
ACSSB systems up to 300 kHz or more for spread spectrum systems.
Furthermore, each multiple access technique ± FDMA, TDMA, CDMA
and a hybrid technique ± is claimed, by various proponents, to have
the highest spectral efficiency when applied to cellular systems.
From (a), (b) and (c) above, it can be clearly seen that both employing
the cellular concept and maximizing the spectrum usage of the present frequency bands are necessary to help alleviate spectral congestion in the land mobile radio environment and to fulfil the increased
demands for service. In fact, higher spectral efficiency leads to more
subscribers, cheaper equipment due to mass production, low call
charges and, overall, lower cost per subscriber.
It is also obvious that a rigorous and comprehensive approach to
the definition and evaluation of spectral efficiency of cellular mobile
radio systems is necessary in order to settle the conflicting claims of
existing and proposed cellular systems, especially if the British government is to go ahead with its plan to involve the private sector in the
management of the radio spectrum [1.8].
To date many methods have been employed in an attempt to
evaluate and compare different modulation and multiple access techniques in terms of their spectral efficiency. These methods include
pure speculation, mathematical derivations, statistical estimations as
well as methods based upon laboratory measurements. Unfortunately, none of the above methods can be said to be rigorous or
4 INTRODUCTION
conclusive. Mathematical methods, for instance, have been used to
predict the co-channel protection ratio, yet this is a highly subjective
system parameter. Other approaches, such as the statistical methods,
are difficult for the practising engineer to apply in general. Results
based on computer simulations must be treated with a degree of
suspicion when the basis of such simulations is not revealed. Not
only have improper ways of comparison appeared in the literature,
such as comparing the spectral efficiency of SSB and FM to that of
TDMA, but there is also a lack of a universal measure for spectral
efficiency within cellular systems. In fact, a comparison between
spectral efficiency values is only meaningful if it refers to:
. the same service;
. the same minimum quality;
. the same traffic conditions;
. the same assumptions on radio propagation conditions;
. the same agreed universal spectral measure.
Thus, it is essential to establish a rigorous and comprehensive set of
criteria with which to evaluate and compare different combinations of
modulation and multiple access techniques in terms of their spectral
efficiency in the cellular land mobile radio environment. This book
discusses such a method which must necessarily embrace the following features.
(a) A measure of spectral efficiency which accounts for all pertinent
system variables within a cellular land mobile radio network. For
such a measure to be successful it must reflect the quality of
service offered by different cellular systems.
(b) Modulation systems, as well as multiple access techniques, must
be assessed for spectral efficiency computation including both
analogue and digital formats.
(c) It is necessary to model the cellular mobile radio system to
account for propagation effects on the radio signal. On the
other hand, it is also necessary to model the relative geographical
locations of the transmitters and receivers in the system so as to
be able to predict the effect of all significant co-channel interfering signals on the desired one.
REFERENCES 5
(d) To include the quality of the cellular systems in terms of the
grade of service, two traffic models are considered. The first one
is a `pure loss' or blocking system model, in which the grade of
service is simply given by the probability that the call is
accepted. The other is a queuing model system in which the
grade of service is expressed in terms of the probability of
delay being greater than t seconds.
(e) The method combines a global approach which accounts for all
system parameters influencing the spectral efficiency in cellular
land mobile radio systems and the ease of a practical applicability to all existing and proposed, digital and analogue, cellular
land mobile radio systems. Hence such systems can be set in a
ranked order of spectral efficiency.
This study also demonstrates the crucial importance of the protection
ratio in the evaluation of the spectral efficiency of modulation systems. It is also argued that since the protection ratio of a given
modulation system inherently represents the voice quality under
varying conditions, it is imperative that such a parameter is evaluated
subjectively. Furthermore, the evaluation of the protection ratio
should be performed under various simulated conditions, e.g. fading
and shadowing, in such a way that the effect of these conditions is
accounted for in the overall value of the protection ratio. In addition,
any technique which improves voice quality or overcomes hazardous
channel conditions in the system should also be included in the test.
Consequently, the effects of amplitude companding, emphasis/deemphasis, coding, etc. will influence the overall value of the protection ratio. A number of current and proposed cellular mobile radio
systems are evaluated using the comprehensive spectral efficiency
package developed.
REFERENCES REFERENCES
[1.1] Jakes, W. C., 1974 `Microwave Mobile Communications' John Wiley and
Sons, New York
[1.2] Young, W. R., 1979 `Advanced Mobile Phone Services: Introduction,
Background and Objectives', Bell Syst. Tech. J., 58 (1) January pp. 1±14
[1.3] Matthews, P. A., 1984 `Communications on the Move' Electron. Power
July pp. 513±8
6 INTRODUCTION
[1.4] MacDonald, V. H., 1979 `Advanced Mobile Phone Services: The
Cellular Concept' Bell Syst. Tech. J., 58 (1) January pp. 15±41
[1.5] Horrocks, R. J. and Scarr, R. W. A., 1994 Future Trends in Telecommun-
ications John Wiley and Sons, Chichester
[1.6] McGeehan, J. P. and Yates, K. W., 1986 `High-Capacity 60 GHz Micro-
cellular Mobile Radio Systems' Telecommunications September pp. 58±64
[1.7] Steele, R., 1985 `Towards a High-Capacity Digital Cellular Mobile
Radio System' IEE Proc., 158 Pt F pp. 405±15
[1.8] Purton, P., 1988 `The American Applaud Trail-Blazing British' The
Times Monday, 12 December 1988, p. 28
2
Measures of Spectral
Efficiency in Cellular Land
Mobile Radio Systems
2.1 INTRODUCTION Introduction
In order to assess the efficiency of spectral usage in cellular land
mobile radio networks, it is imperative to agree upon a measure of
spectral efficiency which accounts for all pertinent system variables
within such networks. An accurate and comprehensive definition of
spectral efficiency is indeed the first step towards the resolution of the
contemporary conflicting claims regarding the relative spectral efficiencies of existing and proposed cellular land mobile radio systems.
An accurate spectral efficiency measure will also permit the estimation of the ultimate capacity of various existing and proposed cellular
systems as well as setting minimum standards for spectral efficiency.
In undertaking the task, the problems currently experienced whereby
some cellular systems claim to have a superior spectral efficiency,
either do not show their measure of spectral efficiency or use a
spectral efficiency measure which is not universally acceptable
could be avoided.
The purpose of this chapter is to survey various possible measures
of spectral efficiency for cellular land mobile radio systems, discussing their advantages, disadvantages and limitations. Our criterion is
to look for a suitable measure of spectral efficiency which is universal
to all cellular land mobile radio systems and can immediately give a
comprehensive measure of how efficient the system is, regardless of
the modulation and multiple access techniques employed. Such a
measure should also be independent of the technology implemented,
Spectralefficiencyin CellularLand Mobile RadioSystems
7
8 SPECTRAL EFFICIENCY IN CELLULAR LAND MOBILE RADIO SYSTEMS
with an allowance for the introduction of any technique which may
improve the spectral efficiency and/or system quality. Furthermore,
no changes or adaptations in the spectral efficiency measure should be
necessary to accommodate any cellular system which may be proposed in the future. With the above considerations, the most suitable
spectral efficiency measure will be adopted to establish a rigorous and
comprehensive set of criteria with which to evaluate and compare
cellular systems which employ different combinations of modulation
and multiple access techniques in terms of their spectral efficiency.
This will be the subject of the following chapters.
2.2 IMPORTANCE OF SPECTRAL EFFICIENCY MEASURES
Measures of spectral efficiency are necessary in order to resolve the
contemporary conflicting claims of spectral efficiency in cellular land
mobile radio systems. In such systems, an objective spectral efficiency
measure is needed for the following reasons.
(a) It allows a bench mark comparison of all existing and proposed
cellular land mobile radio systems in term of their spectral efficiency.
For the GSM (Groupe SpeÂcial Mobile) Pan-European cellular system,
for example, there are conflicting claims regarding the relative spectral efficiencies of proposed digital systems [2.1]. On the other hand,
there are at least seven different analogue cellular land mobile radio
systems in operation throughout the world, including five in Europe
[2.2], which also have conflicting spectral efficiency claims. The resolution of such claims is complicated even further by the present lack
of a precise definition of spectral efficiency within cellular systems
which all parties can agree upon.
(b) An objective measure of spectral efficiency will help to
estimate the ultimate capacity of different cellular land mobile
radio systems. Hence, recommendations towards more spectrally
efficient modulation and multiple access techniques can be put forward. Recommendations of this nature will certainly influence
research and development to move in parallel with more spectrally
efficient techniques and technologies and perhaps reaching higher
spectral efficiency by approaching their limits. Estimates of the ultimate capacity of various cellular systems would also help to forecast
the point of spectral saturation, when coupled with demand growth
projections.
POSSIBLE MEASURES OF SPECTRAL EFFICIENCY 9
(c) An accurate measure of spectral efficiency is also useful in
setting minimum spectral efficiency standards, especially in urban
areas and city centres where frequency congestion is most likely to
occur. Such standards will prevent manufacturers lowering system
costs or offering higher quality services at the expense of squandering
the spectrum. This is particularly necessary with services that are
provided by competitive companies, which is very much the case
nowadays. Setting thse standards will also lead to either more
research and development into systems which do not comply with
the minimum spectral efficiency standards or, perhaps more sensibly,
to concentration of more research on systems which initially comply
with the efficiency standards so as to achieve an even higher spectral
efficiency. The task of setting minimum efficiency standards would be
carried out by independent consultative committees such as the International Radio Consultative Committee (CCIR) and the International
Telegraph and Telephone Consultative Committee (CCITT), and
enforced by regulatory authorities, such as the Radio Regulatory
Division (RRD) of the Department of Trade and Industry (DTI) in
the UK and the Federal Communications Commission (FCC) in
the USA.
2.3 POSSIBLE MEASURES OF SPECTRAL EFFICIENCY Possible Measures ofSpectral Efficiency
The planned spatial re-use of frequency, characteristic to cellular
systems, requires a spectral efficiency measure at the system level.
In this context, spectral efficiency for a cellular system is the way the
system uses its total resources to offer a particular public service to its
highest capacity.
Hatfield [2.3] surveyed various proposed measures of spectral efficiency for land mobile radio systems, reviewing the advantages, disadvantages and limitations of each. In this section possible measures
of spectral efficiency will be examined, paying particular attention to
their relevance and adequacy to cellular systems, both present and
future.
2.3.1 Mobiles/Channel
In the measure `Mobiles/Channel', the number of mobile units per
voice channel is used to indicate the spectral efficiency. The measure
`Users/Channel' has also been used with the same meaning. This is
10 SPECTRAL EFFICIENCY IN CELLULAR LAND MOBILE RADIO SYSTEMS
probably the simplest way of measuring the spectral efficiency of a
mobile radio system. Nevertheless, this measure has certain shortcomings.
(a) In this spectral efficiency measure, traffic considerations are
not taken into account. Take, for example, the case of two systems
being compared, where the mobiles in the two systems do not
generate the same amount of traffic. If the users in one system
generate twice as much busy hour traffic as the other system, for
instance, and both systems could carry the same total traffic,
then that system can appear to be twice as efficient in terms of
mobiles per channel. It is obvious that using the above spectral
efficiency measure, one system can purposely try to inflate its
efficiency by adding more mobiles that generate little or no traffic to
the system.
(b) Channel spacing is not taken into consideration. A wide variety
of cellular land mobile radio systems can be offered as a solution
to spectral congestion. Channel spacings used could vary from
5 kHz for cellular systems employing SSB modulation techniques,
up to 300 kHz or more for spread spectrum systems. Unfortunately, the spectral efficiency measure in terms of Mobiles/Channel
does not account for channel spacing, and hence any advantages or
disadvantages of using one channel spacing over another are simply
not shown in the measure. This problem can be solved by using
mobiles per unit bandwidth as a measure of spectral efficiency. In
fact, both Mobiles/MHz and Users/MHz have been used by some
authors [2.4, 2.5].
(c) The above measure of spectral efficiency does not take into
account the geographic area covered by the system. To exemplify
this, consider two land mobile radio systems, whereby one of them
uses a base station with a very high antenna which covers a large area
of a 50 km radius and the other system uses a base station with a low
antenna covering only a small area of a 10 km radius. The two systems
may be serving the same number of mobiles (or users), however, in
the latter case, more base stations can be spaced at closer distances so
as to re-use the same radio frequencies, and hence serve more mobiles
within the same frequency band allocated for the service. In cellular
land mobile radio systems, the geographic area covered by the system
is a particularly important parameter which needs to be part of the
spectral efficiency measure.
POSSIBLE MEASURES OF SPECTRAL EFFICIENCY 11
2.3.2 Users/Cell
The measure of spectral efficiency as the number of users (or mobiles)
in a cell was introduced to account for cellular coverage, characteristic
to cellular land mobile radio systems. Although used by some authors
[2.5], the users/Cell measure also has certain deficiencies:
(a) The problem of unequal traffic still exists. This problem can be
solved by considering the amount of traffic which a particular system
can provide per cell.
(b) The problem of unequal channel spacings used by different
systems remains unsolved. Even by using the Channels/Cell measure, the number of channels the system can provide per cell raises the
objection of systems operating in different sizes of frequency bands.
Indeed, this can be adjusted by assuming all systems that are being
compared use the same amount of spectrum. Nevertheless, the measure as Channels/Cell does not instantly reflect that.
(c) Adopting Users/Cell seems to overcome the problem of
unequal coverage ± one of the objections of using Users/Channel as
a spectral efficiency measure. Unfortunately, it can still be argued
that different systems may use different cell sizes and different numbers of cells to offer the same service within one region. This is
because cellular systems employing different modulation techniques,
with possibly different channel spacings, may have different immunities against co-channel interference. Consequently, some systems can
employ smaller cells than others to offer the same quality of service. It
is obvious then, that a more accurate measure of the geographic area
covered by the system needs to be used. The most sensible measure of
the service area is to use square kilometres or square miles to replace
the concept of `cell' in the above spectral efficiency measure.
2.3.3 Channels/MHz
The measure of spectral efficiency as the number of channels which a
mobile radio system can provide per MHz appears in the literature
[2.6]. It gets around some of the deficiencies in the previous measures.
It particularly solves the problem of unequal channel spacings
employed by different systems by specifying the number of channels
which a system can provide per given MHz of the frequency band
12 SPECTRAL EFFICIENCY IN CELLULAR LAND MOBILE RADIO SYSTEMS
allocated for the service. The problem of unequal traffic is a minor one
here since the amount of traffic on the channel can be used instead.
Nonetheless, the problem of unequal coverage remains unsolved.
Although the spectral efficiency measure Channel/MHz is suitable
for point-to-point radio communications or one cell mobile radio
systems, it is not adequate for cellular land mobile radio systems.
2.3.4 Erlangs/MHz
In this measure of spectral efficiency, the Erlang* is used as a measure
of traffic intensity. The Erlang (E) measures the quantity of traffic on a
voice channel or a group of channels per unit time and, as a ratio of
time, it is dimensionless. One Erlang of traffic would occupy one
channel full time and 0.05 E would occupy it 5% of the time. Thus,
the number of Erlangs carried cannot exceed the number of channels
[2.7]. Using the above measure of spectral efficiency obviates some of
the shortcomings in the previous measures. It certainly solves the
problem of unequal traffic by using the Erlang as a definite measure
of traffic on a given number of voice channels provided by the system.
It implicitly accounts for the different channel spacings provided by
different systems by measuring the amount of traffic in Erlangs per
MHz of the frequency band allocated for the service. In other words,
the spectral efficiency in Erlangs/MHz is directly related to the measure in Channels/MHz, provided that blocking probabilities or waiting times are equal when systems are being compared. The measure
in Erlangs/MHz seems to be a good one, however, its principal
disadvantage is that the geographic area is still not included.
In the following section, the `spacial efficiency' factor will be added
to the above measure, in an attempt to arrive at the best measure (or
measures) of spectral efficiency in cellular systems.
2.4 BEST MEASURES OF SPECTRAL EFFICIENCY IN
CELLULAR SYSTEMS
Some proposed measures of spectral efficiency for cellular land
mobile radio systems were discussed in the previous section.
Although none of the suggested measures can be said to be totally
* The unit of telephone traffic is the Erlang, named after the Danish telephone engineer
A. K. Erlang, whose paper on traffic theory, published in 1909, is now considered a standard
text.
BestMeasuresof SpectralEfficiency in CellularSystems
BEST MEASURES OF SPECTRAL EFFICIENCY IN CELLULAR SYSTEMS 13
appropriate for cellular systems, it can be deduced that a successful
spectral efficiency measure must have the following features:
(a) It must measure the traffic intensity on the radio channels available for the cellular service. The Erlang as a suitable and definite
measure of traffic intensity will be used for this purpose.
(b) The amount of traffic intensity should be measured per unit
bandwidth of the frequency band allocated for the service (in MHz).
This will inherently account for different channel spacings employed
by various systems.
(c) The spacial efficiency factor or the geographic re-use of frequency must also be included in the measure in terms of unit area
of the geographic zone covered by the service (in km
2
or miles2).
The measure of spectral efficiency as Erlangs/MHz seems to satisfy
both (a) and (b) above. To include the spatial frequency re-use factor,
it is necessary to know the amount of traffic per unit bandwidth per
unit area covered by the service. This leads to the spectral efficiency
measure of
2
Erlangs/MHz/km
.
By Using the above measure of spectral efficiency to compare different cellular systems, the system which can carry more traffic in terms
of Erlangs per MHz of bandwidth in a given unit area of service can
be said to be spectrally more efficient.
2.4.1 Practical Considerations of the Measure Erlangs/MHz/km
2
The measure of spectral efficiency in terms of Erlangs/MHz/km
proves to be adequate, comprehensive and appropriate for cellular
land mobile radio systems. In the following, the choice of units for this
measure is justified and the practical considerations and assumptions
are pointed out.
(a) In the above measure of spectral efficiency, MHz is used as the
bandwidth unit, not kHz or Hz. This is because the measure deals
mainly with voice transmission (telephony), with possible channel
spacings of 5 kHz for SSB cellular systems and up to 300 kHz or
more for spread spectrum. In this case, a MHz can give rise to several
voice channels, and since the number of Erlangs cannot exceed the
2
14 SPECTRAL EFFICIENCY IN CELLULAR LAND MOBILE RADIO SYSTEMS
number of channels, a reasonable number of Erlangs per MHz can be
obtained. However, if kHz or Hz is used in the measure instead of
MHz, a very small fraction of an Erlang per kHz or per Hz is obtained,
which is not favourable for practical systems comparisons.
(b) It is also practicable to use km
2
(or miles2) as a measure of unit
area since it can accommodate a reasonable number of mobiles (or
users), which will in turn give rise to a reasonable spectral efficiency
figure for practical systems.
(c) In the above measure of spectral efficiency, there is an inherent
assumption that the traffic is uniformly distributed across the entire
service area, which is usually not the case in realistic systems. However, his does not seem to be a serious defect in the measure for two
reasons. Firstly, the relative spectral efficiency of cellular systems
under identical conditions is of prime interest, and hence any assumptions made will be equally applicable to all systems under comparison. Secondly, average traffic figures can be adequately used,
assuming uniform traffic within individual cells and not the entire
service area. Conversely, relative and absolute spectral efficiencies are
mostly needed in areas where the greatest demands in terms of
capacity exist. In these areas, such as city centres and metropolitan
areas, the smallest possible cell sizes must be used to give rise to a
maximum capacity, and hence the traffic can be considered to be
uniformly distributed within each individual cell.
(d) The above spectral efficiency measure can be used in such a way
that the efficiency of the multiple access technique employed by the
cellular system is accounted for. This is achieved by considering the
traffic on the voice channels during communication only, hence
excluding guard bands, supervision and set-up channels, etc. This
can be represented by the use of `paid Erlangs' in the above measure,
which reflects the amount of traffic intensity in the channels dedicated
to voice transmission during communication.
2.4.2 Alternative Spectral Efficiency Measures
An alternative and conceptually simpler measure of spectral efficiency in cellular land mobile radio systems is presented in terms of:
2
Voice Channels/MHz/km
.
BEST MEASURES OF SPECTRAL EFFICIENCY IN CELLULAR SYSTEMS 15
In this measure, the more voice channels per MHz a cellular system
can provide in a unit area, the more spectrally efficient it is considered
to be. `Voice Channels' is used in the measure to exclude guard bands,
supervision and set up channels, etc. Hence, the measure accounts for
the efficiency of the multiple access technique employed by the cellular system. This measure is particularly useful for cellular systems
which employ analogue modulation techniques, for which the channel spacing is directly known. Nevertheless, the spectral efficiency
measure in Channels/MHz/km
2
is also applicable to digital systems
if the number of voice channels in the frequency band allocated for the
service is known. This is usually specified in terms of the number of
channels per carrier, where the carrier spacing is given. This is equally
applicable to digital systems which use time division multiple access
(TDMA) techniques.
The spectral efficiency measure in Channels/MHz/km
related to the previous measure in Erlangs/MHz/km
sion from Channels/MHz/km
2
to E/MHz/km2is readily obtained
2
is directly
2
. The conver-
given an equivalent blocking probability or waiting time on the voice
channels, when the service is required (Figure 2.1), depending on the
traffic model used.
Another alternative measure of spectral efficiency for cellular systems
is:
2
Users/MHz/km
.
Figure 2.1 Best Measures of Spectral Efficiency in Cellular Systems
16 SPECTRAL EFFICIENCY IN CELLULAR LAND MOBILE RADIO SYSTEMS
It measures the efficiency of a cellular system in terms of the
number of users per MHz of bandwidth allocated for the service in
a unit area.
Unlike the way the term `user' in the above measure is commonly
used, it is intended to be used in such a way that traffic considerations
are included in the measure. To achieve this, the `user' is defined in
terms of the average traffic which he or she generates in a given
cellular system. Consequently, the spectral efficiency measure in
terms of Users/MHz/km
of E/MHz/km
2
(Figure 2.1). To give an example, if the spectral
efficiency of a cellular system is 5 E/MHz/km
generated per user in the system is say 0.05 E, then the efficiency of
that cellular system is 100 Users/MHz/km
2
is directly related to the measure in terms
2
and the average traffic
2
.
2.5 A POSSIBLE SPECTRAL EFFICIENCY MEASURE FOR
DIGITAL SYSTEMS
Digital cellular land mobile radio systems are becoming increasingly
popular. In fact, various digital cellular systems are being proposed
and some deployed in Europe [2.1, 2.8], North America [2.9] and
Japan [2.10]. In a digital modulation system, the voice channel is
defined in terms of kbits/s (kbps). The bandwidth efficiency of a
digital modulation system can be described in terms of bps/Hz.
This latter parameter can be extended to arrive at the following new
spectral efficiency measure for digital cellular systems:
kbps/MHz/km
2
.
According to this new spectral efficiency measure, the more kbps per
MHz a digital system can provide in a unit service area, the more
spectrally efficient it is considered to be. In the following, the advantages, disadvantages and limitations of the above spectral efficiency
measure are discussed in comparison with the best spectral efficiency
measures in the previous section:
(a) The spectral efficiency measure in terms of kbps/MHz/km
2
attractive to use with digital cellular systems, although it is not particularly useful for analogue systems. On the other hand, the measure
in terms of Channels/MHz/km
2
is equally applicable to both analogue and digital cellular systems, since a voice channel has a definite
meaning whether it is analogue or digitized. Also, the measure in
is
MEASURES OF SPECTRAL EFFICIENCY AND QUALITY CELLULAR SYSTEMS 17
terms of E/MHz/km2is superior to that in terms of kbps/MHz/km
because the former is equally applicable to both analogue and digital
cellular systems. Furthermore, the amount of traffic (in Erlangs)
which can be carried by a group of analogue voice channels is no
different from the traffic which can be carried by the same number of
digitized voice channels.
2
(b) The measure in terms of kbps/MHz/km
the channel spacing or the digitized channel bit rate. This is due to the
fact that the measure kbps/MHz/km
2
was constructed using the
does not account for
spectral efficiency of a digital system in bps/Hz without considering
the bit rate of the digitized channel in kbps. In this case, the spectral
efficiencies of the same digital system employing two different digitized voice channels bit rates will falsely appear to be identical. To give
an example, if a cellular system employs a digital modulation technique with a spectral efficiency of say 2 bps/Hz and uses a channel bit
rate of 16 kbps and another cellular system employs the same digital
modulation technique but uses a different channel bit rate of say 32
kbps, then the spectral efficiencies of the two cellular systems in terms
of kbps/MHz/km
2
will be identical. Nevertheless, considering the
channel bit rate in kbps, it is obvious that the former digital cellular
system can be twice as spectrally efficient as the latter. In fact, the
spectral efficiency of a digital cellular system in terms of kbps/MHz/
2
km
can be presented in terms of Channels/MHz/km2if coupled
with the bit rate of the digitized voice channel in kbps.
For the above reasons, measures in terms of Channels/MHz/km
E/MHz/km
hensive than the measure in kbps/MHz/km
kbps/MHz/km
2
and Users/MHz/km2are superior and more compre-
2
is useful to use with data-based cellular services
2
. Indeed, the measure
such as telex and facsimile.
2
2
,
2.6 MEASURES OF SPECTRAL EFFICIENCY AND THE
QUALITY OF CELLULAR SYSTEMS
From the previous analysis, the best measures of spectral efficiency
for cellular land mobile radio systems are:
. Channels/MHz/km
. Erlangs/MHz/km
. Users/MHz/km
2
2
2
MeasuresofSpectral Efficiencyand Quality CellularSystems
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