WIRELESS COMMUNICATION Unit-2 Final.pptx

EC6801
(Regulation: 2013)
WIRELESS COMMUNICATION

UNIT I WIRELESS CHANNELS
UNIT II CELLULAR ARCHITECTURE
UNIT III DIGITAL SIGNALING FOR FADING CHANNELS
UNIT IV MULTIPATH MITIGATION TECHNIQUES
UNIT V MULTIPLE ANTENNA TECHNIQUES

EC6801
(Regulation: 2013)
WIRELESS COMMUNICATION
UNIT II

Unit Syllabus : CELLULAR ARCHITECTURE
Multiple Access techniques - FDMA, TDMA, CDMA –
Capacity calculations–Cellular concept- Frequency
reuse – channel assignment- hand off- interference &
system capacity- trunking & grade of service –
Coverage and capacity improvement.

TEXTBOOKS:
1. Rappaport,T.S., “Wireless communications”, Second Edition, Pearson
Education, 2010.
2. Andreas.F. Molisch, “Wireless Communications”, John Wiley – India, 2006.

• The ability to communicate with people on the move has evolved
remarkably since Guglielmo Marconi first demonstrated radio's ability to
provide continuous contact with ships sailing the English channel. That
was in 1897.
• Most people are familiar with a number of mobile radio communication
systems used in everyday life., Remote controllers for home entertainment
equipment, cordless telephones, hand-held walkie-talkies, pagers (also
called paging receivers or "beepers"), and cellular telephones are all
examples of mobile radio communication systems.
• The cost, complexity, performance, and types of services offered by each
of these mobile systems are vastly different.
• The term mobile has historically been used to classify any radio terminal
that could be moved during operation. More recently, the term mobile is
used to describe a radio terminal that is attached to a high speed mobile
platform (e.g. a cellular telephone in a fast moving vehicle)`

The term portable describes a radio terminal that can be hand-held and used by
someone at walking speed (e.g. a walkie-talkie or cordless telephone inside a home).
The term subscriber is often used to describe a mobile, or portable user because in
most mobile communication systems, each user pays a subscription fee to use the
system, and each user's communication device is called a subscriber unit.
For a newcomer to the cellular radio field to understand the tremendous impact
that government regulatory agencies and service competitors wield in the evolution
of new wireless systems, services, and technologies.
Radio spectrum usage is controlled by governments, not by service
providers, equipment manufacturers, entrepreneurs, or researchers.
Progressive involvement in technology development is vital for a government to
keep its own country competitive in the rapidly changing field of wireless personal
communications.

• AT&T proposed the concept of a cellular mobile system to the FCC in 1968,
although technology was not available to implement cellular telephony until
the late 1970s.
• In 1983, the FCC (Federal Communications Commission) finally allocated 666
duplex channels (40 MHz of spectrum in the 800 MHz band, each channel
having a one-way bandwidth of 30kHz for a total spectrum occupancy of 60
kHz for each duplex channel) for the U.S. Advanced Mobile Phone System
(AMPS)
• AMPS was the first U.S. cellular telephone system, and was deployed in late
1983.
• In late 1991, the first U.S. Digital Cellular (USDC) system hardware was
installed in major U.S. cities.
• The USDC standard (Electronic Industry Association Interim Standard IS-54)
allows cellular operators to replace gracefully some single-user analog
channels with digital channels which .support three users in the same 30 kHz
bandwidth [EIA90].

• The capacity improvement offered by USDC is three times that of AMPS,
because digital modulation (π/4 differential quadrature phase shift keying).
speech coding, and time division multiple access (TDMA) are used in place of
analog FM and FDMA. Given the rate of digital signal processing
advancements.
• A cellular system based on code division multiple access (CDMA) has been
developed by Qualcomm, lnc.
• CDMA system supports a variable number of users in 1.25 MHz wide
channels using direct sequence spread spectrum.
• The analog AMPS system requires that the signal be at least 18 dB above the
co-channel interference to provide acceptable call quality.
• CDMA systems can operate at much larger interference levels because of their
inherent interference resistance properties.
• The ability of CDMA to operate with a much smaller signal-to-noise ratio than
conventional narrowband FM techniques allows CDMA systems to use the
same set of frequencies in every cell.

Frequency Division Multiple Access (FDMA)
• Frequency division multiple access (FDMA) assigns individual channels
to individual users.
• Each user is allocated a unique frequency band or channel.
• These channels are assigned on demand to users who request service.
• During the period of the call, no other user can share the same
frequency band.

In FDD systems, the users are assigned a channel as a pair of frequencies;
one frequency is used for the forward channel, while the other frequency is
used for the reverse channel.
The features of FDMA are as follows:
• The FDMA channel carries only one phone circuit at a time.
• If an FDMA channel is not in use, then it sits idle and cannot be used
by other users to increase or share capacity. It is essentially a wasted
resource.
• After the assignment of a voice channel, the base station and the
mobile transmit simultaneously and continuously.
• The bandwidths of FDMA channels are relatively narrow (30 kHz) as
• each channel supports only one circuit per carrier.
• That is, FDMA is usually implemnted in narrowband systems.

• The symbol time is large as compared to the average delay spread.
This allows, the amount of inter symbol interference is low and, thus,
little or no equalization is required in FDMA narrowband systems.
• The complexity of FDMA mobile systems is lower when compared to
TDMA systems, though this is changing as digital signal processing
methods improve for TDMA.
• Since FDMA is a continuous transmission scheme, fewer bits are
needed for overhead purposes (such as synchronization and framing
bits) as compared to TDMA.
• FDMA systems have higher cell site system costs as compared to
TDMA systems, because of the single channel per carrier design, and
the need to use costly band pass filters to eliminate spurious
radiation at the base station.

• The FDMA mobile unit uses duplexers since both the transmitter and
receiver operate at the same time. This results in an increase in the cost
of FDMA subscriber units and base stations.
• FDMA requires tight RF filtering to minimize adjacent channel
interference.
Nonlinear Effects in FDMA:
• In a FDMA system, many channels share the same antenna at the base
station. The power amplifiers or the power combiners.
• When operated at or near saturation for maximum power efficiency, are
nonlinear.
o The nonlinearities cause signal spreading in the frequency
domain and generate intermodulation {IM) frequencies.
o IM is undesired RF radiation which can interfere with other
channels in the FDMA systems. Spreading of the spectrum
results in adjacent-channel interference.

o Intermodulation is the generationof undesirable harmonics.
Harmonics generated outside the mobile radio band
o cause interference to adjacent services, while those present inside
the bandcause interference to other users in the mobile system.
Example1:
Find the intermodulation frequencies generated if a base station transmits
two carrier frequencies at 1930 MHz and 1932 MHz that are amplified by a
saturated clipping amplifier. If the mobile radio band is allocated from
1920 MHz to 1940 MHz, designate the IM frequencies that lie inside and
outside the band.
Solution:
Intermodulation distortion products occur at frequencies rnf1 + nf2 for all
integer values of m and n, i.e., -∞><m, n <∞ . Some of the possible
intermodulation frequencies that are produced by a nonlinear device are
(2n + 1)f1- 2nf2, (2n + 2)f1- (2n + 1)f2, (2n + 1)f1-2nf2,
(2n + 2)f2- (2n + 1)f1, etc. for n = 0, 1, 2, ...... .
Table lists several intermodulation product terms.

Table1: Intermodulation Products
The frequencies in the table marked with an asterisk (*) are the frequencies
that lie outside the mobile radio band.
The number of channels that can be simultaneously supported in a FDMA
system is given by
N=
𝑩𝒕−𝟐𝑩𝒈𝒖𝒓𝒅
𝑩𝒄
(1)
where Btis the total spectrum allocation, Bguard is the guard band allocated at
the edge of the allocated spectrum, and Bcis the channel bandwidth.

Example 2.:
If Btis 12.5 MHz, Bguardis 10kHz, and Bc, is 30kHz, find the number of
channels available in an FDMA system.
Solution:
The number of channels available in the FDMA system is given by equation1.
In each cellular carrier is allocated 416 channels.
Example 3. :
In US AMPS, 416 channels are allocated to various cellular operator. the
channel between them is 30 kHz with the guard band of 10 kHz. Calculate
the spectrum allocation given to each operator.
Solution: By some simple manipulation/simplification in equation 1.
We get 𝑩𝒕 = 𝑵𝑩𝒄 + 𝟐𝑩𝒈𝒖𝒂𝒓𝒅 = 𝟒𝟏𝟔 × 𝟑𝟎 × 𝟏𝟎𝟑 + 𝟐 𝟏𝟎 × 𝟏𝟎𝟑 = 12.5MHz
ANSWER: 12.5MHz is band allocated for each simplex band.

Time Division Multiple Access (TDMA)
TDMA systems divide the radio spectrum into time slots, and in each slot only
one user is allowed to either transmit or receive.
Figure 3. TDMA scheme where each channel
occupies a cyclically repeating time slot
From the above Figure we observe fallowing,
• Each user occupies a cyclically repeating time slot, so a channel may be
thought of as particular time slot that reoccurs every frame, where N time
slots comprise a frame.
• TDMA systems transmit data in a buffer-and-burst method, thus the
transmission for any user is noncontinuous.

Unlike in FDMA systems which accommodate analog FM, digital data and
digital modulation must be used with TDMA.
The transmission from various users is interlaced into a repeating frame
structure.
Figure 4.
TDMA frame structure.
From figure 4. we observe fallowing
• Frame consists of a number of slots.
• Each frame is made up of a preamble, an information message, and tail bits.
• In TDMA/TDD (Frequency Division Duplexing), Half of the time slots in the
frame information message would be used for the forward link channels and
half would be used for reverse link channels.

• In TDMA/FDD systems, an identical or similar frame structure would be
used solely for either forward or reverse transmission, but the carrier
frequencies would be different for the forward and reverse links.
• In general, TDMA/FDD systems intentionally induce several time slots of
delay between the forward and reverse time slots of a particular user, so
that duplexers are not required in the subscriber unit.
• In a TDMA frame, the preamble contains the address and
synchronization
• information that both the base station and the subscribers use to identify
each other.
• Guard times are utilized to allow synchronization of the receivers between
different slots and frames.
• Different TDMA wireless standards have different TDMA frame
structures.

The features of TDMA include the following:
• TDMA shares a single carrier frequency with several users, where each user
makes use of nonoverlapping time slots. The number of time slots per frame
depends on several factors, such as modulation technique, available
bandwidth, etc.
• Data transmission for users of a TDMA system is not continuous, but occurs
in bursts. This results in low battery consumption, since the subscriber
transmitter can be turned off when not in use (which is most of the time).
• Because of discontinuous transmissions in TDMA, the handoff process is
much simpler for a subscriber unit, since it is able to listen for other base
stations during idle time slots An enhanced link control, such as that provided
by mobile assisted handoff(MARO) can be carried out by a subscriber by
listening on an idle slot in the TDMA frame.

• TDMA uses different time slots for transmission and reception, thus duplexers
are not required. Even if FDD is used, a switch rather than a duplexer
inside the subscriber unit is all that is required to switch between transmitter
and receiver using TDMA.
• Adaptive equalization is usually necessary in TDMA systems, since the
transmission rates are generally very high as compared to FDMA channels.
• In TDMA, the guard time should be minimized. If the transmitted signal at
the edges of a time slot are suppressed sharply in order to shorten the guard
time, the transmitted spectrum will expand and cause interference to adjacent
channels.
• High synchronization overhead is required in TDMA systems because of
burst transmissions. TDMA transmissions are slotted, and this requires the
receivers to be synchronized for each data burst. In addition, guard slots are
necessary to separate users, and this results in the TDMA systems having
larger overheads as compared to FDMA.

• TDMA has an advantage in that it is possible to allocate different numbers of
time slots per frame to different users. Thus bandwidth can be supplied on
demand to different users by concatenating or reassigning time slots based
on priority.
Efficiency of TDMA –
• The efficiency of a TDMA system is a measure of the percentage of transmitted
data that contains information as opposed to providing overhead for the access
scheme.
• The frame efficiency, 𝜼𝒇 is the percentage of bits per frame which contain
transmitted data.
Note that the transmitted data may include source and channel coding bits, so
the raw end-user efficiency of a system is generally less than 𝜼𝒇 The frame
efficiency can be found as follows.

The number of overhead bits per frame, 𝒃𝑶𝑯 is,
(2)
where, Nr is the number of reference bursts per frame,
N, is the number of traffic bursts per frame,
bris the number of overhead bits per reference burst,
bPis the number of overhead bits per preamble in each slot,
bgis the number of equivalent bits in each guard time interval.
The total number of bits per frame, bT, is
bT= Tf R (3)
where Tfis the frame duration,
R is the channel bit rate.
The frame efficiency 𝜼𝒇 is thus given as
𝜼𝒇 = 𝟏 −
𝒃𝑶𝑯
𝒃𝑻
× 𝟏𝟎𝟎% (4)
𝒃𝑶𝑯 = 𝑵𝒓𝒃𝒓 + 𝑵𝒕𝒃𝒑 + 𝑵𝒕𝒃𝒈 + 𝑵𝒓𝒃𝒈

Number of channels in TDMA system –
The number of TDMA channel slots that can be provided in a TDMA system is found
by multiplying the number of TDMA slots per channel by the number of channels
available
𝑵 =
𝒎 𝑩𝒕𝒐𝒕−𝟐𝑩𝒈𝒖𝒂𝒓𝒅
𝑩𝑪
(5)
where m is the maximum number of TDMA users supported on each
radio channel.
Note that two guard bands, one at the low end of the allocated
frequency band and one at the high end, are required to ensure
that users at the edge of the band do not "bleed over" into an
adjacent radio service.

Example 3.
Consider Global System for Mobile, which is a TDMA/FDD system that uses 25 MHz
for the forward link, which is broken into radio channels of 200 kHz. If 8 speech
channels are supported on a single radio channel, and if no guard band is assumed,
find the number of simultaneous users that can be accommodated in GSM.
Solution
The number of simultaneous users that can be accommodated in GSM is given as
𝑵 =
𝟐𝟓𝑴𝑯𝒛
𝟐𝟎𝟎 𝒌𝑯𝒛/𝟖
= 𝟏𝟎𝟎𝟎
GSM can accommodate 1000 simultaneous users.

Example 4.
If GSM uses a frame structure where each frame consists of 8 time slots, and
each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in
the channel, find
(a) the time duration of a bit,
(b) the time duration of a slot,
(c) the time duration of a frame,
(d) how long must a user occupying a single time slot must wait between two
simultaneous transmissions.
Solution
(a) The time duration of a bit, 𝑻𝒃 =
𝟏
𝟐𝟕𝟎.𝟖𝟑𝟑 𝒌𝒃𝒑𝒔
= 𝟑. 𝟔𝟗𝟐𝝁𝒔
(b) The time duration of a slot, Tslot= 156.25 x Tb= 0.577 ms.
(c) The time duration of a frame, Tr= 8 x Tslot= 4.615 ms.
(d) A user has to wait 4.615 ms, the arrival time of a new frame, for its next
transmission.

Example 5
If a normal GSM time slot consists of 6 stop bit(also called trailing bits), 8.25 guard
bits, 26 training bits, and 2 traffic bursts of 58 bits of encrypted data which is
transmitted at 270.833 kbps. Find
(a) Number of overhead bits per frame, bOH
(b) Total number of bits/frame
(c) Frame rate
(d) Time duration of a slot
(e) Frame efficiency.
Solution
A time slot has 6 + 8.25 + 26 + 2(58) = 156.25 bits.
(a) Number of overhead bits per frame, b0H= 8(6) +8(8.25) +8(26) = 312 bits
(b) Number of bits/frame, 8 ×156.25 = 1250 bits/frame.
(c) Frame rate , 270.833 kbps/1250 bits/frame = 216.66 frame/sec
(d) Time duration of a slot, 156.25 × 1/270.833 kbps = 576.92 𝝁s
(e) Frame efficiency, 𝜼𝒇 =[1-(322/1250)] = 74.24%

Spread Spectrum Multiple Access
• Spread spectrum multiple access (SSMA) uses signals which have a transmission
bandwidth that is several orders of magnitude greater than the minimum
required RF bandwidth.
• A pseudo-noise (PN) sequence converts a narrowband signal to a wide band
noise-like signal before transmission.
• SSMA also provides immunity to multipath interference and robust multiple
access capability.
• SSMA is not very bandwidth efficient when used by a single user. However,
since many users can share the same spread spectrum bandwidth without
interfering with one another, spread spectrum systems become bandwidth
efficient in a multiple user environment.

It is exactly this situation that is of interest to wireless system designers.
There are two main types of spread spectrum multiple access techniques;
 frequency hopped multiple access (FH)
 direct sequence multiple access (DS).
Direct sequence multiple access is also called code division multiple access (CDMA).
Frequency Hopped Multiple Access (FHMA)
• Frequency hopped multiple access (FHMA) is a digital multiple access system
in which the carrier frequencies of the individual users are varied in a
pseudorandom fashion within a wideband channel.
• The digital data is broken into uniform sized bursts which are transmitted on
different carrier frequencies(channel) within the allocated spectrum band.
• The instantaneous bandwidth of any one transmission burst is much smaller than
the total spread bandwidth.

The pseudorandom change of the carrier frequencies of the user randomizes the
occupancy of a specific channel at any given time. thereby allowing for multiple access
over a wide range of frequencies.
In the FH receiver, a locally generated PN code is used to synchronize the receivers
instantaneous frequency with that of the transmitter.
At any given point in time, a frequency hopped signal only occupies a single, relatively
narrow channel since narrowband FM or FSK is used.
The difference between FHMA and a traditional FDMA system is that the frequency
hopped signal changes channels at rapid intervals.
If the rate of change of the carrier frequency is greater than the symbol rate then the
system is referred to as a fast frequency hopping system.
If the channel changes at a rate less than or equal to the symbol rate, it is called slow
frequency hopping.

• A fast frequency hopper may thus be thought of as an FDMA system which
employs frequency diversity.
• FHMA systems often employ energy efficient constant envelope modulation.
inexpensive receivers may be built to provide noncoherent detection of FHMA.
• This implies that linearity is not an issue, and the power of multiple users at the
receiver does not degrade FHMA performance.
• A frequency hopped system provides a level of security, especially when a
large number of channels are used, since an unintended (or an intercepting}
receiver that does not know the pseudorandom sequence of frequency slots must
retune rapidly to search for the signal it wishes to intercept.
• In addition, the FH signal is somewhat immune to fading, since error control
coding and interleaving can be used to protect the frequency hopped signal against
deep fades which may occasionally occur during the hopping sequence.
• Error control coding and interleaving can also be combined to guard against
erasures which can occur when two or more users transmit on the same channel at
the same time.

Code Division Multiple Access (CDMA)
• In code division multiple access (CDMA) systems. the narrowband message
signal is multiplied by a very large bandwidth signal called the spreading signal.
• The spreading signal is a pseudo-noise code sequence that has a chip rate which
is orders of magnitudes greater than the data rate of the message.
• All users in a CDMA system, use the same carrier frequency and may transmit
simultaneously.
• Each user has its own pseudorandom codeword which is approximately orthogonal
to all other codewords.
• The receiver performs a time correlation operation to detect only the specific
desired codeword. All other codewords appear as noise due to decorrelation.
• For detection of the message signal, the receiver needs to know the codeword used
by the transmitter.

Figure 4. CDMA in which each channel is assigned a unique PN code which is
orthogonal to PN codes used by other users.

• Each user operates independently with no knowledge of the other users.
• In CDMA, the power of multiple users at a receiver determines the noise
floor after decorrelation. If the power of each user within a cell is not
controlled such that they do not appear equal at the base station receiver,
then the near-far problem occurs.
• The near-far problem occurs when many mobile users share the same
channel.
• In general, the strongest received mobile signal will capture the demodulator
at a base station.
• In CDMA, stronger received signal levels raise the noise floor at the base
station demodulators for the weaker signals, thereby decreasing the
probability that weaker signals will be received.
• To combat the near-far problem, power control is used in most CDMA
implementations. Power control is provided by each base station in a cellular
system and assures that each mobile within the base station coverage area
provides the same signal level to the base station receiver.

• This solves the problem of a nearby subscriber overpowering the base
station receiver and drowning out the signals of far away subscribers.
• Power control is implemented at the base station by rapidly sampling the
radio signal strength indicator (RSSI) levels of each mobile and then sending
a power change command over the forward radio link. Despite the use of
power control within each cell, out-of-cell mobiles provide interference
which is not under the control of the receiving base station.
The features of CDMA including the following:
• Many users of a CDMA system share the same frequency. Either TDD or
FDD may be used.
• Unlike TDMA or FDMA, CDMA has a soft capacity limit. Increasing the
number of users in a CDMA system raises the noise floor in a linear manner.
Thus, there is no absolute limit on the number of users in CDMA. Rather, the
system performance gradually degrades for all users as the number of
users is increased, and improves as the number of users is decreased.

• Multipath fading may be substantially reduced because the signal is spread over a
large spectrum. If the spread spectrum bandwidth is greater than the coherence
bandwidth of the channel, the inherent frequency diversity will mitigate the
effects of small-scale fading.
• Channel data rates are very high in CDMA systems. Consequently, the symbol
(chip) duration is very short and usually much less than the channel delay spread.
Since PN sequences have low autocorrelation, multi path which is delayed by
more than a chip will appear as noise.
A RAKE receiver can be used to improve reception by
collecting time delayed versions of the required signal.
• Since CDMA uses co-channel cells, it can use macroscopic spatial diversity to
provide soft handoff. Soft handoff is performed by the MSC, which can
simultaneously monitor a particular user from two or more base stations. The
MSC may chose the best version of the signal at any time without switching
frequencies.

• Self-jamming is a problem in CDMA system. Self-jamming arises from the fact
that the spreading sequences of different users are not exactly orthogonal,
hence in the despreading of a particular PN code, non-zero contributions to the
receiver decision statistic for a desired user arise from the transmissions of
other users in the system.
• The near-far problem occurs at a CDMA receiver if an undesired user has a
high detected power as compared to the desired user.
Capacity of Cellular Systems
• Channel capacity for a radio system can be defined as the maximum number
of channels or users that can be provided in a fixed frequency band.
• Radio capacity is a parameter which measures spectrum efficiency of a wireless
system. This parameter is determined by the required carrier-to-interference
ratio (C/I) and the channel bandwidth Bc.

In a cellular system the interference at a base station receiver will come from the
subscriber units in the surrounding cells. This is called reverse channel
interference.
For a particular subscriber unit, the desired base station will provide the desired
forward channel while the surrounding co-channel base stations will provide the
forward channel interference.
Considering the forward channel interference problem,
Let D be the distance between two co-channel cells
R be the cell radius.
Then the minimum ratio of D/R that is required to provide a tolerable level of
co-channel interference is called the co-channel reuse ratio and is given by.
𝑸 =
𝑫
𝑹
(6)

The radio propagation characteristics at a given location along with Noise figure
calculation for link budgetare used to find determine sensible the carrier-to-
interference ratio (C/I).
The M closest co-channel cells (As shown in Figure 5.) may be considered as first
order interference in which case C/I is given by
𝑪
𝑰
=
𝑫𝟎
−𝒏𝟎
𝒌=𝟏
𝑴 𝑫𝒌
−𝒏𝒌
(7)
where
n0 is the path loss exponent in the desired cell
D0is the distance from the desired base station to the mobile
Dk is the distance of the kthcell from the mobile
nk is the path loss exponent to the kthinterfering base station.

Figure 8. Forward channel interference for a cluster size of N = 4. Shown here are four
co-channel base stations which interfere with the serving base station. The distance
from the serving base station to the user is D0 , and interferers are a distance Dk from
the user.

• If only the six closest interfering cells are considered, and all are approximately
at the same distance D and have similar path loss exponents equal to that in the
desired cell, then C/I is given by
𝑪
𝑰
=
𝑫𝟎
−𝒏
𝟔𝑫−𝒏 (8)
• It is assumed that maximum interference occurs when the mobile is at the cell
edge D0= R. and if the C/I for each user is required to be greater than some
minimum ( C/ I) min, which is the minimum carrier-to-interference ratio that
still provides acceptable signal quality at the receiver,
Foe above assumption following equation hold for acceptable performance:
𝟏
𝟔
𝑹
𝑫
−𝒏
≥
𝑪
𝑰 𝒎𝒊𝒏
(9)
Substituting equation 6. in to equation 9., we get the co-channel reuse factor
𝑸 = 𝟔
𝑪
𝑰 𝒎𝒊𝒏
𝟏/𝒏
(10)

The radio capacity of a cellular system is defined as
𝒎 =
𝑩𝒕
𝑩𝒄𝑵
radio channels/cell(11)
where m is the radio capacity metric
Btis the total allocated spectrum for the system
Bcis the channel bandwidth
N is the number of cells in a frequency reuse pattern
N is related to the co-channel reuse factor Q by
𝑸 = 𝟑𝑵 (12)
From Equations (10), (11), and (12), the radio capacity is given as
𝒎 =
𝑩𝒕
𝑩𝒄
𝑸𝟐
𝟑
=
𝑩𝒕
𝑩𝒄
𝟔
𝟑𝒏/𝟐
𝑪
𝑰 𝒎𝒊𝒏
𝟐/𝒏(13)
As per Lee [Lee89a], when n = 4 , the radio capacity is given by
m =
𝑩𝒕
𝑩𝒄
𝟐
𝟑
𝑪
𝑰 𝒎𝒊𝒏
radio channel/cell (14)

In order to provide the same voice quality,
( C/I)min may be lower in a digital systems when compared to an
analog system.
Typically,
The minimum required C/I is 12 dB for narrowband digital systems
18 dB for narrowband analog FM systems,
exact values are determined by subjective listening tests in real-world
propagation conditions.
Each digital wireless standard has a different ( C/I)min , and in order to compare
different systems, an equivalent C/I is used.
If Bt and m are kept constant in Equation (14), then it is clear that Bc and (C/I)min
are related by

𝑪
𝑰 𝒆𝒒
=
𝑪
𝑰 𝒎𝒊𝒏
𝑩𝒄
𝑩𝒄
′
𝟐
(15)
where Bcis the bandwidth of a particular system
( C/I)minis the tolerable value for the same system
𝑩𝒄
′ is the channel bandwidth for a different system,
(C/I)eqis the minimum C/I value for the different system when compared
to the ( C/I)minfor a particular system.
• Notice that for a constant number of users per radio channel, the same voice
quality will be maintained in a different system if (C/I)minincreases by a factor
of four when the bandwidth is halved.
Equation (14) indicates that maximum radio capacity occurs when (C/ I)min
and Beare minimized,
yet equation (15) shows that (C/I)minand Beare inversely related.

Example 6
Evaluate four different cellular radio standards, choose the one with the
maximum radio capacity.
System A: Bc =30.0 kHz, (C/I)min = 18 dB
System B: Bc =25.0kHz, (C/I)min = 14 dB
System C: Bc =12.5 kHz, (C/I)min = 12 dB
System D: Bc =6.25 kHz, (C/I)min = 9 dB
Solution
Consider each system for 6.25 kHz bandwidth, and use equation (15)
System A; Be = 6.25 kHz, (C/I)eq = 18 + 20 log (6.25/30) = 4.375 dB
System B; Be= 6.25 kHz, (C/I)eq = 14 + 20log (6.25/25) = 1.96 dB
System C; Be= 6.25 kHz, (C/I)eq = 12 + 201og (6.25/12.5) = 6 dB
System D; Be =6.25 kHz, (C/I)eq = 9 + 20 log (6.25/6.25) = 9 dB
Based on comparison, the smallest value of (C/I)eq should be selected for
maximum capacity in Equation (14). System B offers the best capacity.

In a digital cellular system, C/I can be expressed as
𝑪
𝑰
=
𝑬𝒃𝑹𝒃
𝑰
=
𝑬𝒄𝑹𝒄
𝑰
(16)
Where Rb is channel bit rate,
E0is the energy per bit,
Rcis the rate of the channel code
Ecis the energy per code symbol
From equations (15) and (16), the ratio of C/I to ( C/I)eqis given
𝑪
𝑰
𝑪
𝑰 𝒆𝒒
=
𝑬𝒄𝑹𝒄
𝑰
𝑬𝒄
′𝑹𝒄
′
𝑰
=
𝑩𝒄
′
𝑩𝒄
𝟐
(17)
The relationship between Rcand Bcis always linear, and if the interference
level I is the same in the mobile environment for two different digital systems,
then equation (17) can be rewritten as

Equation (15) shows that
if Bcis reduced by half, then the energy code symbol increases eight times.
This gives the relationship between EblN0and Bcin a digital cellular system.
A comparison can now be made between the spectrum efficiency for FDMA
and TDMA.
• In FDMA, B, is divided into M channels, each with bandwidth B,.
therefore, the radio capacity for FDMA is given by
𝒎 =
𝑩𝒕
𝑩𝒕
𝑴
𝟐
𝟑
𝑪
𝑰
(19)
Consider the case
where a multichannel FDMA system occupies the same spectrum as a single channel
TDMA system with multiple time slots.
The carrier and interference terms for the first access technique (in this case FDMA)

can be written as,
C = EbRb, I = IoBc,
whereas the second access technique (in this case TDMA) has
carrier and interference terms represented by
C' = Eb𝑹𝒃
′
, 𝑰′ = 𝑰𝒐𝑩𝒄
′
where Rb and 𝑹𝒃
′
are the radio transmission rates of two digital systems,
Ebis the energy per bit,
I0represents the interference power per Hertz.
The terms C'and 𝑰′ are the parameters for the TDMA channels,
The terms C and I apply to the FDMA channels.
Example8.8
Consider an FDMA system with three channels, each having a bandwidth of 10 kHz
and a transmission rate of 10 kbps. A TDMA system has three time slots, channel
bandwidth of 30 kHz, and a transmission rate of 30 kbps.
For the TDMA scheme, the received carrier-to-interference ratio for a single user is
measured for 1/3 of the time the channel is in use. For example, C'/𝑰′can be measured
in 333.3 ms in one second Thus C'/𝑰′If is given by

𝑪′
= 𝑬𝒃𝑹𝒃
′
=
𝑬𝒃𝟏𝟎𝟒𝒃𝒊𝒕
𝟎.𝟑𝟑𝟑 𝒔
= 3RbEb=3C
𝑰′
= 𝑰𝒐𝑩𝒄
′
= 𝑰𝒐30 kHz = 3I
It can be seen that the received carrier-to-interference ratio for a user in this
TDMA system C' /𝑰′
is the same as C/I for a user in the FDMA system. Therefore,
for this example, FDMA and TDMA have the same radio capacity and consequently
the same spectrum efficiency.
However, the required peak power for TDMA is 10logk higher than FDMA, where k
is the number of time slots in a TDMA system of equal bandwidth.
Capacity of Digital Cellular TDMA –
• In practice, TDMA systems improve capacity by a factor of 3 to 6 times as
compared to analog cellular radio systems.
• Powerful error control and speech coding enable better link performance
in high interference environments.
• By exploiting speech activity, some TDMA systems are able to better utilize each
radio channel.

Capacity of Cellular CDMA
• The capacity of CDMA systems is interference limited, while it is bandwidth
limited in FDMA and TDMA. Therefore, any reduction in the interference
will cause a linear increase in the capacity of CDMA. Put another way, in a
CDMA system, the link performance for each user increases as the number of
users decreases.
• A straightforward way to reduce interference is to use multi sectorized
antennas, which results in spatial isolation of users.
• The directional antennas receive signals from only a fraction of the current
users, thus leading to the reduction of interference.
• While TDMA and FDMA reuse frequencies depending on the isolation
between cells provided by the path loss in terrestrial radio propagation,
CDMA can reuse the entire spectrum for all cells, and this results in an
increase of capacity by a large percentage over the normal frequency reuse
factor.

For evaluating the capacity of CDMA system, first consider a single cell system.
The cellular network consists of a large number of mobile users communicating
with a base station (In a multiple cell system, all the base stations
are interconnected by the mobile switching center).
The cell-site transmitter consists of a linear combiner which adds the spread signals
of the individual users and also uses a weighting factor for each signal for forward
link power control purposes. A pilot signal is also included in the cell-site
transmitter and is used by each mobile to set its own power control for the reverse
link.
For a single cell system under consideration, these weighting factors
can be assumed to be equal.
For a single-cell system with power control, all the signals on the reverse channel
are received at the same power level at the base station.

Let the number of users be N.
• each demodulator at the cell site receives a composite waveform containing
the desired signal of power S and (N- I) interfering users, the signal-to-noise
ratio is,
𝑺𝑵𝑹 =
𝑺
𝑵−𝟏 𝑺
=
𝟏
𝑵−𝟏
(20)
• bit energy-to-noise ratio is an important parameter in communication
systems. It is obtained by dividing the signal power by the baseband
information bit rate, R, and the interference power by the total RF
bandwidth, W. The SNR at the base station receiver can be represented in
terms of Eb/N0 given by
𝑬𝒃
𝑵𝟎
=
𝑺/𝑹
𝑵−𝟏 𝑺/𝑾
=
𝑾/𝑹
𝑵−𝟏
(21)
Equation (21) does not take into account the background thermal noise,𝜼, in the
spread bandwidth. To take this noise into consideration, Eb/N0can be
represented as
𝑬𝒃
𝑵𝟎
=
𝑾/𝑹
𝑵−𝟏 + 𝜼/𝑺
(22)

On simplification of equation(22) , The number of users that can access
the system is given as
𝑵 = 𝟏 +
𝑾/𝑹
𝑬𝒃/𝑵𝟎
− 𝜼/𝑺 (23)
where W/R is called the processing gain.
• The background noise determines the cell radius for a given transmitter power.
• In order to achieve an increase in capacity, the interference due to other users
should be reduced.
This can be done by decreasing the denominator of equations (20) or (21).
• The first technique for reducing interference is antenna sectorization.
As an example, a cell site with three antennas, each having a beam width of
120°, has interference 𝑵𝟎
′
which is one-third of the interference received by
an omni directional antenna.
This increases the capacity by a factor of 3 since three times as many users may now
be served within a sector while matching the performance of the omni-directional
antenna system.

• The second technique involves the monitoring of voice activity such that each
transmitter is switched off during periods of no voice activity.
Voice activity is denoted by a factor 𝜶, and the interference term in equation
(21) becomes (NS-1 )𝜶, where NS is the number of users per sector.
With the use of these two techniques, the new average value of Eb/𝑵𝟎
′
within a
sector is given as
𝑬𝒃
𝑵𝟎
′ =
𝑾/𝑹
𝑵𝑺−𝟏 𝜶+ 𝜼/𝑺
(24)
• When the number of users is large and the system is interference limited rather
than noise limited, the number of users can be shown to be
𝑵𝑺 = 𝟏 +
𝟏
𝜶
𝑾/𝑹
𝑬𝒃
𝑵𝟎
(25)
• If the voice activity factor is assumed to have a value of 3/8, and three sectors per
cell site are used, Equation (8.33) demonstrates that the SNR increases by a factor
of 8, which leads to an 8 fold increase in the number of users compared to an
omni-directional antenna system with no voice activity detection.

CDMA Power Control –
• In CDMA, the system capacity is maximized if each mobile transmitter power
level is controlled so that its signal arrives at the cell site with the minimum
required signal-to-interference ratio.
• If the signal powers of all mobile transmitters within an area covered by a cell
site are controlled, then the total signal power received at the cell site from all
mobiles will be equal to the average received power times the number of
mobiles operating in the region of coverage.
A trade-off must be made…
 If a mobile signal arrives at the cell site with a signal that is too weak,
and often the weak user will be dropped.
 If the received power from a mobile user is too great the performance of
this mobile unit will be acceptable, but it will add undesired interference
to all other users in the cell.

Example 7
In IS-95 CDMA system, if W = 1.25 MHz, R = 9600 bps, and N=14 users
Calculate
(a) minimum acceptable Eb/ N0
(b) The maximum number of users that can be supported in a single-cell CDMA
system using
(a) omni-directional base station antennas and no voice activity detection
(b) 3-sectors at the base station and activity detection with a = 3/8.
Assume the system is interference limited.
Solution
(a) Using equation (21),
𝑬𝒃
𝑵𝟎
=
𝑾/𝑹
𝑵−𝟏
= (1.25×106/9600 bps)/(14-1) =10 dB
b) Using equation (25) for each sector we can find NS = 1+
𝟏
𝟎.𝟑𝟕𝟓
[
𝟏.𝟐𝟓×𝟏𝟎𝟔/𝟗𝟔𝟎𝟎
𝟏𝟎
]= 35.7
The total number of users is given by 3NS , since three sectors exist within a cell;
therefore N = 3 x 35.7 = 107 users/ cell.

Capacity of COMA with Multiple Cells
• In actual CDMA cellular systems that employ separate forward and reverse
links, neighboring cells share the same frequency, and each base station
controls the transmit power of each of its own in-cell users.
However, a particular base station is unable to control the power of
users in neighboring cells, and these users add to the noise floor and
decrease capacity on the reverse link of the particular cell of interest.
Figure 9. Illustration of users within a CDMA cellular radio system. Each base station reuses the
same frequency. The dots represent users within the system, which have their transmit power
controlled by their own base station.

• The transmit powers of each out-of-cell user will add to the in-cell interference
(where users are under power control) at the base station receiver.
• The amount of out-of-cell interference determines the frequency reuse factor, f, of
a CDMA cellular system.
• Ideally, each cell shares the same frequency and the maximum possible value of f
(f =1) is achieved.
• In practice, however, the out-of-cell interference reduces f significantly. In
contrast to CDMA systems which use the same frequency for each cell,
narrowband FDMA/FDD systems typically reuse channels every seven cells, in
which case f is simply 1/7.
• The frequency reuse factor for a CDMA system on the reverse link can be defined
as
𝒇 =
𝑵𝟎
𝑵𝟎+ 𝒊 𝑼𝒊𝑵𝒂𝒊
(26)
and the frequency reuse efficiency, F, is defined as
F = f × 𝟏𝟎𝟎 % (27)
N0is the total interference power received from the (N -1) incell users,

Ui is the number of users in the i th adjacent cell,
Nai is the average interference power for a user located in the i th adjacent
cell. Within the cell of interest, the desired user will have the same received
power as the (N-1) undesired in-cell users when power control is employed.

The Cellular Concept—System Design:
• The cellular concept was a major breakthrough in solving the problem
of spectral congestion and user capacity. It offered very high capacity
in a limited spectrum allocation without any major technological
changes.
• The cellular concept is a system-level idea which calls for replacing a
single, high power transmitter (large cell) with many low power
transmitters (small cells), each providing coverage to only a small
portion of the service area.
• Each base station is allocated a portion of the total number of
channels available to the entire system, and nearby base stations are
assigned different groups of channels so that all the available
channels are assigned to a relatively small number of neighboring
base stations.
• Neighboring base stations are assigned different groups of channels
so that the interference between base stations (and the mobile users
under their control) is minimized.

Frequency Reuse:
• Cellular radio systems rely on an intelligent allocation and reuse of channels
throughout a coverage region.
• Each cellular base station is allocated a group of radio channels to be used within
a small geographic area called a cell.
• Base stations in adjacent cells are assigned channel groups which contain
completely different channels than neighboring cells. The base station antennas
are designed to achieve the desired coverage within the particular cell.
• By limiting the coverage area to within the boundaries of a cell, the same group of
channels may be used to cover different cells that are separated from one another
by distances large enough to keep interference levels within tolerable limits.
• The design process of selecting and allocating channel groups for all of the cellular
base stations within a system is called frequency reuse or frequency planning.
• The frequency reuse plan is overlaid upon a map to indicate where different
frequency channels are used.

The actual radio coverage of a cell is known as the footprint and is determined from
field measurements or propagation prediction models.
Figure 10. Illustration of the cellular frequency reuse concept. Cells with the same
letter use the same set of frequencies. A cell cluster is outlined in bold and replicated
over the coverage area. In this example, the cluster size, N, is equal to seven, and the
frequency reuse factor is 1/7 since each cell contains one-seventh of the total number
of available channels.

To understand the frequency reuse concept,
• consider a cellular system which has a total of S duplex channels available for use.
• Each cell is allocated a group of k channels (k < S)
• The S channels are divided among N cells into unique and disjoint channel groups
which each have the same number of channels, the total number of available
radio channels can be expressed as
S = kN (28)
• The N cells which collectively use the complete set of available frequencies is
called a cluster.
• If a cluster is replicated M times within the system, the total number of duplex
channels, C, can be used as a measure of capacity and is given by
C = MkN = MS (29)
• As seen from Equation (29), the capacity of a cellular system is directly
proportional to the number of times a cluster is replicated in a fixed service area.
The factor N is called the cluster size and is typically equal to 4, 7, or 12.
• If the cluster size N is reduced while the cell size is kept constant, more clusters
are required to cover a given area, and hence more capacity (a larger value of C)
is achieved.

A large cluster size indicates that the ratio between the cell radius and the distance
between co-channel cells is small. Conversely, a small cluster size indicates that co-
channel cells are located much closer together.
The value for N is a function of how much interference a mobile or base station can
tolerate while maintaining a sufficient quality of communications.
From a design viewpoint, the smallest possible value of N is desirable in order to
maximize capacity over a given coverage area (i.e., to maximize C in Equation (29)).
The frequency reuse factor of a cellular system is given by 1/N, since each cell within a
cluster is only assigned 1/N of the total available channels in the system.
Due to the fact that the hexagonal geometry of Figure 9 has exactly six equidistant
neighbours and that the lines joining the centers of any cell and each of its neighbours
are separated by multiples of 60 degrees, there are only certain cluster sizes and cell
layouts which are possible.
In order to tessellate—to connect without gaps between adjacent cells—the geometry
of hexagons is such that the number of cells per cluster, N, can only have values which
satisfy Equation (30).

N = i2 + ij + j2 (30)
where i and j are non-negative integers.
To find the nearest co-channel neighbours of a particular cell, one must do the
following:
(1) move i cells along any chain of hexagons and then
(2) Turn 60 degrees counter-clockwise and move j cells.
This is illustrated in Figure 11. for i = 3 and j = 2 (example, N = 19).
Figure 11. Method of locating co-channel cells in a cellular system.
N = 19 (i.e., i = 3, j = 2).

Example 8.
If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular
telephone system which uses two 25 kHz simplex channels to provide full
duplex voice and control channels, compute the number of channels available
per cell if a system uses (a) four-cell reuse, (b) seven-cell reuse, and
(c) 12-cell reuse. If 1 MHz of the allocated spectrum is dedicated to control
channels, determine an equitable distribution of control channels and
voice channels in each cell for each of the three systems.
Solution
Given:
Total bandwidth = 33 MHz
Channel bandwidth = 25 kHz × 2 simplex channels = 50 kHz/duplex channel
Total avalable channels = 33,000/50 = 660 channels
(a) For N = 4,
Total number of channels available per cell = 660/4 ≈ 165 channels.
(b) For N = 7,

(c) For N = 12,
A 1 MHz spectrum for control channels implies that there are:
1000/50 = 20 control channels out of the 660 channels available.
To evenly distribute
the control and voice channels, simply allocate the same number of voice channels
in each cell wherever possible. Here, the 660 channels must be evenly distributed to
each cell within the cluster. In practice, only the 640 voice channels would be
allocated, since the control channels are allocated separately as 1 per cell.
(a) For N = 4, we can have five control channels and 160 voice channels per cell. In
practice, however, each cell only needs a single control channel (the control
channels have a greater reuse distance than the voice channels).Thus, one control
channel and 160 voice channels would be assigned to each cell.

(b) For N = 7, four cells with three control channels and 92 voice channels, two cells
with three control channels and 90 voice channels, and one cell with two control
channels and 92 voice channels could be allocated. In practice, however, each cell
would have one control channel, four cells would have 91 voice channels, and
three cells would have 92 voice channels.
(c) For N = 12, we can have eight cells with two control channels and 53 voice
channels, and four cells with one control channel and 54 voice channels each. In
an actual system, each cell would have one control channel, eight cells would
have 53 voice channels, and four cells would have 54 voice channels.

Channel Assignment Strategies
• For efficient utilization of the radio spectrum, a frequency reuse scheme that is
consistent with the objectives of increasing capacity and minimizing interference
is required.
• A variety of channel assignment strategies have been developed to achieve these
objectives.
• Channel assignment strategies can be classified as either fixed or dynamic.
• The choice of channel assignment strategy impacts the performance of the
system, particularly as to how calls are managed when a mobile user is handed
off from one cell to another.
• In a fixed channel assignment strategy, each cell is allocated a predetermined set
of voice channels. Any call attempt within the cell can only be served by the
unused channels in that particular cell. If all the channels in that cell are
occupied, the call is blocked and the subscriber does not receive service. Several
variations of the fixed assignment strategy exist.

• In one approach, called the borrowing strategy, a cell is allowed to borrow
channels from a neighboring cell if all of its own channels are already occupied.
• The mobile switching centre (MSC) supervises such borrowing procedures and
ensures that the borrowing of a channel does not disrupt or interfere with any
of the calls in progress in the donor cell.
• In a dynamic channel assignment strategy, voice channels are not allocated to
different cells permanently.
• Instead, each time a call request is made, the serving base station requests a
channel from the MSC.
• The switch then allocates a channel to the requested cell following an algorithm
that takes into account the likelihood of future blocking within the cell, the
frequency of use of the candidate channel, the reuse distance of the channel,
and other cost functions.

• The MSC only allocates a given frequency if that frequency is not presently in
use in the cell or any other cell which falls within the minimum restricted
distance of frequency reuse to avoid co-channel interference.
• Dynamic channel assignment reduce the likelihood of blocking, which increases
the trunking capacity of the system, since all the available channels in a market
are accessible to all of the cells.
• Dynamic channel assignment strategies require the MSC to collect real-time
data on channel occupancy, traffic distribution, and radio signal strength
indications (RSSI) of all channels on a continuous basis.
• This increases the storage and computational load on the system but provides
the advantage of increased channel utilization and decreased probability of a
blocked call.

Handoff Strategies
• When a mobile moves into a different cell while a conversation is in progress, the
MSC automatically transfers the call to a new channel belonging to the new base
station.
• This handoff operation not only involves identifying a new base station, but also
requires that the voice and control signals be allocated to channels associated
with the new base station.
• Processing handoffs is an important task in any cellular radio system. Many
handoff strategies prioritize handoff requests over call initiation requests when
allocating unused channels in a cell site.
• Handoffs must be performed successfully and as infrequently as possible, and be
imperceptible to the users.
• In order to meet these requirements, system designers must specify an optimum
signal level at which to initiate a handoff. Once a particular signal level is
specified as the minimum usable.

• signal for acceptable voice quality at the base station receiver (normally taken
as between –90 dBm and –100 dBm), a slightly stronger signal level is used as a
threshold at which a handoff is made.
• This margin, given by Δ = Pr handoff – Pr minimum usable, cannot be too large
or too small. If Δ is too large, unnecessary handoffs which burden the MSC
may occur, and if Δ is too small, there may be insufficient time to complete a
handoff before a call is lost due to weak signal conditions.
• Therefore, Δ is chosen carefully to meet these conflicting requirements. Figure
12 illustrates a handoff situation.
• The case where a handoff is not made and the signal drops below the minimum
acceptable level to keep the channel active.
• This dropped call event can happen when there is an excessive delay by the
MSC in assigning a handoff or when the threshold Δ is set too small for the
handoff time in the system.

Figure 12. Illustration of a handoff scenario at cell boundary.
• Excessive delays may occur during
high traffic conditions due to
computational loading at the MSC or
due to the fact that no channels are
available on any of the nearby base
stations (thus forcing the MSC to
wait until a channel in a nearby cell
becomes free).

• In deciding when to handoff, it is important to ensure that the drop in
the measured signal level is not due to momentary fading and that
the mobile is actually moving away from the serving base station.
• In order to ensure this, the base station monitors the signal level for
a certain period of time before a handoff is initiated.
• This running average measurement of signal strength should be
optimized so that unnecessary handoffs are avoided, while ensuring
that necessary handoffs are completed before a call is terminated
due to poor signal level.
• The length of time needed to decide if a handoff is necessary
depends on the speed at which the vehicle is moving.
• If the slope of the short-term average received signal level in a given
time interval is steep, the handoff should be made quickly.
• Information about the vehicle speed, which can be useful in handoff
decisions, can also be computed from the statistics of the received
short-term fading signal at the base station.

• The time over which a call may be maintained within a cell, without handoff, is
called the dwell time.
• The dwell time of a particular user is governed by a number of factors,
including propagation, interference, distance between the subscriber and the
base station, and other time varying effects.
• Even when a mobile user is stationary, ambient motion in the vicinity of the base
station and the mobile can produce fading; thus, even a stationary subscriber
may have a random and finite dwell time.
• It has been found that the statistics of dwell time vary greatly, depending on the
speed of the user and the type of radio coverage.
Example:
In mature cells which provide coverage for vehicular highway users, most users
tend to have a relatively constant speed and travel along fixed and well-defined
paths with good radio coverage. In such instances, the dwell time for an
arbitrary user is a random variable with a distribution that is highly
concentrated about the mean dwell time.

On the other hand, for users in dense, cluttered microcell environments, there is
typically a large variation of dwell time about the mean, and the dwell times are
typically shorter than the cell geometry would otherwise suggest.
• The statistics of dwell time are important in the practical design of handoff
algorithms.
Mobile Assisted Handoff (MAHO):
• Every mobile station measures the received power from surrounding base stations
and continually reports the results of these measurements to the serving base
station.
• A handoff is initiated when the power received from the base station of a
neighboring cell begins to exceed the power received from the current base station
by a certain level or for a certain period of time.
• MAHO is particularly suited for microcellular environments where handoffs are
more frequent.

• During the course of a call, if a mobile moves from one cellular system to a
different cellular system controlled by a different MSC, an intersystem handoff
becomes necessary.
• An MSC engages in an intersystem handoff when a mobile signal becomes weak
in a given cell and the MSC cannot find another cell within its system to which it
can transfer the call in progress.
There are many issues that must be addressed when implementing an intersystem
handoff.
 A local call may become a long-distance call as the mobile moves out of
its home system and becomes a roamer in a neighboring system.
 Compatibility between the two MSCs must be determined before
implementing an intersystem handoff.

Prioritizing Handoffs
• Giving priority to handoffs is called the guard channel concept, whereby a
fraction of the total available channels in a cell is reserved exclusively for
handoff requests from ongoing calls which may be handed off into the cell.
• Guard channel concept method has the disadvantage of reducing the total
carried traffic, as fewer channels are allocated to originating calls.
• Guard channels, however, offer efficient spectrum utilization when dynamic
channel assignment strategies, which minimize the number of required guard
channels by efficient demand-based allocation, are used.
• Queuing of handoff requests is another method to decrease the probability of
forced termination of a call due to lack of available channels.
• There is a trade-off between the decrease in probability of forced termination
and total carried traffic.
• Queuing of handoffs is possible due to the fact that there is a finite time interval
between the time the received signal level drops below the handoff threshold
and the time, the call is terminated due to insufficient signal level.

• The delay time and size of the queue is determined from the traffic pattern of
the particular service area.
Interference and System Capacity
• Interference is the major limiting factor in the performance of cellular radio
systems.
• Sources of interference
 Another mobile in the same cell,
 A call in progress in a neighboring cell,
 Other base stations operating in the same frequency band,
 Any noncellular system which inadvertently leaks energy into the
cellular frequency band.
• The two major types of system-generated cellular interference
a)co-channel interference
b)adjacent channel interference.

Co-channel Interference and System Capacity
• Frequency reuse implies that in a given coverage area there are several cells that
use the same set of frequencies.
• These cells are called co-channel cells, and the interference between signals from
these cells is called co-channel interference.
• Unlike thermal noise which can be overcome by increasing the signal-to-noise
ratio (SNR), co-channel interference cannot be combated by simply increasing the
carrier power of a transmitter.
This is because an increase in carrier transmit power increases the
interference to neighboring co-channel cells.
• To reduce co-channel interference, co-channel cells must be physically separated
by a minimum distance to provide sufficient isolation due to propagation.

• When the size of each cell is approximately the same and the base stations
transmit the same power, the co-channel interference ratio is independent of the
transmitted power and becomes a function of the radius of the cell (R) and the
distance between centers of the nearest co-channel cells (D).
• By increasing the ratio of D/R, the spatial separation between co-channel cells
relative to the coverage distance of a cell is increased. Thus, interference is
reduced from improved isolation of RF energy from the co-channel cell.
• The parameter Q, called the co-channel reuse ratio, is related to the cluster size
(see Equation (30)). For a hexagonal geometry
𝑸 =
𝑫
𝑹
= 𝟑𝑵 (31)
• A small value of Q provides larger capacity since the
cluster size N is small,
• Large value of Q improves the transmission quality,
due to a smaller level of co-channel interference.
• A trade-off must be made between these two objectives in actual cellular design.

Let i0 be the number of co-channel interfering cells.
Signal-to-interference ratio (S/I or SIR) for a mobile receiver which monitors a
forward channel can be expressed as
𝑺
𝑰
=
𝑺
𝒊=𝟏
𝒊𝟎 𝑰𝒊
(31)
where S is the desired signal power from the desired base station
Iiis the interference power caused by the ith interfering co-channel cell
base station.
If the signal levels of co-channel cells are known, then the S/I ratio
for the forward link can be found.
• Propagation model in a mobile radio channel show that the average received
signal strength at any point decays as a power law of the distance of separation
between a transmitter and receiver.
• The average received power Pr at a distance d from the transmitting antenna is
approximated by

Pr= P0
𝒅
𝒅𝟎
−𝒏
(32)
Pr(dBm) = P0(dBm) - 10× 𝒏𝒍𝒐𝒈
𝒅
𝒅𝟎
(33)
where P0 is the power received at a close-in reference point in the
far field region of the antenna at a small distance d0 from the
transmitting antenna and n is the path loss exponent.
Now consider the
forward link where the desired signal is the serving base station and where the
interference is due to co-channel base stations.
If Di is the distance of the ith interferer from the mobile.
• Received power at a given mobile due to the ith interfering cell will be
proportional to 𝑫𝒊
−𝒏
.
The path loss exponent typically ranges between two and four in
urban cellular systems.

When the transmit power of each base station is equal and the path loss exponent is
the same throughout the coverage area,
S/I for a mobile can be approximated as
𝑺
𝑰
=
𝑹−𝒏
𝒊=𝟏
𝒊𝟎 𝑫𝒊
−𝒏
(34)
Considering only the first layer of interfering cells,
if all the interfering base stations are equidistant from the desired base station and if
this distance is equal to the distance D between cell centers,
then Equation (34) simplifies as
𝑺
𝑰
=
𝑫/𝑹 𝒏
𝒊𝟎
=
𝟑𝑵
𝒏
𝒊𝟎
(35)
Equation (35) relates S/I to the cluster size N, which in turn determines the overall
capacity of the system from Equation (29).

Example 9
If a signal-to-interference ratio of 15 dB is required for satisfactory forward
hannel performance of a cellular system, what is the frequency reuse factor and
cluster size that should be used for maximum capacity if the path loss exponent is
(a) n = 4, (b) n = 3? Assume that there are six cochannel cells in the first tier, and
all of them are at the same distance from the mobile. Use suitable approximations.
Solution
(a) n = 4
First, let us consider a seven-cell reuse pattern.
Using Equation (31), the co-channel reuse ratio D/R = 4.583.
Using Equation (35), the signal-to-noise interference ratio is given by
S/I = (1/6) × (4.583)4 = 75.3 = 18.66 dB
Since this is greater than the minimum required S/I, N = 7 can be
used.

(b) n = 3
First, let us consider a seven-cell reuse pattern.
Using Equation (35), the signal-to-interference ratio is given by
S/I = (1/6) × (4.583)3 = 16.04 = 12.05 dB
Since this is less than the minimum required S/I, we need to use a
larger N.
Using Equation (30), the next possible value of N is 12, (i = j = 2).
The corresponding co-channel ratio is given by Equation (31) as
D/R = 6.0
Using Equation (30), the signal-to-interference ratio is given by
S/I = (1/6) × (6)3 = 36 = 15.56 dB
Since this is greater than the minimum required S/I, N = 12 is used.

Adjacent Channel Interference
• Interference resulting from signals which are adjacent in frequency to the desired
signal is called adjacent channel interference.
• Adjacent channel interference results from imperfect receiver filters which allow
nearby frequencies to leak into the passband.
• The problem can be particularly serious if an adjacent channel user is transmitting
in very close range to a subscriber’s receiver, while the receiver attempts to receive a
base station on the desired channel.
• This is referred to as the near–far effect, where a nearby transmitter (which may or
may not be of the same type as that used by the cellular system) captures the receiver
of the subscriber.
• Alternatively, the near–far effect occurs when a mobile close to a base station
transmits on a channel close to one being used by a weak mobile.
• Adjacent channel interference can be minimized through careful filtering and
channel assignments.

• Thus instead of assigning channels which form a contiguous band of
frequencies within a particular cell, channels are allocated such that the
frequency separation between channels in a given cell is maximized.
• If the frequency reuse factor is large (e.g., small N), the separation between
adjacent channels at the base station may not be sufficient to keep the
adjacent channel interference level within tolerable limits.
• For example, if a close-in mobile is 20 times as close to the base station as
another
• mobile and has energy spillout of its passband, the signal-to-interference ratio
at the base station for the weak mobile (before receiver filtering) is
approximately
𝒔
𝑰
= 𝟐𝟎 −𝒏 (36)
For a path loss exponent n = 4,
𝒔
𝑰
= 10 log(20)-4 = -52 dB

• If the intermediate frequency (IF) filter of the base station receiver has a slope of
20 dB/octave, then an adjacent channel interferer must be displaced by at least
six times the passband bandwidth from the center of the receiver frequency
passband to achieve 52 dB attenuation.
• Here, a separation of approximately six channel bandwidths is required for
typical filters in order to provide 0 dB SIR from a close-in adjacent channel user.
• This implies more than six channel separations are needed to bring the adjacent
channel interference to an acceptable level.
• Tight base station filters are needed when close-in and distant users share the
same cell.
• In practice, base station receivers are preceded by a high Q cavity filter in order
to reject adjacent channel interference..

Power Control for Reducing Interference
• In practical cellular radio and personal communication systems, the power levels
transmitted by every subscriber unit are under constant control by the serving
base stations.
• This is done to ensure that each mobile transmits the smallest power necessary
to maintain a good quality link on the reverse channel
• Power control not only helps prolong battery life for the subscriber unit, but also
dramatically reduces the reverse channel S/I in the system.
• Power control is especially important for emerging CDMA spread spectrum
systems that allow every user in every cell to share the same radio channel.

Trunking and Grade of Service
• Cellular radio systems rely on trunking to accommodate a large number of
users in a limited radio spectrum.
• The concept of trunking allows a large number of users to share the relatively
small number of channels in a cell by providing access to each user, on
demand, from a pool of available channels.
• In a trunked radio system, each user is allocated a channel on a per call basis,
and upon termination of the call, the previously occupied channel is
immediately returned to the pool of available channels.
• Trunking exploits the statistical behaviour of users so that a fixed number of
channels or circuits may accommodate a large, random user community.
• There is a trade-off between the number of available telephone circuits and
the likelihood of a particular user finding that no circuits are available during
the peak calling time.

• To design trunked radio systems that can handle a specific capacity at a specific
“grade of service,” it is essential to understand trunking theory and queuing
theory.
• The fundamentals of trunking theory were developed by Erlang, a Danish
mathematician.
• One Erlang represents the amount of traffic intensity carried by a channel that
is completely occupied (i.e. one call-hour per hour or one call-minute per
minute).
For example, a radio channel that is occupied for thirty minutes
during an hour carries 0.5 Erlangs of traffic.
• The grade of service (GOS) is a measure of the ability of a user to access a
trunked system during the busiest hour. The busy hour is based upon customer
demand at the busiest hour during a week, month, or year.
• The busy hours for cellular radio systems typically occur during rush hours,
between 4 p.m. and 6 p.m. on a Thursday or Friday evening.

Definitions of Common Terms Used in Trunking Theory
• Set-up Time: The time required to allocate a trunked radio channel to a
requesting user.
• Blocked Call: Call which cannot be completed at time of request, due to
congestion. Also referred to as a lost call.
• Holding Time: Average duration of a typical call. Denoted by H (in seconds).
• Traffic Intensity: Measure of channel time utilization, which is the average
channel occupancy measured in Erlangs. This is a
dimensionless quantity and may be used to measure the time
utilization of single or multiple channels. Denoted by A.
• Load: Traffic intensity across the entire trunked radio system, measured in
Erlangs.
• Grade of Service (GOS): A measure of congestion which is specified as the
probability of a call being blocked (for Erlang B), or the
probability of a call being delayed beyond a certain amount
of time (for Erlang C).
• Request Rate: The average number of call requests per unit time. Denoted by λ
seconds–1.

Fore said Definitions of Common Terms used in trunking theory to make capacity
estimates in trunked systems.
• The traffic intensity offered by each user is equal to the call request rate
multiplied by the holding time. That is, each user generates a traffic intensity of
Au Erlangs given by
Au = λH (37)
where H is the average duration of a call
λ is the average number of call requests per unit time for each
user.
• For a system containing U users and an unspecified number of channels, the
total offered traffic intensity A, is given as
A = Uau (38)
• In a C channel trunked system, if the traffic is equally distributed among the
channels, then the traffic intensity per channel, Ac, is given as
Ac = UAu/C (39)

Note that the offered traffic is not necessarily the traffic which is
carried by the trunked system, only that which is offered to the
trunked system. When the offered traffic exceeds the maximum
capacity of the system, the carried traffic becomes limited due to the
limited capacity (i.e. limited number of channels).
• The maximum possible carried traffic is the total number of channels, C, in
Erlangs.
• The AMPS cellular system is designed for a GOS of 2% blocking.
This implies that the channel allocations for cell sites are designed so that 2
out of 100 calls will be blocked due to channel occupancy during the busiest
hour.
There are two types of trunked systems which are commonly used.
• The first type offers no queuing for call requests. That is, for every user who
requests service, it is assumed there is no setup time and the user is given
immediate access to a channel if one is available.

If no channels are available, the requesting user is blocked without access and is free
to try again later.
• This type of trunking is called blocked calls cleared and assumes that calls arrive as
determined by a Poisson distribution.
it is assumed that there are an infinite number of users as well as the following:
(a) there are memoryless arrivals of requests, implying that all users,
including blocked users, may request a channel at any time;
(b) the probability of a user occupying a channel is exponentially
distributed, so that longer calls are less likely to occur as described by an
exponential distribution.
(c) there are a finite number of channels available in the trunking pool.
• This is known as an M/M/m/m queue, and leads to the derivation of the Erlang B
formula (also known as the blocked calls cleared formula).

• The Erlang B formula determines the probability that a call is blocked and is a
measure of the GOS for a trunked system which provides no queuing for blocked
calls.
The Erlang B formula is given by
𝒑𝒓 𝒃𝒍𝒐𝒄𝒌𝒊𝒏𝒈 =
𝑨𝑪
𝑪!
𝒌=𝟎
𝑪 𝑨𝒌
𝒌!
= 𝑮𝑶𝑺, 𝒌 ≤ 𝑪(40)
where C is the number of trunked channels offered by a trunked radio system
A is the total offered traffic.
• While it is possible to model trunked systems with finite users, the resulting
expressions are much more complicated than the Erlang B result.
• The Erlang B formula provides a conservative estimate of the GOS, as the finite
user results always predict a smaller likelihood of blocking.
The capacity of a trunked radio system where blocked calls are lost is tabulated for
various values of GOS and numbers of channels in Table.

Number of Capacity (Erlangs) for GOS
Chennel C = 0.01 = 0.005 = 0.002 = 0.001
----------------------------------------------------------------------------
2 0.153 0.105 0.065 0.046
4 0.809 0.701 0.535 0.439
5 1.36 1.13 0.900 0.762
10 4.46 3.96 3.43 3.09
20 12.0 11.1 10.1 9.41
24 15.3 14.2 13.0 12.2
40 29.0 27.3 25.7 24.5
70 56.1 53.7 51.0 49.2
100 84.1 80.9 77.4 775.2
Table : Capacity of an Erlang B System

• The second kind of trunked system is one in which a queue is provided to hold
calls which are blocked.
• If a channel is not available immediately, the call request may be delayed until a
channel becomes available.
• This type of trunking is called Blocked Calls Delayed.
• Its measure of GOS is defined as the probability that a call is blocked after
waiting a specific length of time in the queue.
• To find the GOS, it is first necessary to find the likelihood that a call is initially
denied access to the system.
• The likelihood of a call not having immediate access to a channel is determined by
the Erlang C formula
𝑷𝒓 𝒅𝒆𝒍𝒂𝒚 > 𝒕 =
𝑨𝑪
𝑨𝑪+𝑪! 𝟏−
𝑨
𝑪 𝒌=𝟎
𝑪−𝟏𝑨𝒌
𝒌!
(41)

• If no channels are immediately available the call is delayed.
• The probability that the delayed call is forced to wait more than t seconds is given
by the probability that a call is delayed, multiplied by the conditional probability
that the delay is greater than t seconds.
• The GOS of a trunked system where blocked calls are delayed is hence given by
𝑷𝒓 𝒅𝒆𝒍𝒂𝒚 > 𝒕 = 𝑷𝒓 𝒅𝒆𝒍𝒂𝒚 > 𝟎 𝑷𝒓 𝒅𝒆𝒍𝒂𝒚 > 𝒕 𝒅𝒆𝒍𝒂𝒚 > 𝟎 (42)
= 𝑷𝒓 𝒅𝒆𝒍𝒂𝒚 > 𝟎 𝒆𝒙𝒑 −𝑪 − 𝑨 𝒕/𝑯)
The average delay D for all calls in a queued system is given by
𝑫 = 𝑷𝒓 𝒅𝒆𝒍𝒂𝒚 > 𝟎
𝑯
𝑪−𝑨
(43)
where the average delay for those calls which are queued is given by H/(C – A).

Improving Coverage and Capacity in Cellular Systems
• As the demand for wireless service increases, the number of channels assigned
to a cell eventually becomes insufficient to support the required number of
users.
• At this point, cellular design techniques are needed to provide more channels
per unit coverage area. Techniques such as
 Cell splitting
 Sectoring
 coverage zone approaches
are used in practice to expand the capacity of cellular systems.
Cell Splitting
• Cell splitting is the process of subdividing a congested cell into smaller cells,
each with its own base station and a corresponding reduction in antenna
height and transmitter power.

• Cell splitting increases the capacity of a cellular system since it increases the
number of times that channels are reused.
• By defining new cells which have a smaller radius than the original cells and by
installing these smaller cells (called microcells) between the existing cells, capacity
increases due to the additional number of channels per unit area.
• The increased number of cells would increase the number of clusters over the
coverage region, which in turn would increase the number of channels, and thus
capacity, in the coverage area.
• Cell splitting allows a system to grow by replacing large cells with smaller cells.
while not upsetting the channel allocation scheme required to maintain the
minimum co-channel reuse ratio Q (see Equation (31) between co-channel cells.
• The base stations are placed at corners of the cells, and the area served by base
station A (In figure) is assumed to be saturated with traffic (i.e., the blocking of
base station A exceeds acceptable rates). New base stations are therefore needed
in the region to increase the number of channels in the area and to reduce the
area served by the single base station.

Figure 13 cell splitting
• Notice in the figure that the original base
station A has been surrounded by six new
microcells.
• The smaller cells were added in such a way
as to preserve the frequency reuse plan of
the system.
• For example, the microcell base station
labelled G was placed half way between two
larger stations utilizing the same channel set
G. This is also the case for the other
microcells in the figure.
• Cell splitting merely scales the geometry of
the cluster. In this case, the radius of each
new microcell is half that of the original cell.

• For the new cells to be smaller in size, the transmit power of these cells must be
reduced.
• The transmit power of the new cells with radius half that of the original cells can
be found by
examining the received power Prat the new and old cell boundaries and setting them
equal to each other. This is necessary to ensure that the frequency reuse plan for the
new microcells behaves exactly as for the original cells.
For Figure 13.
Pr[ at old cell boundary ] ∝ 𝑷𝒕𝟏𝑹−𝒏 (44)
and
Pr [ at new cell boundary ] ∝ 𝑷𝒕𝟐 𝑹/𝟐 −𝒏 (45)
where Pt1 and Pt2 are the transmit powers of the larger and smaller cell base
stations, respectively,
n is the path loss exponent.
If we take n = 4 and set the received powers equal to each
other, then
𝑷𝒕𝟐 =
𝑷𝒕𝟏
𝟏𝟔
(46)

• In other words, the transmit power must be reduced by 12 dB in order to fill in
the original coverage area with microcells, while maintaining the S/I requirement.
• The larger cell is usually dedicated to high speed traffic so that handoffs occur
less frequently.
• Antenna downtilting, which deliberately focuses radiated energy from the base
station toward the ground (rather than toward the horizon), is often used to limit
the radio coverage of newly formed microcells.
Sectoring
• sectoring increases SIR(signal-to-interference ratio) so that the cluster size may
be reduced.
• In this approach, first the SIR is improved using directional antennas, then
capacity improvement is achieved by reducing the number of cells in a cluster,
thus increasing the frequency reuse.
• The co-channel interference in a cellular system may be decreased by replacing a
single omnidirectional antenna at the base station by several directional antennas,
each radiating within a specified sector.

• By using directional antennas, a given cell will receive interference and transmit
with only a fraction of the available co-channel cells. The technique for decreasing
co- channel interference and thus increasing system performance by using
directional antennas is called sectoring.
• The factor by which the co-channel interference is reduced depends on the amount
of sectoring used.
• A cell is normally partitioned into three 120° sectors or six 60° sectors as shown in
Figure below.
Figure 14. (a) 120° sectoring; (b) 60° sectoring.

Repeaters for Range Extension
• We needs to provide dedicated coverage for hard-to-reach areas, such as within
buildings, or in valleys or tunnels.
• Radio retransmitters, known as repeaters, are often used to provide such range
extension capabilities.
• Repeaters are bidirectional in nature, and simultaneously send signals to and
receive signals from a serving base station.
• Repeaters work using over-the-air signals, so they may be installed anywhere and
are capable of repeating an entire cellular or PCS band.
• Upon receiving signals from a base station forward link, the repeater amplifies
and reradiates the base station signals to the specific coverage region.
• Unfortunately, the received noise and interference is also reradiated by the
repeater on both the forward and reverse link, so care must be taken to properly
place the repeaters, and to adjust the various forward and reverse link amplifier
levels and antenna patterns.

• In practice, directional antennas or distributed antenna systems (DAS) are
connected to the inputs or outputs of repeaters for localized spot coverage,
particularly in tunnels or buildings.
• By modifying the coverage of a serving cell, an operator is able to dedicate a
certain amount of the base station’s traffic for the areas covered by the repeater.
• Repeater does not add capacity to the system—it simply serves to reradiate the
base station signal into specific locations.
• Repeaters are increasingly being used to provide coverage into and around
buildings, where coverage has been traditionally weak.
• Many carriers have opted to provide in-building wireless penetration by installing
microcells outside of large buildings, and then installing many repeaters with DAS
networks within the buildings.
• Determining the proper location for repeaters and distributed antenna systems
within buildings requires careful planning, particularly due to the fact that
interference levels are reradiated into the building from the base station and from
the interior of the building back to the base station.

• Repeaters must be provisioned to match the
available capacity from the serving base station.
A Microcell Zone Concept
• The increased number of handoffs required when
sectoring is employed results in an increased load
on the switching and control link elements of the
mobile system.
• In this scheme configuration is shown in figure 15,
each of the three (or possibly more) zone sites
(represented as Tx/Rx) are connected to a single
base station and share the same radio equipment.
• The zones are connected by coaxial cable,
fiberoptic cable, or microwave link to the base
station.
• Multiple zones and a single base station make up a
cell.
Figure 15 The microcell concept

• As a mobile travels within the cell, it is served by the zone with the strongest
signal.
• This approach is superior to sectoring since antennas are placed at the outer
edges of the cell, and any base station channel may be assigned to any zone by
the base station.
• As a mobile travels from one zone to another within the cell, it retains the same
channel. Thus, unlike in sectoring, a handoff is not required at the MSC when
the mobile travels between zones within the cell.
• The base station simply switches the channel to a different zone site.
• In this way, a given channel is active only in the particular zone in which the
mobile is traveling, and hence the base station radiation is localized and
interference is reduced.
• The channels are distributed in time and space by all three zones and are also
reused in co-channel cells in the normal fashion.
• This technique is particularly useful along highways or along urban traffic
corridors.

we need to allingh the above para with suitable
modification.
We left at page 30 on soft copy corresponding
page in pdf file is 60 and corresponding page no
60 is in actual Rappaport book. We need to
start form 60 in actual Rappaport book.

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