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(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II

VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com

CMC

VIDYA SAGAR P

UNIT – II CO-CHANNEL INTERFERENCE Measurement of real time Co-channel interference, Design of antenna system, Antenna parameters and their effects, Diversity technique- Space diversity, Polarization diversity, Frequency diversity, Time diversity. NON CO-CHANNEL INTERFERENCE: Adjacent channel interference, Near end far end interference, cross talk, Effects on coverage and interference by power decrease, Antenna height decrease, Effects of cell site components.



Interference

2.1 Introduction In electronics and communications, especially in the field of telecommunications,

interference is anything which alters, modifies, or disrupts a signal as it travels along a

channel between a source and a receiver. The term typically refers to the addition of

unwanted signals to a useful signal. In a communication environment, there exist both

noise-limited and interference-limited environments. Point-to-point communication

suffers from noise-limited situations, whereas mobile radio environment is interference

limited as several transmitters and receivers are involved in the system.

Sources of interference include another mobile in the same cell, a call in progress in a

neighboring cell, another base station operating in the same frequency band, and any

non-cellular system which leaks energy into the cellular frequency band. The two major

types of system-generated cellular interferences are co-channel interference (CCI) and

adjacent-channel interference (ACI). Although interfering signals are often generated

within a cellular system, in practice they are difficult to control. The chapter introduces

the two types of interferences and estimates the interference levels of either type. It also

introduces the concept of a diversity receiver.

2.2 Types of interferences

Interference in mobile communications is of two types:

Co-channel interference

Adjacent-channel interference

The co-channel interference (CCI) is crosstalk from two different radio transmitters

using the same frequency. In cellular mobile communications (GSM & LTE [Long Term

Evolution] systems, for instance), frequency spectrum is a valuable resource which is

divided into non-overlapping spectrum bands that are assigned to different cells. The

CCI arises in the cellular mobile networks due to the phenomenon of frequency reuse.

Thus, besides the intended signal from the cell, signals at the same frequencies (co-

channel signals) arrive at the receiver from undesired transmitters located (far away) in

some other cells and lead to a deterioration in the receiver performance.

The adjacent-channel interference (ACI), also known as inter-channel interference, is the

interference caused by extraneous power from a signal in an adjacent channel. An ACI

may be caused by inadequate filtering, such as incomplete filtering of unwanted

modulation products in frequency modulation (FM) systems, improper tuning, or poor

frequency control, in either the reference channel or the interfering channel, or in both.

The problem can be particularly serious if an adjacent channel user is transmitting in a

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. The base station may have difficulty in discriminating the desired

mobile user from the “bleed over” caused by the close adjacent-channel mobile.

2.3 Co-channel interference areas in a system

The received voice quality is affected by the grade of coverage and the amount of CCI. In

order to detect channel interference areas in a cellular system, we have to perform two

tasks, discussed in the following.



2.3.1 To find the co-channel interference area from a mobile receiver The CCI can be measured by selecting any one channel (as interference is equal in all

the channels) and transmitting on that channel to all co-channel sites at night while

the mobile receiver is moving in one of the co-channel cells. Co-channel or inter-

channel interference is denoted as carrier-to-interference ratio (CIR) or signal-to-

interference ratio (SIR).

We now look out for any change detected by a field-strength recorder in the mobile unit

and compare the data with the condition of no co-channel sites being transmitted. This

test must be repeated as the mobile unit moves in every co-channel cell. To facilitate

this test, we can install a channel-scanning receiver in a car.

Suppose one channel (f1) which receives the signal level (no co-channel condition),

another channel (f2) which receives the interference level (six-co-channel condition is

the maximum), and a third channel receives f3, which is not transmitting in the air.

Therefore, the noise level is recorded only in f3 (see Fig. 2.1).

Figure 2.1 CCI at the mobile unit

Carrier-to-interference ratio, C/I = f1 − f3 ; Carrier-to-noise ratio, C/N = f2 − f3

The following four conditions are used to compare the results.

If C/I > 18 dB throughout most of the cell, the system is properly designed for capacity.

If C/I < 18 dB and C/N > 18 dB in some areas, it is an indication of the presence of

CCI.

If C/N and C/I are both less than 18 dB and C/N = C/I in a given area, it is an

indication of a coverage problem.

If C/N and C/I are both less than 18 dB and C/N > C/I in a given area, there is a

coverage problem and CCI.

2.3.2 To find the co-channel interference area which affects a cell site

Reciprocity theorem is not applicable for CCI. Hence, the second task should be

performed. In this task, we record the signal strength at every co-channel cell site while a

mobile unit is travelling either in its own cell or in one of the co-channel cells shown in

Figure 2.2.First, we find the areas in an interfering cell in which the top 10 per cent level

of the signal transmitted from the mobile unit in those areas is received at the desired site

(Jth cell in Fig. 2.2). This top 10 per cent level can be distributed in different areas in a

cell. The average value of the received top 10 per cent level signal strength is used as the

interference level from that particular interfering cell. The mobile unit also travels in

different interfering cells. Up to six interference levels are obtained from a mobile unit

running in six interfering cells. We then calculate the average of the bottom 10 per cent



level of the signal strength which is transmitted from a mobile unit in the desired cell (Jth

cell) and received at the desired cell site as a carrier reception level.

Then we can re-establish the CIR received at a desired cell, say, the Jth cell, site as follows.

Figure 2.2 CCI at the cell site

Where: CJ is the desired carrier power from Jth cell base station I is the CCI

Ii is the interference power caused by the ith interfering co-channel cell base station

The number of co-channel cells in the system can be less than six. We then compare

and determine the CCI condition, which will be the same as that in task 1. NJ is the noise

level in the Jth cell assuming no interference exists.

2.4 Estimation of co-channel interference level

For a given cell size, the number of customers that a cellular system can support is

maximized if the cluster size is minimum.

The factor that limits the extent to which cluster size can be reduced is the CCI. This is

because reducing the cluster size has the effect of reducing the frequency reuse ratio,

Where: q is the frequency reuse factor; R is the radius of cell; D is the reuse distance

N is the cluster size or number of cells in the cluster;

With increase in q, spatial separation of co-channel cells increases leading to a decrease

in CCI. With decrease in q, N decreases leading to an increase in the number of replicas of

the cluster (M), which results in an increase in channel capacity (C); however, the CCI

increases.

In earlier days, in order to determine minimum signal-to-noise ratio (SNR) and SIR that

would meet the quality-of-service objectives, simulators were used to conduct tests. The

results of these early studies determined that these quality objectives could be met under

the following conditions

If the SNR is no less than 18 dB over 90 per cent of the coverage area for cells limited by

receiver noise



If the SIR is no less than 17 dB over 90 per cent of the coverage area for cells limited by

interference.

Figure 2.3 begins our analysis by considering the interference from the nearest co-

channel base stations. It is assumed that the receiver noise is negligible compared to the

interference and also the reference base station is in the centre of the diagram.

Figure 2.3 First-tier CCI sources

From the reference base station, it is considered that a mobile unit is located on the cell

boundary at a distance equal to the cell radius R. This is the farthest point that a mobile

unit should be from its serving base station. The nearest co-channel sources are mobile

units in the co-channel cells and are all approximately at the reuse distance D from the

reference base station.

The SIR C/I is given by Equation (2.2) with j = 6 interference sources

Where: P1 is the desired signal power from desired base station

Pj is the interference power caused by Jth interfering co-channel cell base station.

Now if all of the mobile units have the same parameters and the environment is uniform

in all directions, then P2 = P3 = … = P7

The SIR C/I at the desired mobile receiver is given by

Let Di be the distance between the ith interferer and the mobile. The received interference,

Ii, at a given mobile due to ith interfering cell is proportional to (Di)-γ, where γ is the path-

loss exponent and depends upon the terrain environment and 2 ≤ γ ≤ 5.

The received signal power, C is proportional to r-γ where r is the distance between the

mobile and serving base stations. The C/I at the desired mobile receiver is approximated

by

When the mobile is located at the cell boundary (i.e., r = R) and CCI from the second and

other higher tiers is neglected, this means that Ni = 6 and using (Di) ≡ D for i = 1, 2, …, Ni,

we have



and

Using Equations (2.2) and (2.5), we have

Example problem 2.1

Suppose, as in the AMPS system, that a SIR of 18 dB is required. The path-loss exponent

is γ = 4.0. Considering only the nearest CCI sources, find the minimum cluster size.

Solution

Given γ = 4.0,SIR (dB) = 18 dB which implies signal to interference = 63.1.

Using Equation (2.6), we have

Therefore, N = 6.29 ∼ 7.

Table 2.1 includes a column of values of SIR assuming that the path-loss exponent γ = 2.

Only first-tier interference is taken into account, as the first-tier interference

predominates. In the presence of a deep fade, or when no first-tier sources are present,

interference from second or further tiers may be noticeable.

Table 2.1 Approximate signal-to-interference ratio for several reuse ratios with γ = 4

Where N = i2 - j2 - ij and q =

Figure 2.2 shows the geometry of the second and third CCI tiers. The second-tier radius

is the center-to-centre distance between large hexagons, that is,



Figure 2.4 First-, second-, third-tier CCI

From the geometry, the third-tier radius is simply double the side of a large hexagon,

that is, R3 = 2D

For a path-loss exponent of γ = 4, interference from the second tier is

Where : is the level of interference from the first tier.

Equation (2.8) indicates that the interference from the second tier is 9.52 dB below the

level of interference from the first tier. Similarly, the interference from the third tier is

12 dB below the first-tier level. This method is applicable even when there is the need

to consider additional tiers of interference.

2.5 Real-time co-channel interference measurement

The frequency-reuse method is useful for increasing the efficiency of spectrum usage

but results in cochannel interference because the same frequency channel is used

repeatedly in different cochannel cells. Application of the cochannel interference

reduction factor q= D/R = 4.6 for a seven-cell reuse pattern (K = 7).In most mobile radio

environments, use of a seven-cell reuse pattern is not sufficient to avoid cochannel

interference. Increasing K > 7 would reduce the number of channels per cell, and that

would also reduce spectrum efficiency. Therefore, it might be advisable to retain the

same number of radios as the seven-cell system but to sector the cell radially, as if

slicing a pie. This technique would reduce cochannel interference and use channel

sharing and channel borrowing schemes to increase spectrum efficiency.

Real time co-channel interference measured at mobile radio transceiver:

When the carriers are angularly modulated by the voice signal and the RF frequency

difference between them is much higher than the fading frequency, measurement of the

signal carrier-to-interference ratio C/I reveals that the signal is



Design of antenna system:

Design of an Omnidirectional Antenna System in the Worst Case:

The value of q = 4.6 is valid for a normal interference case in a K=7 cell pattern. In this

section we would like to prove that a K=7 cell pattern does not provide a sufficient

frequency re-use distance separation eyen when an ideal condition of flat terrain is

assumed. The worst case is at the location where the weakest signal from its own cell

site but strong interferences from all interfering cell sites. In the worst case the mobile

unit is at the cell boundary R, as shown in Fig.2.5. The distances from all six cochannel

interfering sites are also shown in the figure: two distances of D - R, two distances of D,

and two distances of D + R.

Following the mobile radio propagation rule of 40 dB/dec, we obtain

Then the carrier-to-interference ratio is

Fig.2.5. Cochannel interference (a worst case)

Where q=4.6 is derived from the normal case. Substituting q=4.6 into above eqn. we

obtain C/I =54 or 17 dB, which is lower than 18 dB. To be conservative, we may use

the shortest distance D – R for all six interferers as a worst case; then we have



In reality, because of the imperfect site locations and the rolling nature of the terrain

configuration, the C/I received is always worse than 17 dB and could be 14 dB and

lower. Such an instance can easily our in a heavy traffic situation; therefore, the

system must be designed around the C/I of the worst case. In that case, a cochannel

interference reduction factor of q=4.6 is insufficient.

Therefore, in an omnidirectional-cell system, K = 9 or K 12 would be a correct choice. Then the values of q are

Substituting these values in the equation

Design of antenna system; Design of a Directional Antenna System:

When the call traffic begins to increase, we need to use the frequency spectrum

efficiently and avoid increasing the number of cells K in a seven-cell frequency reuse

pattern. When K increases, the number of frequency channels assigned in a cell must

become smaller (assuming a total allocated channel divided by K) and the efficiency of

applying the frequency reuse scheme decrease.

Instead of increasing the number K in a set of cells, let us keep K =7 and introduce a

directional antenna arrangement. The cochannel interference can be reduced by using

directional antenna. This means that each cell is divided into three or six sectors and

uses three or six directional antennas at a base station. Each sector is assigned a set of

frequencies (channels). The interference between two cochannel cells decreases as

shown Fig.2.6

Directional antennas in K=7 cell patterns:

Three sector case: The three-sector case is shown in Fig.2.6 To illustrate the worst

case situation, two cochannel cells are shown in Fig. 2.6(a). The mobile unit at position

E will experience greater interference in the lower shaded cell sector than in the upper

shaded cell-sector site. This is because the mobile receiver receives the weakest signal

from its own cell but fairly strong interference from the interfering cell.

In a three-sector case, the interference is effective in only one direction because the

front-to-back ratio of a cell-site directional antenna is at least 10 dB or more in a

mobile radio environment. The worst-case cochannel interference in the directional-

antenna sectors in which interference occurs may be calculated. Because of the use of

directional antennas, the number of principal interferers is reduced from six to two

(Fig.2.6). The worst case of C/I occurs when the mobile unit is at position E, at which

point the distance between the mobile unit and the two interfering antennas is roughly

D + (R/2); however, C/I can be calculated more precisely as follows. The value of C/I

can be obtained by the following expression (assuming that the worst case is at position

E at which the distances from two interferers are D + 0.7R and D).



Fig.2.6 interfering cells shown in a seven cell system (two-tiers

Fig.2.7 Determination of C/I in a directional antenna system. (a)Worst case in a 120

directional antenna system (N=7); (b) worst case in a 60 directional antenna system

(N=7)

Let q=4.6; then we have

The C/I received by a mobile unit from the 120° directional antenna sector system

expressed in Eq. above greatly exceeds 18 dB in a worst case. Equation above shows

that using directional antenna sectors can improve the signal-to-interference ratio, that

is, reduce the cochannel interference. However, in reality, the C/I could be 6 dB weaker

than in Eq. given above in a heavy traffic area as a result of irregular terrain contour

and imperfect site locations. The remaining 18.5 dB is still adequate.



Six-sector case: We may also divide a cell into six sectors by using six 60°-beam

directional antennas as shown in Fig.2.6 In this case, only one instance of interference

can occur in each sector as shown in Fig.2.6 Therefore, the carrier-to-interference ratio

in this case is which shows a further reduction of cochannel interference. If we use the

same argument as we did for Eq. above and subtract 6 dB from the result of Eq. the

remaining 23 dB is still more than adequate. When heavy traffic occurs, the 60°-sector

configuration can be used to reduce cochannel interference. However, fewer channels

are generally allowed in a 60° sector and the trunking efficiency decreases. In certain

cases, more available channels could be assigned in a 60° sector.

Directional antenna in K = 4 cell pattern:

Three-sector case: To obtain the carrier-to-interference ratio, we use the same

procedure as in the K = 7 cell-pattern system. The 120° directional antennas used in

the sectors reduced the interferers to two as in K = 7 systems, as shown in Fig.2.8. We

can apply Eq. here. For K = 4, the value of q = 3.46; therefore, Eq. becomes

If, using the same reasoning used with Eq. above, 6 dB is subtracted from the result of

Eq. above, the remaining 14 dB is unacceptable.

Six-sector case: There is only one interferer at a distance of D + R shown in Fig.2.8.

With q=3.46, we can obtain If 6 dB is subtracted from the above result, the remaining

20 dB is adequate.

Fig. 2.8 Interference with frequency reuse pattern K=4.

Under heavy traffic conditions, there is still a great deal of concern over using a K =4

cell pattern in a 60° sector.

Comparing K =7 and N =4 systems:

A K =7 cell pattern system is a logical way to begin an omnicell system. The co-channel

reuse distance is more or less adequate, according to the designed criterion. When the

traffic increases, a three sector system should be implemented, that is, with three 120°

directional antennas in place. In certain hot spots, 60° sectors can be use d locally to

increase the channel utilization.

If a given area is covered by both K=7 and K=4 cell patterns and both patterns have a

six-sector configuration, then the K=7 system has a total of 42 sectors, but the K=4

system has a total of only 24 sectors and, of course, the system of K=7 and six sectors

has less cochannel interference.



One advantage of 60° sectors with K=4 is that they require fewer cell sites than 120

sectors with K=7. Two disadvantages of 60 deg sectors are that (1) they require more

antennas to he mounted on the antenna mast and (2) they often require more frequent

handoffs because of the increased chance that the mobile units will travel across the

six sectors of the call. Furthermore, assigning the proper frequency channel to the

mobile unit in each sector is more difficult unless the antenna height at the cell site is

increased so that the mobile unit can be located more precisely. In reality the terrain is

not flat, end coverage is never uniformly distributed; in addition, the directional

antenna front-to-back power ratio in the field is very difficult to predict. In small cells,

interference could become uncontrollable; then the use of a K = 4 pattern with 60 deg

sectors in small cells needs to be considered only for special implementations such an

portable cellular systems or narrow beam applications. For small cells, a better

alternative scheme is to use a K =7 pattern with 120° sectors plus the underlay-overlay

configuration.

Antenna parameters and their effects:

Lowering the Antenna Height: Lowering the antenna height does not always reduce

the co-channel interference. In some circumstances, such as on fairly flat ground or in

a valley situation, lowering the antenna height will be very effective for reducing the

cochannel and adjacent-channel interference, However, there are three cases where

lowering the antenna height may or may not effectively help reduce the interference.

On a high hill or a high spot: The effective antenna height, rather than the actual

height, is always considered in the system design. Therefore, the effective antenna

height varies according to the location of the mobile unit. When the antenna site is on a

bill, as shown in Fig. 2.9(a), the effective antenna height is h1 + H.

Fig. 2.9.Lowering the antenna height (a) on a high hill and (b) in a valley

If we reduce the actual antenna height to 0.5h1, the effective antenna height becomes

0.5h1 + H. The reduction in gain resulting from the height reduction is

If h1<<H, then the above equation becomes

This simply proves that lowering antenna height on the kill does not reduce the

received power at either the cell site or the mobile unit.



In a valley: The effective antenna height as seen from the mobile unit shown in Fig.

2.9(b) is he1, which is less than the actual antenna height h1. If he1= 2/3 h1, and the

antenna is lowered to ½ h1, then the new effective antenna height is

Then the antenna gain is reduced by

This simply proves that the lowered antenna height in a valley is very effective in

reducing the radiated power in a distant high elevation area. However, in the area

adjacent to the cell-site antenna the effective antenna height is the same as the actual

antenna height. The power reduction caused by decreasing antenna height by half is

only

In a forested area: In a forested area, the antenna should clear the tops of any trees in

the vicinity, especially when they are very close to the antenna. In this case decreasing

the height of the antenna would not be the proper procedure for reducing cochannel

interference because excessive attenuation of the desired signal would occur in the

vicinity of the antenna and in its cell boundary if the antenna were below the treetop

level.

Diversity Techniques:

Diversity: It is the technique used to compensate for fading channel impairments. It is

implemented by using two or more receiving antennas. While Equalization is used to

counter the effects of ISI, Diversity is usually employed to reduce the depth and

duration of the fades experienced by a receiver in a flat fading channel.

These techniques can be employed at both base station and mobile receivers. Spatial

Diversity is the most widely used diversity technique.

Spatial Diversity Technique‐A Brief Description

In this technique multiple antennas are strategically spaced and connected to common

receiving system. While one antenna sees a signal null, one of the other antennas may

see a signal peak, and the receiver is able to select the antenna with the best signal at



any time. The CDMA systems use Rake receivers which provide improvement through

time diversity.

KINDS OF DIVERSITY:



TABLE:



Types of Diversity:

MACROSCOPIC DIVERSITY

•Prevents Large Scale fading.

•Large Scale fading is caused by

shadowing due to variation in both the

terrain profile and the nature of the

surroundings.

Large Scale fading is log normally

distributed signal.

•This fading is prevented by selecting

an antenna which is not shadowed

when others are, this allows increase in

the signal‐to‐noise ratio.

MICROSCOPIC DIVERSITY

•Prevents Small Scale fading.

•Small Scale fading is caused by multiple

reflections from the surroundings. It is

characterized by deep and rapid amplitude

fluctuations which occur as the mobile

moves over distances of a few wavelength.

•This fading is prevented by selecting an

antenna which gives a strong signal that



The receiver branch having the highest instantaneous SNR is connected to the

demodulator. The antenna signals themselves could be sampled and the best one sent

to a single demodulation

Maximal ratio combining:

The signals from all of the M branches are weighted according to their signal voltage to

noise power ratios and then summed

Feedback or scanning diversity: the signal, the best of M signals, is received until it

falls below threshold and the scanning process is again initiated

Polarization diversity: Theoretical model for polarization diversity



RAKE Receiver:

Fig. 16 An M-branch (M-finger) RAKE receiver implementation.

Each correlator detects a time shifted version of the original CDMA transmission, and

each finger of the RAKE correlates to a portion of the signal which is delayed by at least

one chip in time from the other finger.



2.7 Non-co-channel interference

Two major types of non-CCI discussed in this section are as follows:

Adjacent-channel interference

Near-end to far-end ratio interference

2.7.1 Adjacent-channel interference

The ACI can be classified as either in-band or out-of-band interference. The term in-

band is applied when the centre of the interfering signal bandwidth falls within the

bandwidth of the desired signal. The term out-of-band is applied when the centre of

the interfering signal bandwidth falls outside the bandwidth of the desired signal.

In the mobile radio environment, the desired signal and the adjacent-channel signal

may be partially correlated with their fades. When ACI is compared with CCI at the

same level of interfering power, the effects of the ACI are always less.

ACI can be eliminated on the basis of the channel assignment, the filter

characteristics, reduction of near-end to far-end ratio interference, and also by

keeping frequency separation between each channel as large as possible, avoiding

the use of adjacent channels in neighbouring cell sites, and so on. ACI includes

next-channel (the channel next to the operating channel) interference and

neighbouring-channel (more than one channel away from the operating channel)

interference. It can be reduced by frequency assignment.

2.7.1.1 Next-channel interference

For any particular mobile unit, the next-channel interference affecting it cannot be

caused by transmitters in the common cell site but must originate at several other

cell sites. This is due to the fact that any channel combiner at the cell site must

combine the selected channels, normally 21 channels (630 kHz), or at least 10

channels away from the desired one. Without proper system design, next-channel

interference will arrive at the mobile unit from other cell sites. In addition,

interference can be caused by a mobile unit initiating a call on a control channel in

a cell with the next control channel at another cell site. Next-channel interference

reduction methods use the receiving end. Filters with a sharp falloff slope can help

to reduce all the ACI, including the next-channel interference.



2.7.1.2 Neighbouring-channel interference

Neighbouring-channel interference is another type of ACI that is unique to the

mobile radio system. It is caused by the channels that are several channels away

from the desired channel. In general, a fixed set of channels is assigned to each cell

site.

To reduce inter-modulation products, a sufficient amount of band isolation

between channels is required for a multi-channel combiner if all the channels are

simultaneously transmitted at one cell-site antenna. Evolving technologies are

focusing on using multiple antennas instead of one antenna at the cell site with the

assumption that band separation requirements can be resolved.

2.7.1.3 Transmitting and receiving channels interference

Transmitting and receiving channels interference is another type of ACI caused by

the transmitting channels. This is because the transmitting channels are so strong

that they can mask the weak signals received from the receiving channels. This

effect can be reduced by the following means:

In FDMA and TDMA systems, a guard band of 20 MHz is used to separate the

transmitting and receiving channels.

The duplexer can be used but it only provides an isolation of around 30–20 dB.

By band isolation.

2.7.2 Near-end to far-end ratio interference

A type of interference which occurs only in mobile communication systems is the

near-end to far-end type of interference. This kind of interference appears when the

distance between a mobile unit and the base station transmitter becomes critical

with respect to another mobile transmission that is close enough to override the

desired base station signal. This phenomenon occurs when a mobile unit is

relatively far from its desired base station transmitter at a distance d0, but close

enough to its undesired nearby mobile transmitter at a distance d1, and d1 > d0

(refer Fig. 2.6).

The problem in such a situation is whether the two transmitters will transmit

simultaneously at the same power and frequency, thus masking the signals

received by the mobile unit from the desired source by the signals received from the

undesired source. In addition, this type of interference can take place at the base

station when signals are received simultaneously from two mobile units that are at

unequal distances from the base station. The power difference due to the path loss

between the receiving location and the two transmitters is called the near-end to

far-end ratio interference and is expressed by the ratio of path loss at distance d1 to

the path loss at distance d0.



This form of interference is unique to mobile radio systems. It may occur both

within one cell and within cells of two systems.

2.7.2.1 In one cell

When mobile station A is located close to the base station, and at the same time

mobile station B is located far away from the same base station (e.g., at the cell

boundaries), mobile station Acauses ACI to the base station and mobile station B

(Fig. 2.6). The C/I at mobile station B is expressed by the following equation:

Where γ is the path-loss slope

d0 is the distance from base station to desirable mobile station A

d1 is the distance from base station to undesirable mobile station B

Figure 2.6 Near-far interference in one cells

Since d0 > d1, from Equation (2.9) we obtain C/I < 1. This means that the

interfering signal is stronger than the desired signal. This problem can be rectified if

the filters used for frequency separation have sharp cut-off slopes. The frequency

separation can be expressed as follows:

Frequency band separation is 2G−1B; where

B is the channel bandwidth,L is the filter cut-off slope.

2.7.2.2 In cells of two systems

If two different mobile operators cover an area, ACI may occur if the frequency

channels of the two systems are not properly coordinated.

In Figure 2.7, two different mobile radio systems are depicted.

Mobile station A is located at the cell boundaries of system A, but very close to base

station B. In addition, mobile station B is located at the cell boundaries of system B,

but very close to base station A. Interference may occur at base station A from

mobile station B and at mobile station B from base station A. The same interference

will be introduced at base station B and at mobile station A.

This form of interference can be eliminated if the frequency channels of the two

systems are properly coordinated, as mentioned earlier. If such a case occurs, two

different systems operating in the same area may have co-located base stations.



Figure 2.7 Near-far interference in cells of two systems

2.8 Estimation of adjacent-channel interference levels

A receiver in the cellular system must be designed to receive all channels in the

cellular system band, as a telephone connection may be assigned to any of the

possible channels. By using a highly selective filter, the receiver separates one

channel from another. The pass band bandwidth of the filter is equal to the

bandwidth of the channel.

To prevent signals in the adjacent channels from passing to the demodulator, the

filter must cut off sharply at the pass band edges. A “brick wall” filter, which cuts

off abruptly and completely at the pass band edges, is impossible to realize. In

addition, sharp cut-off filters may be too expensive for mass consumer markets. For

both analog and digital implementations, the performance of a filter is very sensitive

to small errors in the component or coefficient values.

ACI can be a problem, even with highly selective channel filtering. There are a few

strategies available for dealing with this problem. A common strategy is to avoid

using adjacent channels in the same market area. This strategy is used in both AM

and FM broadcasting and in television. In cellular systems, however, the number of

channels available translates directly into the number of customers and in turn into

revenue. Channels are too valuable to be set aside for interference avoidance.

With the dynamic control of the power in a mobile unit transmitter, less power can

be transmitted when it is nearer the base station than it does when it is at a cell

edge. In modern cellular systems, to maintain a constant received power level at a

base station, a mobile unit’s transmitted power is adjusted in 1 dB increments

every few milliseconds, as the mobile unit moves over the cell’s coverage area.

Finally, channel partition can be made so that adjacent channels are assigned to

the same cell or to cells that are immediate neighbours. This will guarantee that an

interference source cannot get physically close to a base station receiver. However,

the available channels will be divided up among a relatively small number of cells

when cluster sizes are small. In this case, it may be difficult to avoid assigning

adjacent channels to the same or nearby cells, and ACI may significantly limit how

small the clusters can be made.



Effects on coverage and interference by power decrease and Antenna height decrease



Effects of cell site components :

Channel Combiner:

1. A Fixed Tuned Channel Combiner: At the travelling side, a fixed tunable combined

unit is used. In every cell site, a channel combiner circuit is installed. The transmitted

channels have to be combined based on the following two criteria,

a) The signal isolation between the radio channels must be maximum

b) The insertion loss should be minimum. However, the usage of channel combiner

can be avoided by feeding each channel to its corresponding antenna.

But, if there are I6 channels available in a cell site, there will be requirement of 16

antennas for operation which is bottle neck for real time functionalities. It is not

economical to hive huge hardware setups. Thus, a conventional combiner can he used,

which has 16 channel combining capacity and it is based on the frequency subset of 16

channels of cell site.



The channel combiner would be responsible for each of the 16 channels to exhibit a 3 dB

loss due to the signal insertion in to the channel combiner. The signal isolation would be

17dB, if every channel is separated from its neighboring channels by 630 kHz frequency.

2. Tunable Combiner: Tunable combiner is also referred as frequency agile combiner.

The frequency agile combiner is an advanced combiner circuit with additional features. It

can return any frequency in real time by remote control device, namely microprocessor.

This combiner is essentially a waveguide resonator with a tuning bar facility. A motor

makes the tuning bar to rotate and once the motor starts rotating, the Voltage Standing

Wave Ratio (VSWR) can be measured.

The controller unit has self-adjusting feature and it accepts an optimum value of VSWR

as the motor complete, a full turn. The controller is compatible only with dynamic

frequency assignment.

The cell-sites should be flexible to change their operating frequency ‘f’ that is controlled

by MTSO/MSC. Thus, we can use this frequency agile combiner in the cell site

transceiver setup.

3. Ring Combiner: Ring combiner is used to combine two groups of channels to give one

output. This combiner has an insertion loss of 3 dB. For example, using a ring combiner

two 16 channel groups into one 32 channel output. Even 64 channels can be used with

this combiner if two antennas arc available in the cell site. In case of low transmitter

power more than one ring combiner can be used for combining. However, the demerits of

ring combiners are.

a) It reduces adjacent-channel separation.

b) They may be affected from the problem of power limitations.

Demultiplexer at the Receiving End

The main theme of using demultiplexer at the receiver end is to reduce the non co-

channel interference. A 16:1 demultiplexer is used in between the pre-amplifier stage

and filter stage as shown in figure 10.16 below.

Particularly, 16:1 demultiplexer is used in order to receive 16 channels from a single

antenna. The output of each antenna reaches demultiplexer after passing through a 25

dB gain amplifier. The total split loss of demultiplexer output and due to 16 channels is

given by.

S =10 log 16

= 12.04

S =12.04 dB

Care must be taken such that the intermodulation product at the demultiplexer output

is 65 dB down and the space diversity antennas connected to an umbrella filter must

have a 55 dB rejection from other systems band, otherwise in case. if a dummy mobile

unit is close to the cell site then the preamplifier generates intermodulation frequency at

the amplifiers output which may lead to cross talk.



Example problem 2.1

If signal-to-noise interference ratio of 15 dB is required for a satisfactory forward channel

performance of a cellular system, what is frequency reuse factor and cellular size that

should be used for maximum capacity if the path-loss exponent is (a) γ = 4 and (b) γ = 3?

Assume that there are six co-channel cells in the first tier and all of them are at the same

distance from the mobile. Use suitable approximations.

Solution

1. Path-loss exponent γ = 4

First, let cluster size N = 7

And

Since this is greater than minimum required , N = 7 can be used.


Let N = 7

which is less than minimum required , hence we need to use larger N.

∴ Next possible value of N = 12 for i = j = 2. (N = i2 - j2 - ij)

For N = 12,

Since this is greater than minimum required , N = 12 is used.

Example problem 2.2

If signal-to-noise interference ratio of 20 dB is required for a satisfactory forward channel

performance of a cellular system, what is the frequency reuse factor and the cellular size

that should be used for maximum capacity if the path-loss exponent is (a) γ = 6 and (b) γ

= 4? Assume that there are six co-channel cells in the first tier and all of them at the

same distance from the mobile. Use suitable approximations.

Solution


First, let cluster size N = 7.

and

Since this is greater than minimum required , N = 7 can be used.


Let N = 7

which is less than minimum required , hence we need to use larger N.

Next possible value of N = 12 for i = j = 2 (N = i2 - j2 - ij)

For N = 12,



which is less than minimum required hence we need to use larger N. Continue this for

lager values of N until is greater than the minimum required.

Objective type questions and answers

1. Anything which alters, modifies, or disrupts a signal as it travels along a channel between

a source and a receiver is called as

1. noise

2. interference

3. crosstalk

4. deterioration in receiver

2. For hexagonal cellular systems, the interference that results from the first tier is

1. next-channel interference

2. near-end to far-end interference

3. co-channel interference

4. adjacent-channel interference

3. Co-channel interference can be reduced by

1. decreasing D/R

2. increasing co-channel interference ratio, q

3. reducing number of channels

4. increasing cluster size (N)

4. Adjacent-channel interference is caused by signals from

1. same frequencies

2. same cell site

3. neighbouring frequencies

4. neighbouring cell site

5. Adjacent-channel interference is reduced if the separation between adjacent

channels in a cell is

1. maximum

2. minimum

3. unaltered

4. doubled

6. Co-channel interference limits the extent to which

1. cluster size can be reduced

2. transmit power can be used

3. co-channel interference ratio can be increased

4. number of channels that can be used in a cell site

7. Interference caused by the channels that are several channels away from the desired

channel is known as

1. next-channel interference

2. transmitting and receiving channel interference

3. neighbouring-channel interference

4. intra-channel interference

Answers: 1. (b), 2. (c), 3. (b), 2. (c), 5. (a), 6. (a), 7. (c).



Key equations

1. Carrier-to-interference ratio received at a desired cell

2. Frequency reuse ratio.

3. The signal-to-interference ratio C/I is given by equation with J = 6 interference sources

4. The signal-to-interference ratio C/I at the desired mobile receiver is given by

5. Frequency band separation 2G-1B

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