Upload
others
View
6
Download
0
Embed Size (px)
Citation preview
(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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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,
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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).
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
any time. The CDMA systems use Rake receivers which provide improvement through
time diversity.
KINDS OF DIVERSITY:
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
TABLE:
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
Effects on coverage and interference by power decrease and Antenna height decrease
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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.
2. Path-loss exponent γ = 3
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
1. Path-loss exponent γ = 6
First, let cluster size N = 7.
and
Since this is greater than minimum required , N = 7 can be used.
2. Path-loss exponent γ = 4
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,
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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).
(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-II
VIDYA SAGAR.P VBIT Potharajuvidyasagar.wordpress.com
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