INTRODUCTION TO CELLULAR SYSTEMS
1. BASIC CELLULAR SYSTEMS (R.G.P.V. Dec 2012, June 2013, Dec 2014)
Q. Describe the basic cellular system?
A basic analog cellular system consists of three subsystems: a mobile unit, a cell site, and a mobile telephone switching office (MTSO), as Fig. 1 shows, with connections to link the three subsystems.
1. Mobile units. A mobile telephone unit contains a control unit, a transceiver, and an antenna system.
2. Cell site. The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets, antennas, a power plant, and data terminals.
3. MTSO. The switching office, the central coordinating element for all cell sites, contains the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing, provides operation and maintenance, and handles billing activities.
4. Connections. The radio and high-speed data links connect the three subsystems. Each mobile unit can only use one channel at a time for its communication link. But the channel is not fixed; it can be any one in the entire band assigned by the serving area, with each site having multichannel capabilities that can connect simultaneously to many mobile units.
Fig. 1. Basic cellular System
The MTSO is the heart of the analog cellular mobile system. Its processor provides central coordination and cellular administration. The cellular switch, which can be either analog or digital, switches calls to connect mobile subscribers to other mobile subscribers and to the nationwide telephone network.
It uses voice trunks similar to telephone company interoffice voice trunks. It also contains data links providing supervision links between the processor and the switch and between the cell sites and the processor.

2. PERFORMANCE CRITERIA (R.G.P.V. Dec 2010)
Q. What are the performance criteria of cellular system?
There are three categories for specifying performance criteria.
2.1 Voice Quality
Voice quality is very hard to judge without subjective tests for users’ opinions. In this technical area, engineers cannot decide how to build a system without knowing the voice quality that will satisfy the users. In military communications, the situation differs: armed forces personnel must use the assigned equipment. CM: For any given commercial communications system, the voice quality will be based on the following criterion: a set value x at which y percent of customers rate the system voice quality (from transmitter to receiver) as good or excellent; the top two circuit merits (CM) of the five listed below.
CM Score Quality scale
CM5 5 Excellent (speech perfectly understandable)
CM4 4 Good (speech easily understandable, some noise)
CM3 3 Fair (speech understandable with a slight effort)
CM2 2 Poor (speech understandable only with considerable effort)
CM1 1 Unsatisfactory (speech not understandable)
2.2. Service Quality
Three items are required for service quality.
1. Coverage. The system should serve an area as large as possible. With radio coverage, however, because of irregular terrain configurations, it is usually not practical to cover 100 percent of the area for two reasons: a. The transmitted power would have to be very high to illuminate weak spots with sufficient reception, a significant added cost factor. b. The higher the transmitted power, the harder it becomes to control interference. Therefore, systems usually try to cover 90 percent of an area in flat terrain and 75 percent of an area in hilly terrain. Cellular systems3 state that 75 percent of users rate the voice quality between good and excellent in 90 percent of the served area, which is generally flat terrain. The voice quality and coverage criteria would be adjusted as per decided various terrain conditions. In hilly terrain, 90 percent of users must rate voice quality good or excellent in 75 percent of the served area.
2. Required grade of service. For a normal start-up system, the grade of service is specified for a blocking probability of .02 for initiating calls at the busy hour. This is an average value.
However, the blocking probability at each cell site will be different. At the busy hour, near freeways, automobile traffic is usually heavy, so the blocking probability at certain cell sites may be higher than 2 percent, especially when car accidents occur. To decrease the blocking probability requires a good system plan and a sufficient number of radio channels.

3. Number of dropped calls. During Q calls in an hour, if a call is dropped and Q−1 calls are completed, then the call drop rate is 1/Q. This drop rate must be kept low. A high drop rate could be caused by either coverage problems or handoff problems related to inadequate channel availability or weak reception.
3. UNIQUENESS OF MOBILE RADIO ENVIRONMENT (R.G.P.V. Dec 2012, June 2011)
Q. Determine the path losses in real mobile radio environment?
The Propagation Attenuation. In general, the propagation path loss increases not only with frequency but also with distance. If the antenna height at the cell site is 30 to 100 m and at the mobile unit about 3 m above the ground, and the distance between the cell site and the mobile unit is usually 2 km or more, then the incident angles of both the direct wave and the reflected wave are very small, as Fig. 2.4 shows. The incident angle of the direct wave is θ1, and the incident angle of the reflected wave is θ
2
. θ
1
is also called the elevation angle. The propagation path loss would be 40dB/dec. This means that a 40-dB loss at a signal receiver will be observed by the mobile unit as it moves from 1 to 10 km. Therefore C is inversely proportional to R 4 .
( 1 )
Where C = received carrier power
R = distance measured from the transmitter to the receiver
α = constant
Fig. 2. Transmission Model
The difference in power reception at two different distances R 1 and R 2 will result in and the decibel expression of Eq. is
This 40 dB/dec is the general rule for the mobile radio environment and is easy to remember. It is also easy to compare to the free-space propagation rule of 20 dB/dec. The linear and decibel scale expressions are

In a real mobile radio environment, the propagation path-loss slope varies as
( 3 )
γ usually lies between 2 and 5 depending on the actual conditions,
γ cannot be lower than 2, which is the free-space condition.
C = 10 log α − 10γ log R dB (4)
4. OPERATION OF CELLULAR SYSTEMS (R.G.P.V. Dec 2010)
Q. Write the different operations of basic cellular system?
The operation can be divided into four parts and a handoff procedure.
Mobile unit initialization . When a user activates the receiver of the mobile unit, the receiver scans the set-up channels. It then selects the strongest and locks on for a certain time. Because each site is assigned a different set-up channel, locking onto the strongest set-up channel usually means selecting the nearest cell site. This self-location scheme is used in the idle stage and is user-independent. It has a great advantage because it eliminates the load on the transmission at the cell site for locating the mobile unit. The disadvantage of the self-location scheme is that no location information of idle mobile units appears at each cell site. Therefore, when the call initiates from the land line to a mobile unit, the paging process is longer. For a large percentage of calls originates at the mobile unit, the use of self-location schemes is justified. After a given period, the self-location procedure is repeated.
Mobile originated call . The user places the called number into an originating register in the mobile unit, and pushes the “send” button. A request for service is sent on a selected set-up channel obtained from a self-location scheme. The cell site receives it, and in directional cell sites (or sectors), selects the best directive antenna for the voice channel to use. At the same time, the cell site sends a request to the mobile telephone switching office (MTSO) via a high-speed data link. The MTSO selects an appropriate voice channel for the call, and the cell site acts on it through the best directive antenna to link the mobile unit. The MTSO also connects the wire-line party through the telephone company zone office.
Network originated call . A land-line party dials a mobile unit number. The telephone company zone office recognizes that the number is mobile and forwards the call to the MTSO. The MTSO sends a paging message to certain cell sites based on the mobile unit number and the search algorithm. Each cell site transmits the page on its own set-up channel. If the mobile unit is registered, the registered site pages the mobile. The mobile unit recognizes its own identification on a strong set-up channel, locks onto it, and responds to the cell site. The mobile unit also follows the instruction to tune to an assigned voice channel and initiate user alert.
Call termination. When the mobile user turns off the transmitter, a particular signal (signaling tone) transmits to the cell site, and both sides free the voice channel. The mobile unit resumes monitoring pages through the strongest set-up channel.
Handoff procedure . During the call, two parties are on a traffic channel. When the mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak. The current cell site requests a handoff. The system switches the call to a new frequency channel in a new

cell site without either interrupting the call or alerting the user. The call continues as long as the user is talking. The user does not notice the handoff occurrences.
EX.1. Let the maximum calls per hour Qi in one cell be 3000 and an average calling time T be
1.76 min. The blocking probability B is 2 percent find the offered load A.
(R.G.P.V. Dec 2010)
Solution:
Given Data Qi=3000 calls/hour, T = 1.76 min.


3000

1 .
76
A
 
88
60
5. CONCEPT OF FREQUENCY REUSE CHANNELS (R.G.P.V. Dec 2011, June 2014)
Q. Discuss the reuse concept of frequency?
A radio channel consists of a pair of frequencies, one for each direction of transmission that is used for full-duplex operation. A particular radio channel, say F1, used in one geographic zone as named it a cell, say C1, with a coverage radius R can be used in another cell with the same coverage radius at a distance D away.
Frequency reuse is the core concept of the cellular mobile radio system. In this frequency reuse system, users in different geographic locations (different cells) may simultaneously use the same frequency channel (see Fig. 3). The frequency reuse system can drastically increase the spectrum efficiency, but if the system is not properly designed, serious interference may occur.
Interference due to the common use of the same channel is called cochannel interference and is our major concern in the concept of frequency reuse.
Fig. 3. The ratio of D/R

Fig. 4. N-cell reuse Pattern
Frequency Reuse Distance
The minimum distance that allows the same frequency to be reused will depend on many factors, such as the number of cochannel cells in the vicinity of the center cell, the type of geographic terrain contour, the antenna height, and the transmitted power at each cell site.
The frequency reuse distance D can be determined from:
D

3 K R
Where K is the frequency reuse pattern shown in Fig. 4, then

D


3


.
46
4
6
.
6
RforK
RforK
RforK



4
7
12
(1.1)
If all the cell sites transmit the same power, then K increases and the frequency reuse distance D increases. This increased D reduces the chance that cochannel interference may occur.
Theoretically, a large K is desired. However, the total number of allocated channels is fixed.
When K is too large, the number of channels assigned to each of K cells becomes small. It is always true that if the total number of channels in K cells is divided as K increases, trunking inefficiency results.
6. COCHANNEL INTERFERENCE REDUCTION FACTOR (R.G.P.V. june 2010)
Q. Derive the formula for minimum cochannel interference?
Assume that the size of all cells is roughly the same. The cell size is determined by the coverage area of the signal strength in each cell. As long as the cell size is fixed, cochannel interference is independent of the transmitted power of each cell. It means that the received threshold level at the mobile unit is adjusted to the size of the cell. Actually, cochannel interference is a function of a parameter q defined as q

D
R
The parameter q is the cochannel interference reduction factor. When the ratio q increases, cochannel interference decreases. Furthermore, the separation D in Eq. (2.6-1) is a function of K
I and C/I
D
 f ( K
I
,
C
I
)
Where K
I
is the number of cochannel interfering cells in the first tier and C/I is the received carrier-to-interference ratio at the desired mobile receiver.
C

I K
I 
K

1
C
D
 
K
In a fully equipped hexagonal-shaped cellular system, there are always six cochannel interfering cells in the first tier, as shown in Fig. 2.15; that is, K
I
= 6 Cochannel interference can be experienced both at the cell site and at mobile units in the center cell.
C
I

R
K
I 
K

1
 
D
 
K
Therefore, the cochannel interference from the second tier of interfering cells is negligible.
C

1

1
I K
K
 I

1


D
K
R


 
K
K
 I

1
( q
K
)
 

Fig. 5. Six effective interfering cells of cell 1
Where q k
is the cochannel interference reduction factor with k th
cochannel interfering cell q
K

D
K
R
7. DESIRED C/I FROM A NORMAL CASE IN AN OMNIDIRECTIONAL ANTENNA
SYSTEM
Q. Find the desired C/I For K=7?
There are two cases to be considered: (1) the signal and cochannel interference received by the mobile unit and (2) the signal and cochannel interference received by the cell site. Both cases are shown in Fig. 6.
Nm , and Nb are the local noises at the mobile unit and the cell site, respectively.
Usually, Nm and Nb are small and can be neglected as compared with the interference level.
Fig. 6. Cochannel interference from six interferers, (a) Receiving at the cell site; (b) receiving at the mobile unit.
Assume that all that all D k are the same for simplicityD k
then D = D k
, and q = q k
, and
C

R
 
 q

I 6 D
 
6
(1)
Thus q
 
6
C
I
(2)

And q

6



C
I


1 /

(3)
Normal cellular practice is to specify C/I to be 18 dB or higher based on subjective tests Because a C/I of 18 dB is measured by the acceptance of voice quality from present cellular mobile receivers. q

D / R


6

63 .
1

1 / 4 
4 .
41 (4)
This is because q is not a function of transmitted power. The computer simulation described in the next section finds the value of q = 4 .
6, which is very close to Eq. (1.1). The factor q can be related to the finite set of cells K in a hexagonal-shaped cellular system by q



3 K
(5)
Substituting q from Eq. (4) into Eq. (5) yields
K

7 (6)
Equation (6) indicates that a seven-cell reuse pattern is needed for a C/I of 18 dB.
The seven-cell reuse pattern is shown in Fig. 4.
9. HANDOFF MECHANISM (R.G.P.V. june 2010, Dec 2013)
Q. Write short note on Handoff?
It is a unique feature that allows cellular systems to operate as effectively as demonstrated in actual use. There are two kinds of handoffs, hard and soft. The hard handoff is “brake before make.” The soft handoff is “make before brake.” Two cochannel cells using the frequency
F 1 separated by a distance D are shown in Fig. 7 a . The radius R and the distance D are governed by the value of q . Now we have to fill in with other frequency channels such as F 2, F 3, and F 4 between two cochannel cells in order to provide a communication system in the whole area.
The fill-in frequencies F 2, F 3, and F 4 are also assigned to their corresponding cells C 2, C 3, and
C 4 (see Fig. 7 b ) according to the same value of q . Suppose a mobile unit is starting a call in cell
C 1 and then moves to C 2. The call can be dropped and reinitiated in the frequency channel from
F 1 to F 2 while the mobile unit moves from cell C 1 to cell C 2. This process of changing frequencies can be done automatically by the system without the user’s intervention.
Fig. 7. Handoff mechanism. (a) Cochannel interference reduction ratio q (b) Fill-in frequencies

10. CELL SPLITTING (R.G.P.V. june 2013)
Q. Describe the different type of cell splitting?
The motivation behind implementing a cellular mobile system is to improve the utilization of spectrum efficiency. The frequency reuse scheme is one concept, and cell splitting is another concept. When traffic density starts to build up and the frequency channels Fi in each cell Ci cannot provide enough mobile calls, the original cell can be split into smaller cells. Usually the new radius is one-half the original radius (see Fig. 2.18). There are two ways of splitting.
The following equation is true
Let each new cell carry the same maximum traffic load of the old cell;
There are two kinds of cell-splitting techniques:
1. Permanent splitting. The installation of every new split cell has to be planned ahead of time; the number of channels, the transmitted power, the assigned frequencies, the choosing of the cell-site selection, and the traffic load consideration should all be considered. When ready, the actual service cut-over should be set at the lowest traffic point, usually at midnight on a weekend. Hopefully, only a few calls will be dropped because of this cut-over, assuming that the downtime of the system is within 2 h.
2. Dynamic splitting. This scheme is based on using the allocated spectrum efficiency in real time. The algorithm for dynamically splitting cell sites is a tedious job, as we cannot

2.1 Introduction (R.G.P.V. June 2011)
Q. How to plan signal coverage in a different environmental conditions?
Coverage is the extent of a geographical area in which a wireless network provider offers cellular service for mobile phone users. It is usually a measure of cell phone connectivity and depends on several factors such as orography (i.e., study of mountains), buildings, technology, and radio frequency. Some frequencies provide better coverage while other frequencies penetrate better through obstacles, such as buildings in cities. The ability of a mobile phone to connect to a base station depends on the strength of the signal which may be boosted by higher power transmissions, taller antenna masts, and better antennae. This is also a problem when designing networks for large metropolitan areas with modern skyscrapers. Also, signals do not travel deep underground. Hence, specialized transmission solutions are used to deliver mobile phone coverage into areas such as underground parking garages and subway trains.
The extent of coverage for a given building is represented in the form of percentage of the area covered at a certain height above the ground level. The task is to cover the whole area with a minimum number of cell sites. Since 100 per cent cell coverage of an area is not possible, the cell sites must be engineered so that the holes are located in the no-traffic locations. The coverage area will be examined in environments such as in an open area, in a suburban area, in an urban area, over flat terrain, over hilly terrain, over water, and through foliage areas.
This chapter discusses the prediction model, which is a point-to-point model, followed by a study of propagation over water or flat open area and the loss due to foliage. The cell-site antenna heights and signal coverage cells are then briefly discussed. Finally, the chapter concludes with an overview of near and long-distance propagation. The results generated from the prediction model will differ depending on the coverage area being used. There are many field-strength prediction models that provide more or less an area-to-area prediction.
2.2 Point-to-point model (R.G.P.V. June 2013, Dec 2013)
Q. Write in detail about the Point-to-point model?
The free-space propagation model does not apply in a mobile radio environment. Propagation path loss depends on distance of the mobile from its serving cell-site, carrier frequency of transmission, f c
(or wavelength λ c
), the antenna heights of cell-site and mobile unit and the local terrain characteristics such as buildings and hills. Mobile radio propagation path loss is uniquely different from that experienced in other communication media. The signal received by a mobile unit remains constant only over a small operating area and varies as the mobile unit moves.
This is mainly due to the variations in the terrain conditions and presence of man-made structures surrounding the mobile unit.
The point-to-point propagation prediction model reduces the uncertainty range by including the terrain contour profiles in the path-loss predictions. In a non-obstructive condition, the direct radio path from the cell-site to the mobile unit is not obstructed by the terrain contour but the radio path may be obstructed by man-made structures. In the mobile radio environment, line-ofsight condition is generally not available. Under these conditions, the antenna height gain is calculated for every location of the mobile unit it covers. Taking into account the antenna-height

gain at various mobile locations, the path-loss slope will have a standard deviation of only less than 2–3 dB instead of 8 dB as observed in area-to-area prediction model.
In obstructive condition (i.e., the direct path from the cell-site to the mobile unit is obstructed by the terrain contour), first the area-to-area prediction model is applied. This is then followed by diffraction loss. When the heavy foliage is close in at the mobile unit, the loss due to the foliage can be obtained from the diffraction loss. In either ease, according to the theoretical model, the
40 dB per decade path loss slope applies.
The mobile point-to-point prediction model provides a standard deviation of less than 3 dB from the predicted value. It is very useful for designing a mobile cellular system with a radius of 15 km or less for each cell. Since the path loss or received signal strength data follows the lognormal distribution, 68 per cent of predicted values obtained from a point-to-point propagation model are within 2–3 dB. In irregular terrain conditions, the signal received by a mobile varies due to the change in terrain conditions and the presence of natural or man-made obstructions.
The features of point-to-point prediction model are as follows: o o o
In point-to-point model, the large range of uncertainty is reduced.
For prediction of path loss, it applies the detailed terrain contour information.
In the analysis of point-to-point method, the predicted values were placed at x -axis and o o o o actual measured values at y -axis and it was a simple method.
A line of 45° as shown in Figure 2.1 is drawn between these two values and this 45° line is the errorless prediction line.
This prediction model is very useful for cellular mobile communication.
But the largest difference between the predicted and measured values is around 3 dB and the accuracy is less than that of area-to-area prediction model.
The occurrence of handoffs in cellular mobile communication can be predicted with more accuracy in this model.
Figure 2.1
Point-to-point prediction curves in no obstruction
2.3 Propagation over water or flat open area (R.G.P.V. June 2008)
Q. Write short notes on (a) Propagation over water or flat open area (b) Propagation between two fixed stations
The mobile signal propagation over water or flat open area has to be given proper design setups because probability of occurrence of interference is high if necessary arrangements are not done.

For fresh as well as the sea water, the value of relative permittivity (
ε r
) is same whereas their conductivities and dielectric constant (
ε c
) are different from one another.
Figure 2.2 shows two antennas; one at mobile unit and the other at sea level with two reflection points. One point ( P
1
) is close to mobile unit and the other point ( P
2
) is away from the mobile unit.
Figure 2.2
Propagation over water
2.3.1 Propagation between two fixed stations
Consider two fixed stations that use point-to-point communication between them. Here, the received power P r
will be, where
P t
is transmitted power
λ is the wavelength and d distance between two stations b and φ v
are the amplitude and phase of the complex reflection coefficient, respectively.
Note: The phase difference Δφ, phase differenceis due to the path difference Δ d (between direct and reflected waves)
The path loss for the land-to-mobile propagation over the land is 40 dB per decade and it is different for the land-to-mobile propagation over water. In land-to-mobile propagation over water, path loss is 20 dB per decade.
2.4 Foliage loss (R.G.P.V. June 2012)
Q. What is foliage loss in cellular radio environment?
Foliage loss is the loss that occurs due to trees. It includes many parameters and variations pertaining to the size; the density; the distribution of leaves, branches, trunks; the height of the trees relative to the antenna height; and so on. Figure 2.3 illustrates the problem.

Figure 2.3
Foliage environment
Consider three levels: trunks, branches, and leaves. Each level includes a distribution of sizes of trunks, branches, and leaves and also of the density and spacing between the adjacent trunks, branches, and leaves. The texture and thickness of the leaves are also taken into account.
However, the estimate of the signal reception due to foliage loss does not need any degree of accuracy for system design.
However, a rough estimate should be sufficient. In tropical zones, the signal can hardly penetrate due to the large and thick sizes of tree leaves. It will then propagate from the top of the tree and deflect to the mobile receiver.
When the foliage is uniformly heavy and the path lengths are short, the foliage loss can be treated as a wire-line loss, in decibels per foot or decibels per metre. Decibels per octaves or decibels per decade are used when the path length is long and the foliage is non-uniform. In general, foliage loss is proportional to the frequency to the fourth power ( ∼ f 4 ).
2.5 Cell-site antenna heights and signal coverage cells (R.G.P.V. Dec 2013, Dec 2014)
Q. Determine Cell-site antenna heights and signal coverage cells?
The term effective antenna height refers to the height of the centre of radiation of an antenna above the effective ground level.
Antenna height unchanged
If the antenna height is unchanged then the whole signal-strength map can be linearly updated according to the change in power of the cell-site transmitter. If the transmitted power increases by 3 dB, just add 3 dB to each grid in the signal-strength map. The relative differences in power among the grids remain the same.
Antenna height changed
If the antenna height changes (±Δ h ), then the whole signal-strength map obtained from the old antenna height cannot be updated with a simple antenna gain formula as

where h
1
is the old actual antenna height and h
′
1 is the new actual antenna height. However, we can still use the same terrain contour data along the radio paths (from the cell-site antenna to each grid) to figure out the difference in gain resulting from the different effective antenna heights in each grid. where, h e
is the old effective antenna height and h
′ e
is the new effective antenna height. The additional gain (increase or decrease) will be added to the signal-strength grid based on the old antenna height.
Antenna location changed
If the location of the antenna is changed, then the point-to-point program has to start all over again. The old point-to-point terrain contour data are no longer useful. The old effective antenna height seen from a distance will be different when the location of the antenna changes and there is no relation between the old effective antenna height and the new effective antenna height. Therefore, every time the location of the antenna changes, the new point-to-point prediction calculation starts.
Visualization of signal coverage cells (R.G.P.V. Dec 2013, Dec 2011)
Q. Discuss in detail Visualization of signal coverage cells?
A physical cell is usually visualized as a signal-reception region around the cell site. Within the region, there are weak spots called holes. This is always true when a cell covers a relative flat terrain.
However, if a cell covers a hilly area, then the coverage patterns of the cell will look like those shown in Figure 2.4(b). Here, the two cell sites are separated by a river. Because of the shadow loss due to the river bank, cell site A cannot cover area A
′, but cell site
B can. The same situation applies to cell site B in area B
′. Now every time the vehicle enters area
A
′, a handoff is requested as if it were in cell B .

Figure 2.4
(a) Signal coverage due to effective antenna heights (b) Signal coverage served by two cell sites
Therefore, in most cases, the holes in one cell are covered by the other sites. As long as the processing capacity at the mobile telephone switching office (MTSO) can handle excessive handoff, this overlapped arrangement for filling the holes is a good approach in a noninterference condition.
Example problem 2.1
When the cell-site antenna height is 30 m (100 ft), the mobile unit 8 km (5 m) away sees this cell-site effective antenna height as 60 m. Now if the antenna height is changed to 45 m, then find the changed effective antenna height seen by the same mobile unit.
Solution
Given
Old cell site antenna height, h
1
= 30 m
New cell site antenna height, h
′
1
= 45 m
Effective antenna height seen by the mobile unit, h e
= 60 m
The new cell-site effective antenna height, h
′ e
seen from the mobile unit can be derived h ′ e
= h e
+ ( h ′
1
− h
1
) = 60 + (45 − 30) = 75 m
11.6 Near and long-distance propagation
In this section, the cases of near-in and long-distance propagation are considered.
2.6.1 Near-in distance propagation (R.G.P.V. June 2010, Dec 2011)
Q. Write in short about the Near-in distance propagation?
A high-gain omnidirectional antenna has narrow beamwidth in the vertical plane within the radius of 1 mi (mile). The amount of signal received at the mobile unit will be reduced when the mobile unit is 1 mi away (it is said to be in the shadow region outside the main beam of the antenna).

Because of the antenna’s vertical pattern, the elevation angle will be larger resulting in a weak reception level. There will be significant difference in the signal reception level say in the range of 10–20 dB due to road structures that would be in-line and perpendicular orientations.
Also the nearby surrounding objects of the cell site could influence the signal level reception, when the mobile unit is available within the 1 mi radius. As the distance of objects is more than 1 mi from the cell site, they may not affect the signal as with the previous case. For the land-tomobile propagation, the height of antenna at cell site will definitely affect the signal reception.
Here, the antenna height at base station is an important criterion to consider.
Near-in propagation has also to be considered as that of the far-in distance case. A suburban area can be assumed for analysis. The signal received level is −61.7 dBm for a suburban area at 1 mi intercept, and it is dependent on the reference set of the parameters, including antenna height.
2.6.2 Long-distance propagation (R.G.P.V. June 2012)
Q. Write about the Long-distance propagation?
In cellular mobile communication, for short-and long-distance propagation, there are some advantages and disadvantages. In the case of a high cell site, it can cover large areas even in a noisy environment. Apart from the interference generated from co-channels and adjacent channels in the cell site, the long-distance propagation also affects the interference. As the traffic increases, the noise limited system becomes interference limited.
Considering an area within cell radii of 50 and 200 mi
Within a 50 mi radius, for the high cell site, the low-atmospheric region would influence the ground-wave path to travel in a non-straight line. This effect is more dominant over sea water than over a flat land profile. The signal may be very strong at one point of measurement and weak at another because the wave path can bend either upward or downward.
Within a radius of 200 mi, the case is different. The tropospheric signal propagation will prevail at a frequency of 800 MHz for long-distance propagation and the signal may sometimes reach
200 mi (320 km) away. This is due to the sudden changes in dielectric constant values of the troposphere which is 10 km above the earth’s surface. The change in temperature also will result with a change in dielectric constant value. In tropospheric signal propagation, the wave may be divided by the mechanisms of reflection and refraction. In addition, moisture also affects signal propagation. The pressure of water vapour will decrease as the antenna height increases. If the value of refractive index starts to decrease with height over a particular range, then the rays get curved downwards. This condition is called as duct propagation or trapping.
Thus, the two cases discussed above on signal reflections due to flat terrain and hilly structure show how reflection takes place. The height of antenna above the hilly structure is accounted along with the height of the hill.
In both the cases, there are reflections but if the contour structure is with more irregularities, the amount of reflection is also severe and will result in more propagation path loss.
Consider Figure 2.5 for measurement of antenna heights. The effective antenna height h e
can be calculated from the point where the reflected ground plane and antenna meet. The effective antenna height is 40 m in case (a) and 200 m in case (b). The actual antenna height h a
is 100 m.
Example problem 2.2
From Figure 2.5, find the antenna height gain for both cases (a) and (b).
solution
In case (a) h a
= 100 m h′ a
= 140 m
Δ h = 40 m

In case (b) h a
= 100 m h′ a
= 300 m
Δ h = 200 m
The antenna height gain (Δ
G ) measurement for case (a) and case (b) is given by
Gain = Δ G = 20log ( h e
/h a
)
For case (a) Δ G = 20log (40/100) = −8 dB (negative gain value)
For case (b) Δ
G = 20log (200/100) = 6 dB (positive gain value)
Figure 2.5
Antenna height gain
From the above example, it is evident that as the mobile unit travels along the road according to the land profile whether hilly or quasi-flat terrain the gain value changes. In addition, the effective antenna height changes as the mobile unit moves.
If the changes in antenna height gain are not considered as the mobile travels then the path loss slope will 8 dB standard deviation. Otherwise it would be in range 2–3 dB.
In Figure 2.6(a), if the height h
1
<< H then the length l of the floor will be approximately equal to the length of the moving vehicle. In the case of free propagation shown, a strong signal reception is possible.

In Figure 2.6(b), the length l when compared to previous length is longer and the antenna set-up remains the same. Since the l is longer, the ground reflection takes place by which there are two waves – direct and reflected – are generated.
The stronger the reflected wave, the larger will be the path loss. This wave occurs at a small angle
θ
. Thus, owing to a small incident angle, a large reflection coefficient is possible due to reflection mechanism. In addition, the presence of a stronger reflected wave tends to weaken the direct wave in propagation.
Figure 2.6
Effective antenna heights