Cellular System Design Trade Offs

7Cellular System Design Trade-offs

As the demand for wireless communication service increases, the number of channels assigned to a cell eventually becomes insuffi-cient to support the required number of subscribers to establish simultaneous communi cation links. There are several cellular-design techniques which help to achieve enhanced system capacity within the allocated limited frequency spectrum at an acceptable signal quality. This chap-ter covers numerous methods and system parameters to achieve these objectives. The emphasis here is to understand the cellular-system design trade-offs among various techniques, ranging from cell split-ting for varying traffic densities; modula-tion and speech-coding techniques for spectrum efficiency; equalisation, diver-sity, and channel-coding techniques to combat errors, and hand-off mechanisms, leading to successful planning, design and implementation of cellular mobile com-munication systems.

7.1 SYSTEM PARAMETERS TO INCREASE CELL COVERAGE

The following approaches are used in a cellular system to increase the radio coverage within a cell.

– Increasing the transmitted power at the cell-site – Increasing cell-site antenna height – Using a high-gain omnidirectional antenna at the cell-site – Using directional antennas at the cell-site – Proper selection of cell-site locations – Engineering the antenna patterns at the cell-site – Using a diversity receiver – Using a low-noise receiver – Lowering the threshold level of received signals – Using wireless repeaters or/and signal enhancers

From the knowledge of propagation models in a mobile oper-ating environment, the received signal power at the mobile subscriber, Pr is given by

Pr ∝ Pt Gt ht r −4 (7.1)

where Pt is the cell-site transmitter power, Gt is the cell-site transmitter antenna gain, ht is the cell-site transmitter antenna height, and r is the distance between cell-site transmitter and mobile receiver, which signifies the coverage radius. The path-loss exponent γ in a mobile radio environment is taken as 4. Keeping all other factors unchanged, the increase of transmitter power will result into increase in coverage area.

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184 Wireless Communications

EXAMPLE 7.1 Increase in cell coverage by increasing transmitted power

The cell-site transmitted power increases by 3 dB. For the same minimum acceptable received signal power and all other factors remaining unchanged, prove that the coverage area increases by 1.4 times. Assume mobile radio operating environment conditions.

Solution

Step 1. Let the initial cell-site transmitted power be Pt1 and the minimum acceptable received signal power be Pr1 at a

cell radius of r1 . Then,

Pr1 ∝ Pt1

r1−4 (7.2)

Step 2. Let the changed cell-site transmitted power be Pt2 and the corresponding minimum acceptable received signal power be Pr2 and the cell radius be r2 . Then,

Pr2 ∝ Pt2

r2−4 (7.3)

Step 3. It is given that the minimum acceptable received signal power remains unchanged even after increasing the transmitted power, that is,

Pr1 = Pr2

Step 4. Therefore, equating Eq. (7.2) and Eq. (7.3),

Pt1 r1

−4 = Pt2 r2

−4

Or, Pt1 r2

4 = Pt2 r1

4

Or, r24 = (Pt2

/ Pt1) r1

4

Or, r2 = (Pt2 / Pt1

) 1/4 r1 (7.4)

Step 5. It is given that the cell-site transmitted power increases by 3 dB, which means it is doubled. That is, Pt2

= 2 Pt1

Or, (Pt2 / Pt1

) = 2

Step 6. Substituting it in Eq. (7.4), we get r2 = (2 )

1/4 r1

Or, r2 = 1.19 r1 (7.5)

Step 7. Let the initial coverage area be A1 and the new coverage area be A2 . As we know that the coverage area (circular or hexagonal) is directly proportional to square of the radius,

A2 = (r2 / r1)2 A1

Using Eq. (7.5), A2 = (1.19)2 A1

Hence, A2 = 1.4 A1 (7.6)

Comments on the result It is proved that when the transmitted power is increased by 3 dB (or doubled), the coverage area increases by 1.4 times for the same minimum acceptable received signal power and all other factors remaining unchanged.

In a flat operating terrain, doubling the cell-site antenna height results into an increase in gain by 6 dB (or four times), that is, the 6-dB/octave rule is applicable. In a hilly terrain contour, the increase in gain may be more or less than 6 dB for doubling the cell-site antenna height, depending on the location of the mobile unit. In such situations, an effective antenna height should be used to compute the increase in coverage area.


Cellular System Design Trade-offs 185

Using a high-gain omnidirectional antenna or a direc-tional antenna at the cell-site, the similar increase in cov-erage area would be obtained as seen in case of increasing transmitted power. Similarly, use of multiple directional antenna patterns offered by adaptive antenna arrays and smart antennas at the cell-site can also result into increase in coverage area.

A diversity receiver is very useful in reducing the mul-tipath fading, and thereby increase in received signal level at the mobile subscriber unit. For the same minimum acceptable received signal level, effective radio coverage area is increased. In the same way, use of a low-noise receiver can result into increase in radio coverage area because it can receive the minimum acceptable received signal at the farther distance. When the minimum acceptable threshold signal level is reduced at the receiver, the radius of the cell increases, thereby increasing the radio coverage area. The extent of increase in coverage area for a specified reduction in the threshold received signal level is illustrated in Example 7.2.

Example 7.2 Increase in coverage by reducing threshold received level

Let the minimum acceptable threshold signal level be reduced at the receiver by 6 dB. For the same cell-site transmitted power and all other factors remaining unchanged, prove that the coverage area is doubled. Assume mobile radio operating environment conditions.

Solution

Step 1. Let the initial cell-site transmitted power be Pt1 and the minimum acceptable received signal power be Pr1 at a

cell radius of r1. Then, Pr1

∝ Pt1 r1

−4

Or, Pt1 ∝ Pr1

r14 (7.7)

Step 2. Let the changed cell-site transmitted power be Pt2 and the corresponding minimum acceptable received signal power be Pr2 and the cell radius be r2. Then,

Pr2 ∝ Pt2

r2−4

Or, Pt2 ∝ Pr2

r24 (7.8)

Step 3. It is specified that the cell-site transmitted power remains unchanged though the minimum acceptable received signal power is reduced, that is,

Pt1 = Pt2

Step 4. Therefore, equating Eq. (7.7) and Eq. (7.8),

Pr1 r1

4 = Pr2 r2

4

Or, r24 = (Pr1

/ Pr2 ) r1

4

Or, r2 = (Pr1 / Pr2

) 1/ 4 r1 (7.9)

Step 5. It is given that the minimum acceptable received signal power is reduced by 6 dB that means it is reduced by four times (every 3-dB reduction corresponds to half of the original signal power). That is, Pr2

= (1/4) Pr1

Or, Pr1 = 4 Pr2

Or, (Pr1 / Pr2

) = 4

Step 6. Substituting it in Eq. (7.9), we get r2 = (4 )

1/ 4 r1

Facts to Know !

With a given transmitted power and actual antenna height at the cell-site, coverage area can be increased by proper selection of cell-site location.

In principle, a high location around the planned site location should always be selected.



Or, r2 = 1.414 r1

Or, r2 / r1 = 1.414 (7.10)

Step 7. Let the initial coverage area be A1 and the new coverage area be A2. As we know that the coverage area (circular or hexagonal) is directly proportional to square of the radius, A2 = (r2 / r1)

2 A1

Step 8. Using Eq. (7.10), we get A2 = (1.414)2 A1 Hence, A2 = 2 A1

Comments on the results Hence it is proved that when the minimum acceptable received signal power is reduced by 6 dB (or four times), the cell coverage area is doubled for the same cell-site transmitted power and all other factors remaining unchanged.

Wireless repeaters or signal enhancers are often used to extend the coverage area. Repeaters are usually bi-directional in nature, and simultaneously send signals to and receive signals from a serving cell-site. Wireless repeaters may be installed anywhere and are capable of repeating the complete allocated cellular frequency band. Upon receiving signals from a cell-site forward link, the repeater amplifies and reradiates the cell-site signals to the specific extended coverage area.

7.2 COVERAGE HOLE FILLERS AND LEAKY FEEDERS

Coverage hole is an area within the radio coverage footprint of a wireless communication system in which the received RF signal level is below the specified threshold value. Coverage holes, also called weak received signal spots, are usually caused by physical obstructions such as buildings, hills, dense foliage as well as hard-to-reach areas such as within buildings (indoor), or in valleys or in tunnels. Because the earth is not flat, many coverage holes are created during transmission of radio signals.

The radio coverage is sometimes blocked in the outdoor wireless network applications which contain high buildings, hills and tunnels, and thus shadow regions are created. It is most desirable to fill in these cover-age holes. The received signals are required to be extended by simple means into these coverage hole areas. Deploying a new cell-site in this area could be one possible solution but it is not only expensive, but also requires new channel assignment or rearranging the frequency plan. So it may not be economically justifiable from the revenue point of view from that area which may still be strategically important.

Among the various techniques available for filling the coverage holes such as wireless enhancers or repeaters (wideband and channelised), passive reflectors, diversity receivers, and cophase combiners (feedforward and feed-back), wireless repeaters can provide the simplest and cost-effective solution to reradiate the amplified signal so as to reach the coverage hole areas. Unfortunately, the received noise and interference is also reradiated by the repeater on both the forward and reverse link. Therefore, care must be taken to properly place the repeaters, and to adjust the vari-ous forward and reverse-link amplifier levels and antenna patterns. In practice, particularly in tunnels or high-rise buildings, directional antennas or distributed antenna sys-tems are connected to the inputs or outputs of repeaters for localised weak-spot coverage.

Facts to Know !

In the deployment of any wire-less communication network, the radio coverage provided by any given cell-site does not cover

100% locations within the cell boundary, deter-mined by the minimum acceptable received signal levels.

Facts to Know !

The repeater does not add capacity to the system. It simply serves to reradiate the received signal into specific coverage-hole locations.



By modifying the coverage of a serving cell by using wireless repeaters, a service provider is able to dedicate a certain amount of the cell-site’s traffic for the areas covered by the repeater. The two physical considerations for a successful repeater deployment are isolation and line-of-sight conditions. There must be sufficient isolation between the donor antenna and the coverage antenna to prevent feedback oscillation for on-frequency repeaters where the transmitted frequency is the same as the received frequency. In order to meet this requirement, the two antennas — the donor antenna and the coverage antenna — must be physically separated from each other. This physical separation can be realised if

• there is a tower of sufficient height to separate the antennas vertically while pointed in opposite direc-tions—one towards the actual cell-site transmitter and another towards the coverage hole region,

• the antennas are mounted on separate masts with sufficient horizontal separation and pointed in opposite directions as stated above, or

• there is a physical structure such as a building that can provide the needed physical isolation when the antennas can be mounted on opposite sides of the building.

At the repeater location, there must be line-of-sight condition to both the coverage hole area and the desired donor cell-site. It is desirable that the downlink signal from the repeater must be strong enough to be received by the mobile subscribers located in the coverage hole region while the receiver sensitivity of repeater plus that of the cell-site must be sufficient enough to process the signal received from the mobile subscribers.

Figure 7.1 illustrates deployment of the repeater with two different directional antennas mounted on the same tower, one in the direction of the main cell-site and another in the direction of a mobile operating in a coverage hole region.

One of the main concerns is that there is overlap between the repeated signal and the primary cell-site signal. The repeated signal has significant added delay. In some cases, it is important to limit the coverage antenna pattern to the area needed and minimise the overlap. If the undesired signal received by the donor directional antenna is transmitted back to the cell-site, cochannel and adjacent-channel interference may occur. For shadow areas such as behind the hills or inside tunnels, this is generally not a problem. Otherwise, an alternative way is to use a high-gain donor antenna to bring in the signal from a neighbour cell. In this case, the mobile subscribers located in coverage-hole regions would hand-off to the neighbour cell in the repeater cover-age area and there would be no interference.

Instead of using a wideband enhancer, a channelised enhancer can be effectively deployed which would amplify only the selected channels. Geographic terrain contour should be considered in proper installation of a channelised enhancer. The separation between two antennas at the enhancer is very critical. If this separa-tion is inadequate, the signal from the coverage antenna can be received by the donor antenna or vice versa. This may result in jamming of the system instead of filling the coverage hole. Likewise, the distance between the enhancer location and the serving cell-site should be as minimum as possible to avoid spread of radiations into a large area in the vicinity of the serving cell-site and beyond.

Leaky Feeders In some areas such as in tunnels, underground garages, coal mines or within a cell of less than 1-km radius, leaky-feeder techniques provide adequate coverage with minimum interference. The most popular types of leaky feeders are

• Leaky waveguide or fast-wave antenna• Leaky coaxial feeder cable

Wirelessrepeater

Towardsmain cell

site

Donor directionalantenna

Mobile located incoverage-hole

region

Coverage holedirectionalantenna

Fig. 7.1 Illustration for deployment of wireless repeater



In leaky waveguides, only the fractional energy will be leaking constantly through the opening slots (apertures) of the waveguide structure supporting λg > λc, where λg is the wavelength of the transmitted waveguide and λc is the wavelength of the transmitted signal in free space. The leakage rate is a function of position of the slot in the waveguide. The radiation pattern of a leaky waveguide can serve a larger area along the waveguide. These are generally used above 3 GHz.

Leaky cables are easily implemented in the tunnels because their energy is confined within the tunnel. Leaky coaxial feeder cables are used at frequencies below 1 GHz because the cable loss is 6 dB/100 m only.

The RF powers cannot exceed a maximum of 500 mW in order to prevent any inflammatory sparks for safety consid-erations. The leaky feeder is characterised by transmission and coupling losses. For proper operation of a leaky coaxial feeder cable, use of high-coupling loss cables near the trans-mitter is recommended as little energy will leak out and thus have low-transmission loss.

7.3 SYSTEM PARAMETERS TO REDUCE INTERFERENCE

In a wireless communication environment, no matter whether the signal is transmitted outdoors or indoors, the phenomenon of multipath interference is unavoidable. In a cellular system network, the frequency reuse is exploited extensively in order to increase system capacity by efficient use of allocated RF spectrum, thereby causing significant system interference. Even though interfering signals are often generated within the cellular system, they are difficult to control in practice due to random propagation effects. Moreover, the transmitters from

competing cellular service operators are often a significant source of out-of-band interference that is even more difficult to control, since other cellular service operators often locate their cell-sites in close proximity to one another in order to provide comparable cell coverage to mobile subscribers.

A mobile subscriber unit is capable of receiving a down-link signal from each of a number of cell-sites as well as

transmitting an uplink signal to a number of cell sites through a wireless channel. The combined effect of multipath interference and cochannel interference becomes a challenging task to maintain the desired perfor-mance of cellular systems. Thus, interference is the major limiting factor in achieving the desired performance of cellular radio systems.

Sources of interference may include another mobile active in the same cell, a call in progress in an adja-cent cell, other cell-sites operating in the same frequency band, or any non-cellular wireless communication system, which inadvertently leaks energy into the cellular frequency band. Interference on voice channels causes cross talk, and interference on control channels leads to missed and blocked calls due to errors in the signaling. Interference is more severe in urban areas, due to the greater RF noise floor and availability of a large number of active cell-sites and mobiles. Interference has been recognised as a major bottleneck in increasing system capacity and is often responsible for dropped calls.

In most situations, the methods adopted for increasing the coverage area also causes additional interference if cochannel and adjacent channels are used in the system. So there is always a trade off between the meth-ods used for increasing coverage area and reducing interference so as to ensure the overall system quality.Therefore, it is desirable to reduce interference in the cellular network. The following approaches are used in a cellular system to reduce the interference.

– A good frequency-management plan – An intelligent real-time channel assignment scheme

Facts to Know !

Low temperature and snow accu-mulation around slots causes an increase in transmission loss. Periodic spacing of slots along the

leaky coaxial feeder cable causes the intensive radiation pointing to a specific direction.

Facts to Know !

The two major types of system-generated cellular interference are cochannel interference and adjacent-channel interference



– Assignment of a proper voice channel to a particular mobile unit – Design of an antenna pattern on the basis of a desired direction – Tilting antenna patterns by electronic or mechanical downtilting techniques – Reducing the antenna height – Choosing the cell-site location properly – Reducing the transmitted power at the cell-site

Frequency management, or radio resource management, is the system-level control of cochannel interference and other radio-transmission characteristics in wireless communication systems. Frequency management involves strategies and algorithms for controlling system parameters such as radiated power, channel assign-ment, handover criteria, modulation scheme, error-coding scheme, etc. The objective of a good frequency management scheme should be to utilise the limited RF spectrum resources and wireless network infrastruc-ture as efficiently as possible without exceeding the accept-able limits of interference.

Intelligent frequency planning is needed in order to avoid problems of cochannel and adjacent-channel interference in a cellular system. The channels may be assigned dynamically in such a way so as not to cause any interference to on-going communications. Depending on the active channels at any time, some free channels may be noisy, some free channels may be quiet, and some free channels may be vulnerable to channel interference. These factors should be considered in the assigment of voice channels to a particular mobile subscriber unit among a set of available channels.

The design of an antenna pattern—omnidirectional or directional—including downward tilting will enable to confine the radiated signal energy within a small area, thereby reducing interference. Proper selection of cell-site location as well as optimum antenna height depending upon the terrain conditions along with reducing transmit-ted power can be more effective in reducing interference while trading off with the coverage area. In practical cellular communication systems, the power levels transmitted by every mobile subscriber unit are under constant control by the serving cell-site. This is done to ensure that each mobile subscriber transmits the minimum power necessary to maintain a good quality link on the reverse channel as well as to reduce the interference.

7.4 METHODS TO INCREASE TRAFFIC CAPACITY

Traffic capacity, or user capacity, can be simply defined as the maximum number of simultaneous users that can be supported by a system for a given performance requirement. The capacity of a cellular system is directly proportional to the number of times a cluster is replicated in a designated service area. As the demand for cellular service increases, the number of channels assigned to a cell eventually becomes insufficient to serve the required number of subscribers. At this point, various cellular-design methods are needed to provide more number of channels per unit coverage area to expand the capacity of cellular systems. The following methods may be considered to increase the traffic capacity of a cellular system.

– Enhanced frequency spectrum for new subscribers – Proper channel-assignment strategies – Mutli-access and modulation schemes – Use of smart antennas – Cellular hierarchy

Obtaining enhanced frequency spectrum for new subscribers is a very simple but too expensive method to increase traffic capacity. For example, an additional spectrum allocation of 10 MHz means an increase of 166 voice channels in US AMPS system.

Facts to Know !

The type and installation of a cell-site antenna system plays a critical role in determining the impact of system interference. Depending

on the traffic demand, a strong signal may be needed in some directions and no signal may be needed in some other directions.



Changing the channel-assignment strategies can result in increase in capacity of a cellular network. In practice, the distribution of the subscribers in the area is not uniform over all cells at all time. So, instead of distributing available channels equally among all cells, it is possible to use a nonuniform distribution of chan-nels among different cells according to their respective traffic requirements. The user capacity of each cell is dynamically changing by the geography of the service area and with time. The dynamic channel assignment to different cells also reduces the probability of call blockage which results in accepting a higher number of mobile subscribers over the coverage area. This can be considered equivalent to expansion of the network capacity

through availabilty of additional channels. Another effective method to increase the

capacity is to change the mutli-access and modu-lation scheme. Use of digital cellular technology such as TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access) air interface with digital modulation schemes results into significant increase in system capacity as compared to analog cellular technology using FDMA (Frequency Division Multiple Access) with FM modulation. However, this changeover requires the installation of new infrastructure by service providers as well as use of a new mobile terminal by the subscribers.

Another method to increase the capacity of the cellular network is changing the cellular architecture and hierarchy such as cell sectoring using directional antennas, cell splitting (use of small cell size), microcell zone techniques, and using multiple reuse factors called reuse partitioning. These techniques change the size and shape of the coverage of the cells by adding new cell-sites or modifying the type and installation of cell-site antennas to increase the overall capacity.

7.5 CELL SPLITTING

The prime objective of implementing a cellular mobile system is to enhance the system capacity along with improvement in the spectrum efficiency. Frequency reuse and cell splitting are two main concepts in cellular sys-tems required to achieve this objective. Cell splitting is the process of dividing a larger congested cell into smaller cells, each with its own cell-site with a corresponding reduction in transmitter power and antenna height. This is usually done to make more voice channels available to accommodate traffic growth in the area covered by the original cell. When the traffic in an area increases, larger cells are split into smaller cells so that frequency can be reused more frequently. By defining new smaller cells which have a smaller radius than the original larger cells and by installing these smaller cells (called microcells) between the existing cells, the system user capacity increases due to availability of additional number of channels per unit service area as well as the number of times that frequency channels are reused. In other words, the increased number of cells would increase the number of channels as well as number of clusters over the coverage area, which in turn would increase the overall system capacity.

It has been seen earlier how the concept of frequency reuse leads to a cellular architecture that can allow for almost limitless expansion in the geographic area and the number of mobile subscribers that the system can serve. In configuring a cellular layout, there are two most important key parameters—the cell radius R and the cluster size K. Although a cellular system can be expanded in a geographic coverage area simply by adding cells at the periphery, yet it is worth considering as to how a system can expand in meeting the requirement of increasing subscriber density.

The cell radius governs both the geographic radio coverage area served by a cell-site located at its centre and also, for a given subscriber density, the number of subscribers that the cell must service. Simple economic

Facts to Know !

By using smart antennas at cell-sites, mobile subscribers in the same cells can use the same physical communication channel as long as they are not located in the same

angular region with respect to a cell-site. Such a multi-access scheme, referred to as space division multiple access, can be achieved by the cell-site directing a narrow beam toward a mobile subscriber unit communicating with it. The interference between cochannel cells is also greatly reduced because the antenna radiation patterns are extremely narrow.



considerations suggest that the cell size should be as large as possible. Since installation and commis-sioning of every cell requires an investment in a antenna tower, land on which the tower is placed, and radio transmission equipment, a large cell size minimises the average infrastructure cost per subscriber. The cell size is ultimately determined by the received signal quality requirement that an acceptable signal-to-noise ratio be maintained over the coverage area. A number of system parameters such as transmitter power, cell-site antenna height, receiver noise figure, and receiver sensitivity are involved in determining the signal-to-noise ratio. Transmitter power is particularly limited in the reverse direction, as the mobile subscriber units have low transmitted power, are battery powered, and small in size.

Cell splitting allows an orderly growth of the cellular system as per the requirement of varying traffic density. Cell splitting increases the number of cell sites in order to increase the system capacity. Thus, cell splitting allows a system to grow by replacing large cells with smaller cells, with changing the frequency reuse plan or channel-allocation scheme required to maintain the minimum cochannel reuse ratio between cochannel cells.

7.5.1 Techniques of Cell Splitting

There are two techniques of cell splitting — permanent cell splitting and dynamic cell splitting. In permanent cell splitting, the installation of every new split cell has to be planned in advance. The transmitter power, the number of channels per cell, the assigned frequencies, selection of the cell-site, and the traffic load should all be considered. The channel assignment scheme should follow the rule based on frequency-reuse ratio with the transmitter power adjusted accordingly. Selecting a proper site location for a small cell is not easy. The cell-site antenna can be mounted on a monopole or erected by a mastless antenna arrangement. When ready with the plan of split cells, the actual changeover of service with split cells should take place during the lowest traffic period, usually at a midnight or at a weekend, to avoid no-service conditions or a large number of call drops.

The dynamic cell-splitting scheme is based on utilising the allocated spectrum efficiency in real time. The algorithm for dynamically splitting the cells is a tedious job and it should be implemented gradually to pre-vent dropped calls at heavy traffic hours. The size of the splitting cells is also dependent on the radio aspect and the capacity of the switching processor. The radio aspects include how well the coverage pattern can be controlled and how accurately mobile locations would be known. A high capacity of the switching processor is needed to handle more hand-offs due to smaller cells.

7.5.2 Effects of Cell Splitting

Cell splitting involves the changes in the following aspects of cellular architecture.

– Reduction in the coverage area of a split cell – Reducion in the cell-site transmitter power of a split cell – Increase in traffic load after cell splitting – Changing frequency reuse plan – Changing the channel assignment

When the traffic density starts to build up and the number of assigned channels in each cell cannot provide enough mobile calls simultaneously, the original larger cell can be split into smaller cells. Usually, the radius of a new split cell is one-half of the original larger cell, that is,

New cell radius = (original cell radius) / 2

Facts to Know !

Cell splitting is an attractive feature of the cellular concept. When the call traffic in a particular cell could no longer support a rea-sonable grade of service with the existing

allocation of channels in that cell, that cell would be subdivided into a number of smaller cells — with lower transmitter power and new (on a smaller scale) frequency reuse pattern — fitting within the area of the former cell.



If the radius of the original cell is designated as R0 and the radius of the split cell is designated as R1 then

R1 = R0 / 2

There are two ways of cell splitting:

• The one in which the original cell-site is not used and is completed replaced with new split cells (Fig. 7.2 depicts this situation with circular cell areas).

• The second way of cell splitting is in which the original cell is also used along with new cells, as shown in Fig. 7.3.

Originalcell

Splitcells

R0

R1

R1 = R0/2

Fig. 7.3 Cell splitting using original cell

Original cell

Split cells

R0

R1

R1 = R0/2

Fig. 7.2 Cell splitting without using original cell

EXAMPLE 7.3 Coverage area after cell splitting

Let the radius of split cells be 50% of the radius of the original cell (before splitting). Show that the coverage area of a split cell is one-fourth the coverage area of the original cell. Comment on the results obtained.

Solution

Step 1. Let the radius of the original cell = R0

The reduction in the radius of split cell is 50% of the radius of the original cell.Then, the radius of the split cell, R1 = R0/2

Step 2. Assuming the regular hexagonal cells, Area of a regular hexagonal original cell, A0 = (3√3 / 2) × R0

2

Step 3. Area of a regular hexagonal split cell, A1 = (3√3 / 2) × (R1)2

Substituting R1 = R0/2, A1 = (3√3 / 2) × (R0/2)2

Or, A1 = [(3√3 / 2) × R02 ] / 4

Hence, Area of a split cell, A1 = A0 / 4

Comment on the results It is seen that the coverage area of the split smaller cell is one-fourth of the cover-age area of the larger original cell. It means that in order to cover the original service area (being served by original larger cell with radius R0) with split cells (having radius R0/2), four times as many split cells would be required. The increased number of cells would increase the number of clusters over the coverage region, which in turn would increase the number of channels, and thus capacity, in the coverage area. However, the increase in capacity due to split cells is achieved at the cost of installing additional infrastructure.



EXAMPLE 7.4 Cell-site transmitter power after cell splitting

For an identical received power at the boundaries of original larger cell with radius R0 and the new split cell with radius R0 /2, prove that the cell-site transmitter power of the split cell must be 12 dB less than the cell-site transmitter power of the original larger cell. Assume path-loss exponent as 4 in a typical mobile environment. Comment on the results obtained.

SolutionFor the split cells of smaller size, the transmit power of these cells must be reduced for an identical received power at the boundaries of the cells. The transmit power of the new split cells with radius half that of the original larger cells can be computed by examining the received signal power, Pr at the cell boundaries of the original and split cells respectively, and then setting them equal to each other. This is necessary to ensure that the frequency reuse plan for the new split cells behaves exactly as for the original cells in order to maintain the cochannel interference levels.

Step 1. Let Pt0 be the cell-site transmitter power of the larger original cell with radius R0 ,

Then, at the cell boundary of the original cell, Pr ∝ Pt0

R0−4 (7.11)

Step 2. Let Pt1 be the cell-site transmitter power of the smaller split cell with radius R0 /2,Then, at the cell boundary of the split cell, Pr ∝ Pt1

(R0/2)−4 (7.12)

Step 3. Equating Eq. (7.11) and Eq. (7.12), we get Pt1

(R0/2)−4 = Pt0 R0

−4

Or, Pt1 (1 / 2−4 ) = Pt0

Or, Pt1 = Pt0

/16

Step 4. Expressing it in logarithmic form, we get

Pt1 (dBm) = Pt0

(dBm) – 10 log (16)

Or, Pt1 (dBm) = Pt0

(dBm) – 12 (dB) (7.13)

Comment on the results It means that the cell-site transmit power of a split cell with radius R0/2 must be reduced by 12 dB than the cell-site transmitter power of original cell with radius R0 in order to fill in the original coverage area with split cells, while keeping the signal-to-interference ratio requirement same.

In case an original larger cell is split repeatedly ‘n’ number of times, and every time the radius of the split cell is one-half of its immediate previous cell, that is,

the radius of nth split cell, Rn = R0 /n

where R0 is the radius of the original cell, and Rn is the radius of the nth split cellThen, cell-site transmitter power of the nth split cell can be given by

Ptn (dBm) = Pt0 (dBm) – 12 (n) dB (7.14)

EXAMPLE 7.5 Cell-site transmitter power after multiple cell splitting

For an identical received power at the boundaries of the original larger macrocell with radius R0 and the new split minicell with radius R0 /2 and further split microcell with radius R0 /4, prove that the cell-site transmitter power of the split microcell must be 24 dB less than the cell-site transmitter power of the original larger macrocell. Assume path loss exponent as 4 in a typical mobile environment.

SolutionRadius of the original larger macrocell = R0 (given)

Radius of the new split mini cell = R0 / 2 (given)Radius of the split microcell = R0 / 4 (given)



Step 1. Since the cell splitting is carried out from original radius R0 to R0 /2 and then once again to R0 /4, the cell split-ting is carried out two times, which means n = 2.

Step 2. Figure 7.4 illustrates the cell splitting from radius R0 to R0 /2 and then to R0 /4.

Step 3. The reduction in transmitted power of small-est microcell after two times splitting is given by the Eq. (7.14) as Pt2

(dBm) = Pt0 (dBm) – 12 × 2 dB

Or, Pt2 (dBm) = Pt0

(dBm) – 24 dB

Hence, the reduction in transmitted power of the split cell should be by 24 dB.When cell splitting occurs, the value of the fre-quency-reuse factor or cochannel interference reduc-tion factor q = D/R is always held constant since both D and R are split by the same factor.

EXAMPLE 7.6 Increase in system capacity after cell splitting

The radius of the split cell is one-half of that of the original cell (before splitting). Show that the overall system capacity increases by four times.

SolutionRadius of the original larger macrocell = R0 (given) Radius of the new split minicell = R0 / 2 (given)

Step 1. The area of a cell is proportional to the square of the radius of the cell.

Step 2. Therefore, when the original cell with radius R0 is split into smaller cells, each with radius R0 /2, the coverage area of the split cell is one-fourth of the coverage area of the original cell.

Step 3. It means that in order to cover the original service area with these split cells, four times as many split cells would be required, while maintaining the same reuse factor or cochannel interference ratio as well as the minimum acceptable received signal level at the boundary of the split cell as that of the original cells.

Step 4. Assuming that the number of channels assigned per cell remains same in split cells as that of original cell, the frequency reuse planning is redone.

Step 5. This will result into proportionate increase in the number of clusters with split cells to cover the entire region as designated with the original cells.

Step 6. Thus, the total number of available channels with split-cell configuration increases by four times.

Step 7. Therefore, the overall system capacity also increases by four times in the coverage area. Hence, new system capacity = 4 × (system capacity with original cells)

If each split cell can again be split into four sub-split cells with further radius reduction by one-half then the traffic load would increase by 16 times. As the cell splitting continues every time with reduction in radius of a split cell being one-half of that of its immediate previous cell, the general formula for increase in overall system capacity with cell splitting can be expressed as

New system capacity = 4n × (system capacity of original cell) (7.15)where ‘n’ is the number of times of uniform cell splitting.

Small cells

Medium cells

Large cellsR0/4

R0

R0/2

Fig. 7.4 Repetitive cell splitting



For example, if n = 4, it means an original cell has been split four times. The new capacity will be New system capacity = 44 × (system capacity of original cell)

Or, New system capacity = 256 × (system capacity of original cell)

EXAMPLE 7.7 Channel capacity after cell splitting

Determine (a) The channel capacity for a cellular system service area comprised of seven macrocells with 16 channels per macrocell(b) Channel capacity if each macrocell is split into four minicells(c) Channel capacity if each minicell is further split into four microcells

Solution(a) To determine the channel capacity for macrocell configurationNumber of macrocells per system = 7 (given)Number of channels per macrocell = 16 (given)Number of channels per macrocell system = 7 × 16Hence, the channel capacity of macrocell system = 112 channels

(b) To determine the channel capacity for minicell configurationNumber of macrocells per system = 7 (given)Number of channels per macrocell = 16 (given)Number of minicells per macrocell = 4 (given)Number of channels per minicell system = 7 × 16 × 4Hence, the channel capacity of minicell system = 448 channelsAlternately, New channel capacity = 4 × (channel capacity with original cells)Hence, New channel capacity = 4 × 112 channels = 448 channels

(c) To determine the channel capacity for microcell configurationNumber of macrocells per system = 7 (given)Number of channels per macrocell = 16 (given)Number of minicells per macrocell = 4 (given)Number of microcells per minicell = 4 (given)Number of channels per minicell system = 7 × 16 × 4 × 4Hence, the channel capacity of minicell system = 1792 channelsAlternately, New channel capacity = 4n × (channel capacity of original cell) where ‘n’ is the number of times of uniform cell splitting. Here a macrocell is split into 4 minicells and each minicell is further split into 4 microcells. Therefore, n = 2 New channel capacity = 42 × (channel capacity of original cell)Hence, New channel capacity = 16 × 112 channels = 1792 channels

Example 7.8 Increase in system capacity with cell splitting

Consider the cellular system shown in Fig. 7.5, in which the original cells with a radius of 2 km are split into smaller cells, each with a radius of 1 km.Let each cell-site be assigned 120 channels regardless of the cell size. How many times will the number of channels contained in a 6 × 6 km2 area (shown with dotted lines in Fig. 7.5) centered around small cell ‘S’ be increased with cell splitting as compared to without cell splitting?



SolutionThe number of channels assigned in a large cell = 120 (given)

Step 1. Since the number of channels assigned in a split cell is the same as that in a large cell, therefore,The number of channels assigned in a split cell = 120

Step 2. Coverage area centered around the small cell ‘S’ = 6 × 6 km2 (given)From the figure, it is seen that to cover the given area, it is needed to cover 3 km each to the left, right, top and bottom of the cell-site.

Step 3. As evident from the given figure, it can be easily observed that the 6 × 6 km2 centered around the small cell ‘S’ contains more number of split cells than that of large cells.

Step 4. However, because of difference in edges of a square area and a regular hexagonal area, the number of either types of cells contained within the given square area cannot be accurate, it can only be an estimate. A reasonable approxi-mation from visual observation is that there are nearly 4 large cells within this area. Number of large cells within the given area = 4Number of channels per large cell = 120Number of channels without cell splitting = 4 × 120 = 480 channels

Step 5. Radius of the large cell, R0 = 2 km (given)Radius of the split cell, R1 = 1 km (given)

Number of split cells within the square = (R0 / R1)2 × number of large cells

Or, Number of split cells within the square = (2 / 1)2 × 4 = 16 small cells

Step 6. With a finite square area under consideration, it is necessary to take care of edge effect. So the number of small cells contained within the given square area would be slightly less than 16. A reasonable estimate would be 15 small cells.

Step 7. Thus, Number of split cells within the given area = 15 Number of channels per split cell = 120 Number of channels with cell splitting = 15 × 120 = 1800 channels

Step 8 Hence, increase in number of channels = 1800 / 480 = 3.75 times

Comments on the results Therefore, with an estimate of 15 split cells in the given square area, the number of channels contained in that area, with cell splitting, is 3.75 times more than the number of channels without cell splitting. This is quite close to the theoretical limit of 4 times increase in system capacity as given by Eq. (7.15) by putting n = 1. It is obvious that cell splitting increases the number of channels within the same coverage area, and thereby the overall system capacity.

Antenna downtilting, which deliberately focuses radiated energy from the cell-site towards the earth (rather than towards the horizon), is often used to limit the radio coverage of newly formed microcells.

7.5.3 Cell Sizes in Cellular Architecture

In practice, all cells in a cellular system are not split at the same time. Due to difficulties in installation of split cells at certain locations, different cell sizes will exist simultaneously. In such situations, special care in the system design needs to be taken to keep the distance between cochannel cells at the minimum required value. Hence, channel assignments become more complicated. To the same extent, large and small cells

SR1 = R0/2

R0

Fig. 7.5 An example of cell splitting



can be isolated by selecting a group of frequencies that will be used only in the cells located between the large cells on one side and the small cells on the other side. This is needed in order to eliminate the interfer-ence being transmitted from the large cells to the small cells. Hand-off issues must also be addressed so that high-speed and low-speed mobile subscribers can be simultaneously accommodated.

Figure 7.6 shows a typical cell distribution in a cel-lular network, in which the service area includes low-traffic (rural), medium-traffic (suburban), high-traffic (town or highways areas), thereby requiring a mix of large-as well as small-sized cells.

There are a number of cell sizes which are deployed in a cellular network to provide comprehensive area coverage. This is needed to support traffic variations in different geographical areas having various applications. The cells are distributed to form a hierarchy with different cell sizes such as femtocells (smallest cell used for interconnecting personal wireless devices such as cellphones, laptops, notepads, within a few metres), picocells (small cells inside a building to support WLANs within tens of metres), microcells (used in urban areas to support PCS within hundreds of metres), macrocells (to cover metropolitan areas of several kilometres), and megacells (used with satellites to cover nationwide areas within hundreds of kilemetres).

An ideal cellular network has a mixed hierarchy of different sizes of cells to achieve a comprehensive coverage of a variety of applications. For example, femtocells are used to connect personal wireless devices, picocells for indoor users, microcells for pedestrians in the streets or office/residential buildings in dense urban areas, macrocells for vehicle drivers in suburban/rural areas, and megacells to cover users in airplanes.

There are certain advantages of using different sizes of cells in a hierarchical cellular architecture such as

– to extend the coverage of the areas that are difficult to cover by a large cell – to increase the capacity of the cellular network for those areas that have a higher density of subscribers – to provide services to increased number of wireless devices and the need for communication between

these devices, that is, connecting laptops with cellphones

Microcells cover small distances, usually less than 100 metres, using cell-sites with antennas mounted on street light posts or on the sides of buildings. The reason for this is that the cells designed to cover suburban areas have antennas on tall towers or rooftops of high buildings to cover a large area. However, signals from these antennas cannot propagate into urban areas or indoor environments. So there is a need to install low-power base stations with their antennas mounted on the walls to cover a smaller area, resulting in the creation of a smaller sized cell. The limitation of a small cell is based on the accuracy of cell-site locations and control of the radiation patterns of cell-site antennas. In microcell systems, a practical hand-off problem called cell dragging results from pedestrian users which provide a very strong signal to the cell-site. To resolve this issue, hand-off threshold levels and radio-coverage parameters must be adjusted carefully.

7.6 REVIEW OF MODULATION TECHNIQUES

Modulation is the process of encoding information signals from a message source in a manner suitable for wireless transmission. It involves translating a baseband information signal to a bandpass signal at radio carrier frequencies that are very high compared to the baseband frequency. The baseband signal is called the

Rural Town

Fig. 7.6 Cell distribution in a cellular network


Cellular System Design Trade Offs

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