Unit-2 Basic Cellular Concept

Que 1. Explain in brief frequency reuse concept in wireless communication.

1. Frequency Reuse is the scheme in which allocation and reuse of channels throughout a
   coverage region is done.
2. Each cellular base station is allocated a group of radio channels or Frequency sub-bands to be used within a small geographic area known as a cell.
3. The process of selecting and allocating the frequency sub-bands for all of the cellular base station within a system is called Frequency reuse or Frequency Planning. The same set of frequency is reused after a specific distance to ensure increase in capacity and coverage.

4. In Figure 1, all cells marked as ‘A’ will be allotted the same group of channels. i.e. cells which have been given the same letter in the diagram have the same group of channels.
5. Cells which have been allotted the same group of frequency channels are called Cochannel cells. Cell A-Cell G have unique channels and there are no repetitions. Group of cells in which every channel is unique is called as a Cluster.
6. To find the total number of channel allocated to a cell: S = Total number of duplex channels available to use, k = Channels allocated to each cell (k<S), N = Total number of cells or Cluster Size. Then Total number of channels (S) will be, S = kN
7. The number of cells after which a frequency channel can be reused is called as the Frequency reuse factor (R.F.), and it is given as Frequency Reuse Factor = 1/N. In the above diagram cluster size is 7 (A,B,C,D,E,F,G) thus frequency reuse factor is 1/7.
8. If a Cluster is replicated or repeated M times within the cellular system, then Capacity, C will be, C = MkN = MS
9. Since co-channel cells use the same set of channels, there is always possibility of interference in these cells. Interference between the co-channel cells is called as Cochannel interference.
10. There should be a minimum Distance after which the same channel can be reused with minimum interference. This distance is called as Minimum safe distance and is given by,

If D is the minimum safe distance and R is the radius of each cell, then the ratio of D/R is termed
as Reuse factor Q and is given by

The Reuse Factor Q has a very important significance in deciding the capacity improvement
techniques.

Que 2. Define handoff. Explain basic process of handoff with neat diagram.

1. When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. This process of transferring Mobile Station’s call from one frequency channel or Base Station
to another is called as Handoff/Handover.

2. Handoff parameters

The following basic parameters are needed to determine whether a handoff is required or not.

1. Signal strength of the BS with which communication is being made.
2. Signal strengths of the surrounding BSs.
3. Availability of channels.

The handoff parameters are measured in the following way:

1. Signal strengths of BSs are measured by the mobile devices.
2. Channel availability status is known at the cellular network.
3. Cellular network makes the decision about when the hand over is to take place in
   which channel of which cell.

3. When a mobile user moves in a different cell while a conversation is in progress, the received signal strength from Base Station becomes weak. BS is not able to detect the signal transmitted by the MS and vice versa. Thus, the call may be dropped (discontinued). Therefore, to maintain the continuity of a call, it is necessary to allocate new frequency channel from the cell where the MS is currently located. In cellular
system it is performed by the process of Hand off/ Handover.

4. Handoff should be performed successfully and as infrequently as possible to minimize load on the system and must be unknown to the user. Hence, system designers have to decide on optimum power levels at which Hand-off should be initiated. Also, Hand-off must be complete at the right time in order to minimize call drops. To achieve all this, a complete information of the minimum detectable signal level, handoff threshold is required.

5. Figure 2(a) shows a handoff is not made and the signal drops below the minimum acceptable level required to keep the channel active. This dropped call event can happen when there is a large delay by the MSC in assigning a handoff or when the threshold is set too small for the handoff time in the system. Excessive delays may occur during high traffic conditions due to computational loading at the MSC or due to the fact that no channels are available on any of the nearby Base Stations (thus forcing the MSC to wait until a channel in a nearby cell becomes free).

6. Figure 2(b) shows a proper hand off scenario in which the handoff threshold was correctly defined and the signal did not fall beyond the threshold. This handoff threshold level should not be very high to avoid unnecessary handoff scenarios. It should also not be very low as there will be insufficient time given to complete the process handoff which may cause call drops.

7. Hence the delta value i.e. difference between the minimum detectable signal level and Handoff threshold is usually kept in the range of 0-6 dBs in modern cellular communication systems. Practical value taken by most of the service providers is 2 dBs.

8. The minimum usable signal level for acceptable voice quality is usually seen to be -90 dBm to -100 dBm. If the received signal falls below this level, it cannot be detected by the receiver. A slightly stronger signal level is used as threshold at which the Hand-off is initiated.

Que 3. Explain the type (scenarios) of Handoff/Handover. OR
Explain in brief classification of Handoff process.

1. When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. This procedure is called handoff.
2. Depending on different purpose the handoff is classified are as below

A. Depending on nature of handoffs the classification is as

1. Hard Handoff
2. Soft Handoff

1. Hard Handoff:

• It is also known as “Break before make” connection.
• In this type of handoff the link to the old base station is terminated before the mobile station establishes a link with the new base station.
• It is used in FDMA and TDMA based mobile system.
• Figure shows the mechanism of hard handoff.

2. Soft Handoff:

• It is also called as Mobile Directed handoff or make before Break Connection.
• In these types of handoff the link to the old base station is not terminated before the mobile station established a link with the new base station.
• Once the link is established the connection to old BS is terminated.
• It is used in UMTS to improve the signal quality
• It is more seamless handover.
• Figure shows the mechanisms of soft handoff.

B. Depending on purpose of Handoff classification are as

a. Intra cell handoff
b. Inter cell handoff
c. Inter System Handoff

a. Intra-cell handover

Such a kind of handover is performed to optimize the traffic load in the cell or to improve
quality of a connection by changing carrier frequency.

b. Inter-cell handover

It is also known as Intra-BSC handover.

Here the mobile moves from one cell to another but remains within the same BSC (Base station controller).

Here the BSC handles the handover process

c. Inter System Handoff

i) Inter-BSC handover

• It is also called as Intra-MSC handover.
• As BSC can control only a limited number of cells, we might usually need to transfer a mobile from one BSC to another BSC.
• Here the MSC handles the handover process.

ii) Inter-MSC handover

• It occurs when a mobile moves from one MSC region to another MSC.
• MSC cover a large area. It can be imagined as a handover from Maharashtra MSC to Gujarat MSC while travelling.

C. Handoff schemes based on algorithms of handoff (handoff protocols)

The mobile unit and the BS are connected via radio links which carry data as well as signalling information. There are three different handoff strategies based on algorithms of handoff, which have been proposed for transferring the connection to a new BS.

1. MCHO (mobile-controlled handoff)
2. NCHO (network-controlled handoff)
3. MAHO (mobile-assisted handoff)

Since the number of handoffs increases with decreasing cell size, it will be an almost impossible task to make a handoff decision for every mobile by one central switch (centralized). Moreover, in microcells the connection between MS and BS can deteriorate very quickly. Fast handoff decisions required in such situations can be achieved more readily by decentralizing the handoff decision process.

Network-controlled handoff (NCHO)

1. NCHO is a centralized handoff protocol. In this type of handoff the network (surrounding BS, the MSC or both) makes a handoff decision based on measurements of the RSS (received signal strength) of mobile and the interferences from different BS.
2. The signal-to-interference ratio (SIR) is measured by means of a supervisory audio tone
   (SAT).
3. If the mobile is measured to have a weaker signal in its old cell, while a stronger signal in a neighboring cell, then a handoff decision could be made by the network to switch BS from the old cell to the new cell. Such a type of handoff in general takes 100–200 ms and produces a noticeable “interruption” in the conversation.
4. However, overall delay of such a type of handoff is in general in the range of 5–10 s. Thus, this type of handoff is not suitable to a rapidly changing environment and to a high density of users due to the associated delay.
5. The NCHO is widely used in the first-generation cellular systems, such as AMPS, Total Accesses Communications System (TACS) and Nordic Mobile Telephone (NMT). In NCHO, the MSC is solely in charge of the handoff process and the MSs are completely passive.

Mobile-assisted handoff (MAHO)

1. MAHO is a variant of NCHO strategy. To improve the handoff reaction time and to reduce the handoff administration load of the MSC, the handoff decisions should be distributed towards the mobile phones.
2. One way to achieve this could be to let the mobile phones make the measurements and the MSC make the decisions.
3. In the MAHO strategy, the network (BS and/or MSC) directs the mobile to measure the signal strengths from the surrounding BSs and to report those measurements back to the network.
4. The network then uses these measurements to determine where a handoff is required with which channel.
5. The delay in this protocol starting from the handoff initiation till the handoff execution is around 1 s. This time may still be too long to avoid dropping a call due to street corner effect.
6. Some examples of present cellular networks which implement MAHO are the GSM system and the IS-95 system.

Mobile-controlled handoff (MCHO)

1. In this case, the mobile phone is the only entity which measures the handoff criteria and makes a decision based on them.
2. The MSC is not involved in the handoff process resulting in reduced burden on the MSC. The mobile has to choose the optimum BS based on the measurements.
3. Since the handoff process is implemented in the mobile itself, the delay is usually smaller with a typical value of 0.1 s and is suitable for microcellular systems.
4. In this strategy, the mobile continuously monitors the radio signal strengths and quality of surrounding BSs.
5. A handoff can be initiated if the signal strength of the serving BS is lower than that of another BS by a certain threshold. Then the mobile requests the target BS for a channel with the lowest interference and handoff mechanism will take place.
6. In such a case, the MS does not have any information about the signal quality of other users, but handoff must not cause interference to other users.
7. MCHO is the highest degree of handoff decentralization. Some of the advantages of handoff decentralization are as follows:

1. Handoff decisions can be made fast.

2. MSC does not have to make handoff decisions for every mobile, which is a very difficult task for the MSC of high-capacity microcellular systems (radius < 1 km).

An example of a MCHO-based handoff control network is the standard for cordless
phones in Europe – digital European cordless telephone (DECT).

Que 4. Differentiate between soft hand off and hard hand off.

Que 5. Explain the important techniques to enhance the cellular coverage capacity. OR
Write a Short note on Cell Splitting, cell sectoring, repeaters for extending range and Microcell zone concept.

There is a performance criterion of cellular mobile systems like:

a) Voice quality.
b) Service Quality like coverage and quality of service.
c) Number of Dropped calls.
d) Special features like call forwarding, call diverting, call barring.

As the demand for wireless service increases, the number of channels assigned to cell becomes insufficient to support required number of users.

At this point, cellular design techniques are needed to provide more channels per unit coverage area.

Following techniques used to improve cell capacity in cellular system,

• Cell Splitting.
• Sectoring.
• Coverage Zone Approach.
• Repeaters

A) CELL SPLITTING:

It is process of subdividing a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitter power.

Cell splitting increases capacity of cellular system since it increases number of times that channels are reused, it preserves frequency reuse plan.

It defines new cells which have smaller radius than original cells and by installing these smaller cells called microcells between existing cells, that is radius will be half of the original cell.

Thus capacity increases due to additional number of channels per unit area, but does not disturb the channel allocation scheme required to maintain the minimum co-channel reuse ratio Q between co-channel cells.

B) SECTORING:

This is another method to increase cellular capacity and coverage by keeping cell radius unchanged and decreasing D/R ratio.

In this approach, capacity improvement is achieved by reducing the number of cells in a cluster and thus increasing the frequency reuse.

The co-channel interference in a cellular system may be decreased by replacing a single Omni-directional antenna at the base station by several directional antennas, each radiating within a specified sector.

The factor by which the co-channel interference is reduced depends on the amount of
sectoring used.

a) 1200 sectoring b) 600 sectoring

1) 3 Sectors 1200each

2) 6 Sectors 600 each

Advantages:

• Improvement in Signal capacity.
• Improvement in signal to interference ratio.
• Increases frequency reuse.

Disadvantages:

• Increase in number of handoffs.
• Increase in number of antenna at each base station.

C) COVERAGE ZONE/ MICROCELL ZONE CONCEPT

This approach was presented by Lee to solve the problem of an increased load on the switching and control link elements of the mobile system due to sectoring.

• It is based on a microcell concept for 7 cell reuse.
• In this scheme, each of the three zone sites are connected to a single base station and share the same radio equipment.
• Multiple zones and a single base station make up a cell. As a mobile travels within the cell, it is served by the zone with the strongest signal.
• This approach is superior to sectoring since antennas are placed at the outer edges of the cell, and any base station channel may be assigned to any zone by the base station.

D) Repeaters

1. The users of wireless networks require dedicated coverage for coverage in areas like mountains, valleys, buildings, etc. To provide such range capabilities radio retransmitters called as “repeaters” are used.
2. Repeaters are bidirectional. They simultaneously transmit and receive signals from the serving base station. They operate using over-the-air signals so that they can be installed anywhere.
3. Repeaters are capable of repeating an entire cellular band.
4. When the signals from base station are received, the repeater amplifies and reradiates the base station signals to that particular coverage region.
5. The noise that is received and interference are reradiated by the repeater on the forward and the reverse link. So, repeaters must be carefully installed to adjust the forward and reverse link amplifier levels and antenna patterns.
6. Directional antennas are connected to the inputs or outputs of repeaters for localized spot coverage in tunnels or buildings.
7. If the coverage of a cell that is in use is modified then the user can allocate some base stations traffic to areas covered by the repeater.
8. The repeater does not add capacity to the system. They are used to provide coverage into and around buildings where the coverage is weak. Repeaters with DAS (Distributed Antenna Systems) network are installed within the buildings to provide coverage into targeted areas.
9. Knowledge of correct location for repeaters and distributed antenna systems within the building needs planning, because into the building the interference levels are reradiated into the building from the base station and from the interior of the building to the base station.

Que 6. What is frequency planning in cellular system? Draw frequency reuse pattern for N = 4 and N = 7.

1. The design process of selecting and allocating channel groups for all cellular base stations within a system is called as frequency reuse or frequency planning.

2. Cellular radio systems rely on an intelligent allocation and reuse of channels throughout a coverage region. Each cellular base station is allocated a group of radio channels to be used with small geographic area called a cell. Base stations in adjacent cells are assigned channel group which contains completely different channels than neighboring cells.

3. By limiting the coverage area to boundaries of a cell, same group of channels may be used to cover different cells that are separated from each other by distances large enough to keep interference levels within tolerance limits.

4. If each cell is allotted K sets of channels and if S channels are divided among N cells in unique and disjoint groups which have same number of channels, then total number of available channels can be S=KN.

Thus, a small value of Q means larger capacity as cluster size N is small whereas large value of Q improves transmission quality. To improve capacity N is usually 4, 7 or 12.

5. To improve capacity and efficiency of frequency reuse plan, cell sectoring is employed with Omni directional antenna.

frequency reuse pattern for N = 4 and N = 7

• Each cellular base station is allocated group of radio channels to be used within a small geographic area called “cell”. Base stations in adjacent cells are assigned channel group which contains completely different channels than neighboring cell.
• By limiting coverage area to within the boundaries of cell, the same group of channels may be used to cover different cells that are separated from one another by distance large enough to keep interference level within tolerable limits.
• The design process of selecting and allocating channel groups for all the cellular base station within a system is called frequency reuse or frequency planning. Frequency reuse is important as the spectrum allocated for cellular transmission is limited and demand is increasing rapidly.

For cluster size N = 7For cluster size N = 4

Problem: 1

Problem: 2