1.

Explain about the concept of Frequency Management concern to the

numbering the channels and grouping into the subsets?
A. The function of frequency management is to divide the total number of available
channels into subsets which can be assigned to each cell either in a fixed fashion or
dynamically. Frequency management refers to designating set-up channels and voice
channels, numbering the channels, and grouping the voice channels into subsets. Channel
assignment refers to the allocation of specific channels to cell sites and mobile units.
Allocation of specific channels to cell site on long term basis is called ‘fixed channel
assignment’. Allocation of specific channels to cell site on short term basis is called ‘dynamic
channel assignment’.

NUMBERING THE RADIO CHANNELS

Many cellular mobile systems operate on 666 channels. Each channel consists of two
frequency channel bandwidths to allow duplex operation. These two channel bandwidths
must be separated in frequency in order to avoid interference. The sub-sections that follow
describe the numbering of radio channels by discussing the frequency management and
grouping of channels into subsets.

Ex:

In channel 1, the two frequencies available for mobile and cell-site transmit are

1. 825.030 MHz (mobile transmit)

2. 870.030 MHz (cell-site transmit)

In channel 666, the two frequencies available for mobile and cell-site transmit are

1. 844.98 MHz (mobile transmit)

2. 889.98 MHz (cell-site transmit)

GROUPING INTO SUBSETS

Since there are 21 set-up channels for each block, it is logical to group 312 voice channels
into 21 subsets of 15 voice channels. Each subset then consists of 16 channels – –15 voice
channels and one control channel. In each set, the closest adjacent channel is 21 channels
away. The channel separation is provided in such a way that it is sufficient to meet the
adjacent channel isolation requirement. The 16 channels in each subset can be mounted on
a frame and connected to a channel combiner. Wide separation between adjacent channels
is required for meeting the requirement of minimum isolation.

2. Summarize how set-up channels, paging channels and voice channels

are used in cellular mobile communication.
A. SET-UP CHANNELS
The set-up channels are also called as control channels. They are designated to set-up calls in
the system. But even without set-up channels, a system could work where all the channels
are in either block A or block B which will be used as voice channels. If a frequency reuse
technique is applied to a cellular system, the set-up channels act as control channels. The set-
up channels are classified with respect to their application. They are
• access channels
• paging channels
ACCESS CHANNELS
Access channels are used for calls originating from mobile. When a mobile set scans all the 21
set-up channels, two conditions are considered:
• If no set-up channels are operational in block A, then the mobile unit switches automatically
to block B.
• If there is a strong set-up channel with no message detected then within the second setup,
it will be selected by the scanner.
PAGING CHANNELS
Paging channels are used for calls originating from land. Every cell site is assigned its own
control or set-up channels. For example, FOCC is the forward set-up channel in which every
cell site are mainly used to page the mobile unit with control message of same mobile station.
• The same message is transmitted by different set-up channels and there is no simulcast
interference.
• A better algorithm is used to page from all the cell sites.
3. Discuss in detail about access channels and paging channels.
ACCESS CHANNELS: In mobile-originating calls, the mobile unit scans its 21 set-up channels
and chooses the strongest one. Because each set-up channel is associated with one cell, the
strongest setup channel indicates which cell is to serve the mobile-originating calls. Th. mobile
unit detects the system information transmitted from the cell site. Also, the mobile unit
monitors the Busy/Idle status bits over the desired forward setup channel. When the idle bits
are received, the mobile unit can use the corresponding reverse set-up channel to initiate a
call. Frequently only one system operates in a given city; for instance, block B When the
mobile unit first scans the 21 set-up channels in block A, two conditions can occur.
1. If no set-up channels of block A are operational, the mobile unit automatically switches to
block B.
2. If a strong set-up signal strength is received but no message can be detected, then the
scanner chooses the second strongest set-up channel. If the message still cannot be detected,
the mobile unit switches to block B and scans to block B set-up channels.
3. Change power at the mobile unit: When the mobile unit monitors the strongest signal
strength from all Set-up channels and selects that channel to receive the messages.
4. Direct call retries. When a cell site has no available voice channels, it can send a direct call
retry Message through the set-up channel. The mobile unit will initiate the call from a
neighboring cell which is on the list of neighboring cells in the direct call-retry message.
PAGING CHANNELS: Each cell site has been allocated its own setup channel (control channel).
The assigned forward set-up channel (FOCC) of each cell site is used to page the mobile unit
with the same mobile station control message. Because the same message is transmitted by
the different set-up channels, no simulcast interference occurs in the system. The algorithm
for paging & mobile unit can be performed in different ways. The simplest way is to page from
all the cell sites. This can occupy a large amount of the traffic load. The other way is to page
in an area corresponding to the mobile unit phone number. If there is no answer, the system
tries to page in other areas. The drawback is that response time is sometimes too long. When
the mobile unit responds to the page on the reverse set-up channel, the cell site which
receives the response checks the signal reception level and makes a decision regarding the
voice channel assignment based on least interference in the selected sector or underlay-
overlay region.
4. Differentiate Fixed and Dynamic channel assignment methods
Fixed Channel Assignment Dynamic Channel Assignment
1. Fixed number of channels or 1. Fixed number of channels are not
voice channels are allocated to cells allocated to cells
2. If all the channels are occupied 2. If all the channels are occupied and
and user make a call then the call will user make a call then Base
be blocked in FCA. Station(BS) request more channel to
the Mobile Station Center(MSC).
3. Frequency reuse is maximum 3. Frequency reuse is not that
because cells channels are maximum in DCA because of
separated by minimum reuse channel randomness allocation.
distance.
4. In FCA no such complex 4. In DCA complex algorithms are
algorithms are used. used to decide which available
channel is most efficient.
5. Fixed Channel Allocation 5. Dynamic Channel Allocation
Strategy is less costly than the DCA Strategy is costly because lot of
computation is required in real-time.
6. In FCA allocated channels 6. In DCA once the call is completed
remains to the cell, once the call is then the channel or the voice channel
completed return to the MSC.
7. Mobile Station Center(MSC) 7. Mobile Station Center(MSC) has
has less responsibilities. more signalling load and
responsibilities
5. Write short notes on channel sharing and borrowing and explain about the
non-fixed channel assignment algorithms.

Channel Sharing:

When a particular cell needs more channels in order to meet the increased traffic demand,
the channels of another sector at the same cell site can be shared to meet the short-term
overload traffic. Channel sharing can be done from one of the two adjacent sectors of the
neighbouring cells in a sectored cellular system configuration. Shared channels can be
returned back when the channels become available in the shared sector. This scheme is called
the ordered channel assignment scheme with rearrangement. An alternate scheme is channel
assignment with sharing and reassignment. This scheme makes sure that channel-sharing
arrangement causes minimum impact on call-blocking probability in neighbouring cells.
Reassignment of shared channels is done in order to provide maximum assistance to the
neighbouring cells to meet the temporary increased traffic demand. The channel can also be
ordered based on which channels provide better performance. Channel sharing always
increases the trunking efficiency of the channels.

Channel Borrowing:

The channel-borrowing scheme is used for slow growing systems on a long-term basis as an
alternate to the costly cell-splitting technique to handle increased traffic. One approach to
address increased traffic of either mobile originating calls or handoff calls in a cell is to borrow
free available channels from neighbouring cells. A simple channel-borrowing scheme implies
that if all channels assigned to a cell have already been used, then additional channels as per
the current need can be borrowed from any other cell (preferably adjacent cells) that has
some free unused channels. In addition, the central cell site can also borrow channels from
neighbouring cells. The extent of borrowing channels depends on the traffic density in the
area.
NONFIXED CHANNEL ASSIGNMENT ALGORITHMS
There are many different ways of implementing FBCA. In a general sense, FBCA can also be
applied while accounting for the forcible borrowing of the channels within a fixed channel set
to reduce the chance of cochannel assignment in a reuse cell pattern.
•The FBCA algorithm is based on assigning a channel dynamically but obeying the rule of reuse
distance. The distance between the two cells is reuse distance, which is the minimum distance
at which no cochannel interference would occur.
•Very infrequently, no channel can be borrowed in the neighboring cells. Even those channels
currently in operation can be forcibly borrowed and will be replaced by a new channel in the
neighboring cell.

6. Illustrate briefly about (i) Cell Spliting (ii) Overlaid Cells (iii) Sectorization.

Cell Spliting:

1) It is the process of subdividing a congested cell into smaller cell with its own base
station having the corresponding reduction in the antenna heights & the transmitted
power.

2) There are two cell splitting techniques,

a. Permanent splitting

b. Dynamic splitting

3) This technique utilizes the allocated spectrum efficiency in real time. In situations
such as traffic jams, the ideal small cell sites may be rendered operative in order to
increase the cell’s traffic capacity

 Cell splitting increases the capacity of a system since it increases number of

times that channels are reused.
 In cell splitting original cell is split in to smaller cells. New cell radius is half of
the original radius.

 In this the cell boundaries need to be revised so that the local area which was
earlier considered as a single cell can now contain number of smaller cell ,these new
cells are called microcells

Overlaid Cells:

In the overlaid cell concept, the cellular network is seen as a superposition of the smaller cell
pattern on top of the complete larger cell pattern. Each cell face will divide its channels between
a larger and a smaller cell group. The selected channels in the larger group will be used in all
the coverage areas of a larger cell. The selected channels installed in the smaller cell will make
the smaller cell group. The formation and use of the channel groups is then governed by the
presence or absence of real smaller cell neighbours. For example, the use of any channel
installed in a smaller cell must be restricted to the other smaller cell overlay areas in the nearest
larger co-channels. If a mobile user using a smaller cell channel goes out of the perimeter of
the small cell overlay area on the larger cell, it needs to handoff to a channel of the larger group
or to a neighbouring cell. Therefore, the presence of two sizes of cells in a network reduces the
capacity of some of the larger cells. This can force their cells to split even if they would not
consider the growth of traffi c in their areas.

SECTORIZATION:

One approach to increase the subscriber capacity of a cellular network is replacing the
omnidirectional antenna at each BS by three or six directional antennas with a 120° or 60°
opening of the corresponding receivers to illuminate a fraction of a cell. Each sector can be
recognized as a new cell, with its own (set of) frequency channels. By using directional antenna
on a BS, each pointing in different directions, it is possible to sectorize the BS so that several
different cells are served from the same location. This increases the traffi c capacity of the BS.
An example of a cell with three sectors is shown in Figure 18.7. In this case, each sectoral
antenna uses a 120° beamwidth directional antenna pattern in the horizontal plane. Assuming
that the antenna radiation pattern is such that the gain is virtually zero outside the beam, the
interference outside of the 120° sector radiation angle is essentially suppressed, which reduces
the relative other cell interference by a factor of 3 and improves CDMA capacity by the same
factor. Three-sector cells are widely deployed in CDMA cellular systems, except for indoor or
picocell systems where distributed antenna systems may be better suited. The use of directional
sector antennas substantially reduces the interference among cochannel cells. This allows
denser frequency reuse.