Cellular concept• Interference and system capacity, Improving system capacity and cell coverage.
Cellular System fundamentals
• Cellular technology aims at achieving high system capacity with limited radio spectrum.
• Cellular design makes use of the fact that the power of a transmitted signal falls off with distance.
• Two users at spatially-separate locations can operate on the same frequency with minimal interference between them.
• This allows very efficient use of cellular spectrum so that a large number of users can be accommodated.
Cellular System fundamentals
• Cellular concept is a system level idea which uses many low power transmitters called base stations, each providing coverage to only a small portion of the total service area.
• Each base station is assigned a portion of the total number of channels available to the entire system and neighbouring base stations are assigned different groups of channels.
• System capacity increases by reusing the frequency within the region of service
Cellular System fundamentals
• Cells use the same set of frequencies are known as co-channel cells
• Reuse of frequency may give rise to interference between signals of these cells, called co-channel interference, if the power levels of the transmitters as well reuse distance are not properly designed.
• A regular cell shape is needed for systematic system design
• When considering tessellating structures, geometric shapes which cover entire region without overlap and with equal area there may be several choices
Different candidate geometric shapes
Typical radio coverage in a cell
The MSC acts as a central controller for the network. It allocates channels within each cell, coordinate handoffs between cells when a mobile traverses a cell boundary, and route calls to and from mobile users. The MSC can route voice calls through the public switched telephone network (PSTN) and it can also provide Internet access.
Frequency Reuse
duplex channels
The N cells which collectively use this set of S
channels are said to form a cluster

 such that .
can be repeated times giving an -fold
capacity increase with same set of channels
M M
and are non-negative integers with i j i j
Allowable cluster sizes 1, 3, 4, 7, 9, 12......N
Coverage of a given region using 7-cell reuse pattern
Nearest co-channel neighbour
ratio of the distance of the centers of the nearest
co-channel
Cell splitting and sectoring
Cell splitting and sectoring are the two cellular design techniques for increasing system capacity and improving coverage
Cell splitting subdivides a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitter power.
In the sectoring approach, omni directional antennas at the base station are replaced by directional antennas, each radiating in a specific sector
Cell splitting
Channel assignment
Channel assignment strategies deal with the efficient utilization of radio spectrum through a frequency reuse scheme whose objective is to increase capacity as well as keep interference level as low as possible.
In fixed assignment, each cell is assigned a predetermined set of voice channels. In this assignment scheme, if all the channels in the cell are occupied, a newly generated call is blocked and the subscriber does not receive the service until a channel becomes free.
In another approach, a cell is allowed to borrow channels from a neighboring cell if all its own channels are already occupied. The mobile switching center (MSC) supervises such borrowing and also ensures that services in the donor cell are not disrupted due to borrowing.
In dynamic channel assignment, channels are not assigned to the cells permanently. Instead, serving base station, on receipt of a call request, asks for a channel in the MSC. MSC based on certain considerations allocates channel
Dynamic channel assignment reduces the likelihood of blocking. However in this scheme MSC is required to collect real time data on channel occupancy and traffic distribution.
Handoff
Handoff or hand over is generally used in the context of transferring an ongoing call to a different channel or a cell.
The main purpose of handover is to maintain an ongoing call. The handoff has to be transparent to the user and typically will not result in a loss of service
The handoff decision may be made at the MS, BS, or mobile switching center, MSC.
Larger results in unnecessary handoffs, burdens MSC
Too small may result in loss of calls


MAHO
• In mobile assisted handoff (MAHO) every mobile station measures the received signal from the surrounding base stations and reports the measurement results to the serving base station.
• A handover is initiated when received signal power from the neighboring base station exceeds the power received from the current base station of attachment by a predefined amount and for a certain period of time.
• Handoff time is reduced in case of MAHO.
• There are a number of parameters that need to be known to determine whether a handover is required.
• It is required to the signal strength of the base station with which communication is being made, along with the signal strengths of the surrounding stations.
• Additionally the availability of channels also needs to be known.
• The mobile is obviously best suited to monitor the strength of the base stations.
• But only the cellular network knows the status of channel availability and the network makes the decision about when the handover is to take place and to which channel of which cell.
• In the context of GSM system, there are four different types of handovers, which involve transferring a call between:
– Channels in the same cell
– Cells under the control of the same Base Station Controller (BSC),
– Cells under the control of different BSCs, but belonging to the same Mobile services Switching Center (MSC), and
– Cells under the control of different MSCs.
• The first two types of handover, called internal handovers, involve only one Base Station Controller (BSC).
• To save signaling bandwidth, they are managed by the BSC without involving the Mobile services Switching Center (MSC), except to notify it at the completion of the handover.
• The last two types of handover, called external handovers, are handled by the MSCs involved.
Unlike wireless communication system that assigns different radio channel during handoff, also called hard handoff, for systems like CDMA where a mobile uses the same channel in every cell, Handoff is not a physical change but assigning a new BS to handle control functions for the mobile.
For a given user, MSC can use received signal strength from multiple neighbouring BSs to determine the best version of the signal at any moment.
This ability to selecting among instantaneous received signals from multiple base stations is called soft handoff.
Interference: Co-channel and adjacent channel interference
• Interference is a major performance-limiting factor for cellular radio system.
• Interference often limits the system capacity.
• There are two major types of interference: adjacent channel and co-channel.
• Adjacent channel interference (ACI) results due to interference of signals, which are adjacent in frequency to the desired channel.
• Adjacent channel interference results when the receiver filters are imperfect.
• Adjacent channel interference becomes particularly serious when an adjacent channel user is transmitting in very close range to a subscriber’s receiver while the subscriber attempts to receive a base station on the desired channel.
• Such situations are referred to as near far effect.
• ACI can be minimized through proper filtering and channel assignment.
• Co-channel interference refers to the interference that results from those cells, which use same frequency.
• As several cells use the same frequency, there will be several tiers of such interfering cells; the cells at particular reuse distance constitute a particular tier of interfering cells.
• CCI can be reduced by repeating frequency at larger distances, i.e. is using larger clusters that increase reuse distance.
• But increasing reuse distance reduces the system capacity.
• Thus, we have contradictory requirements, high capacity demands smaller cluster size while maintaining proper signal to interference power ratio demands larger reuse distance.
• CCI should be properly taken into account while designing cellular systems.
• As the power of the interfering signals decrease rapidly with distance, the most significant portion of the interfering power comes from the first tier of interfering cells.
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Co-channel Interference