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EE 494 | Wireless Communications
Lecture # 4 and 5: Cellular Concept and System Design Fundamentals
Dr. Abid Ullah
Assistant Professor, Department of Electrical Engineering, Jalozai Campus
University of Engineering and Technology, Peshawar
Week # 4 and 5:
Cellular Concept and System Design Fundamentals
Outline:
โ Introduction to cellular concept โ Frequency Reuse โ Channel Assignments Strategies โ Handoff Strategies โ SIR and System Capacity โ Trunking and Grade of Service โ Improving Coverage and Capacity of Cellular Systems
Introduction to Cellular Concept:
- Cellular concept is a system-level idea which calls for replacing a single high powered transmitter (large cell) with many low powered transmitters (small cells), each providing coverage to only a small portion of the service area
- Offered very high capacity in a limited spectrum allocation without any major technological changes
- helped in solving the problem of spectral congestion and user capacity
- Each base station is allocated a portion of the total number of channels available to the entire system
- nearby base stations are assigned different groups of channels
- interference between base stations is minimized
- Systematically spacing base stations and their channels groups throughout the geographical region
- frequencies are reused as many times as necessary, so long as the interference in co-channel base stations is kept below acceptable levels
- As demand for services increase, the number of base stations may be increased, thereby providing additional radio capacity with no additional increase in radio spectrum.
Frequency Reuse:
- Design process of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning
- Each cellular base station is allocated a group of radio channels to be used within a small geographical area called cell
- Base stations in adjacent cells are assigned channel groups which contain completely different channels than neighboring cells
- Base station antennas are designed to achieve the desired coverage within the particular cell
Figure 3.1 Illustration of the cellular frequency reuse concept. Cells with the same letter use the same set of frequencies. A cell cluster is outlined in bold and replicated over the coverage area. In this example, the cluster size, N, is equal to seven, and the frequency reuse factor is 1/7 since each cell contains 1/7 of the total number of available channels
Frequency Reuse:
- Consider a cellular system which has a total of S duplex channels
- each cell is allocated a group of k channels (k<S)
- S channels are divided among N cells into unique and disjoint channel groups
- total number of available channels can be expressed as S = kN
- The N cells which collectively use the complete set of available frequencies is called a cluster
- If a cluster is replicated M times within the system, the total number of duplex channels C , can be used as a measure of capacity C = MkN = MS
- This equation shows that capacity of a cellular system is directly proportional to the number of times a cluster is replicated in a fixed service area
- Factor N is called the cluster size and is typically equal to 4,7, or 12.
- Value for N is a function of how much interference a mobile or base station can tolerate while maintaining a sufficient quality of communications.
- The frequency reuse factor of a cellular system is given by 1/N , since each cell within a cluster is only assigned 1/N of the total available channels in the system
Frequency Reuse:
- In order to tessellate - to connect without gaps between adjacent cells - the geometry of hexagons is such that the number of cells per cluster N, can have values from the following equation N = i 2 + ij + j 2
- where i and j are non-negative integers
Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19, (i.e., i =3, j=2)
Channel Assignment Strategies:
- Channel assignment strategies can be classified as either fixed or dynamic
- Fixed channel assignment strategy, each cell is allocated a predetermined set of voice channels
- call attempts within the cell can only be served by the unused channels in that particular cell
- if no channels are available in a cell then the call is blocked
- several variations of the fixed channel assignment
- Borrowing strategy, a cell is allowed to borrow channels from neighboring cell if all of its own channels are already occupied
- MSC supervises such borrowing procedures, keeping in view no disruption or interference in other cells
- Dynamic channel assignment, voice channels are not allocated permanently to different cells.
- Each time a call request is made, serving base station requests a channel from the MSC
- MSC then allocates a channel to the requested cell following an algorithm that takes into account the likelihood of future blocking within the cell, the frequency use of the candidate channel, the reuse distance of the channel, etc.
- Dynamic channel assignment strategy requires the MSC to collect real-time data on channel occupancy, traffic distribution, and RSSI of all channels on a continuous basis
Handoff Strategies:
- 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
- handoff operation involves identifying a new base station, and
- voice and control signals allocated to channels in new base station
- handoff requests have priority over initiation requests
- Handoff is requested when the signal level goes below a particular threshold ๐บ to initiate handoff
- The handoff margin ๐บ is calculated by equation ๐บ = P (^) r handoff - P (^) r minimum usable
- If handoff threshold is too large unnecessary handoff occurs, but its too small, there may be insufficient time to complete a handoff before a call is lost due to weak signal conditions
- Dynamic channel assignment strategy requires the MSC to collect real-time data on channel occupancy, traffic distribution, and RSSI of all channels on a continuous basis
Handoff Strategies:
- When deciding handoff, it is important to ensure that the drop in the measured signal level is not due to momentary fading and that the mobile is actually moving away from the serving base station.
- base station monitors the signal level for a certain period of time
- running average signal strength measurements to avoid unnecessary handoffs
- length of time needed to decide handoff depends on the speed of the vehicle
- time over which a call may be maintained within a cell, without handoff is called the dwell time
- Dwell time for a particular user is governed by the following factors,
- propagation,
- interference,
- distance between subscriber and the base station, and
- time varying effects
- First generation cellular systems, a locator receiver is used by the MSC to monitor the signal strength of users in neighboring cells, which appear to be in need of handoff
- Based on locator receiver signal strength information from each base station, MSC decides if handoff is necessary or not.
Handoff Strategies:
- In Mobile Assisted HandOff (MAHO), every mobile station measures the received power from the surrounding base stations and reports the results to serving base station
- handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from current base station by a certain level or for a certain period of time
- MAHO enables call handoff at much faster rate then 1st generation systems
- Intersystem handoff is necessary if the mobile moves from coverage area of one cellular system controlled by one MSC to different cellular system controlled by another MSC
- Local calls can become long distance calls in intersystem handoffs
- Compatibility between the two MSCs must be determined,
- Different systems have different policies and methods for managing handoff requests
- from user perspective having an ongoing call disrupted is more annoying than being blocked occasionally on a new call attempt
Hard Handoff and Soft Handoff:
- Hard handoff happens in systems with channelized wireless systems that assigns different radio channels during handoff
- Ability to select between the instantaneous received signals from a variety of base stations is called soft handoff
- CDMA systems share the same channel everywhere
- term handoff does not mean a physical change in assigned channel but rather that a different base station handles the radio communication tasks
- mobile measures the received signal strength of different base stations
- MSC decides the handoff base station based on mobile measurements
Week # 4 and 5:
Cellular Concept and System Design Fundamentals
Outline:
โ SIR and System Capacity โ Trunking and Grade of Service โ Improving Coverage and Capacity of Cellular Systems
Co-Channel Interference and System Capacity:
- Frequency reuse implies that in a given coverage area there are several cells that use the same set of frequencies.
- These are called co-channel cells , and their interference is called co-channel interference
- Unlike thermal noise which can be reduced by increasing SNR
- Co-channel interference can not be reduced by increasing transmit power
- Co-channel interference can be reduced by physically separating the co-channel cells
- Physical separation provides sufficient isolation due to propagation
- Cells with same size and transmit power, co-channel interference ratio is a function radius of cell R and distance between centers of the nearest co-channel cells D
- Increasing the ratio of D/R, the spatial separation between co-channel cells relative to the coverage distance of a cell is increased
- Interference is reduced from improved isolation of RF energy from co-channel cells
- The ratio of D/R is called the co-channel reuse ratio Q Q = D/R = sqrt(3N) - Small value of Q provides larger capacity since the cluster size N is small - large value of Q improves the transmission quality, due to a smaller level of co-channel interference
Interference:
- Let i 0 be the number of co-channel interference cells. Signal-to-interference ratio is given by
- where S is the signal power from desired base station and I (^) i is the interference power from i th^ interfering co-channel cell base station
- Average received signal strength at any point decays as a power law of the distance of separation between a transmitter and receiver
- P (^) r: average received power
- P 0 : power received at a close-in reference point in the far-field region of antenna at a distance d (^0)
- n: pathloss exponent
- If D (^) i is the distance of the ith interferer from the mobile, the received power at a given mobile due to the i th^ interfering cell will be proportional to (Di) -n
- Pathloss exponent typically ranges between 2 and 4 in urban cellular system
