Cellular Telephone System - Book 2025 PDF

# Unit-II ## Cellular Telephone System The system which provides a wireless connection to the PSTN for any user location within the radio range of the system called as cellular telephone system. ### Advantages of cellular telephone system 1. Accomodate large number of users over a large geograph...

# Unit-II ## Cellular Telephone System The system which provides a wireless connection to the PSTN for any user location within the radio range of the system called as cellular telephone system. ### Advantages of cellular telephone system 1. Accomodate large number of users over a large geographic area, within a limited frequency spectrum. 2. Provides high quality of service 3. High capacity is achieved by limiting coverage of each base station to small geographical area called cell. 4. All processing is uninterrupted due to handoff mechanism ### Limitations of conventional mobile 1. **Limited service capability:** - High power - Large cell - Reinforcing call - Cell site The communication zone was very large & transmitter were working on high powers. The user has to reinitialize the call when moving into new zone. This was reason for call drop. Also the number of active users were limited to number of channels assigned to perticular freq. zone. 2. **Poor service performance:** - Large number of subscribers created high blocking probability during busy hrs. 3. **Insufficient frequency spectrum utilization:** - Conventional system does not utilize the spectrum effectively since each channel can serve only one customer at a time in a whole area. ...There was a need of efficient mobile communication system. As per FCC Federal communication commission ideal system should give minimum usage & customer satisfaction with minimum B.W. so the idea mobile system would operate within a limited assigned frequency band & would serve an almost unlimited numbers of users in unlimited areas. Three major approaches are: - Single sideband (SSB) which divides allocated frequency band into more number of channels. - Cellular, which reuses the allocated freq. band in different geographical locations. - Spread Spectrum, frequency hopped, which generates many codes over a wide frequency band. ### Cellular Concept Cellular concept is system-level idea which calls for replacement of single, high power transmitter (large cell) with many low power transmitter (small cells), each providing coverage to only a small portion of the service area. ### Cellular System In cellular system each base station is allocated a portion of the total number of channels available to entire system & nearby base stations are assigned different group of channels so that all available channels are assigned to relatively small no. of neighboring base stations. **Basic cellular system consists of 3 parts -** - Mobile unit - control unit + TxRx+ Antenna system - Cell site - Provides interface between TMTSO or MU - MTSO - Switching office. ### Performance Criterion 1. **Voice Quality:** - Expressed in terms of CM1 to CM5 - CM1(Excellent) CM5 (poor). As % of customers choosing CM4 & CM5 increases, the cost of building the system rises. 2. **Service Quality:** - Coverage - Grade of service - No of dropped calls 3. **Special features:** - Call waiting, call forwarding, USR etc ### Frequency Reuse & Frequency Planning - **Cell**: It is a small geographical area. Each base station in this cell is allocated a group of radio channels - Base stations in adjacent cells are assigned channel groups which contain completely different channel than neighboring cells. - By Limiting the coverage area to within the boundaries of a cell, the same group of channels may be used to cover the different cells that are separated from one another. - By distances large enough to keep interference levels within tolerable limits. The design process of selecting & allocating channel groups for all the cellular base stations within the system is called frequency reuse or frequency planning.. - Fig. shows the concept frequency reuse, where cells labeled with the same letters use the same group of channels. - Hexagonal model is conceptual & simplistic model adopted universally. The actual radio coverage of a cell is known as the footprint & It is determined from field measurement or propagation prediction models. **Why Hexagonal Model adopted?** - It is natural to choose circular shape cell for representation of coverage area of a base station. - Adjacent circles cannot be overlaid upon a map without leaving a gap or overlap. Thus while considering the geographical shapes region without overlapping with equal area there were 2 choices - A square, Equilateral A & hexagon. - A cell must be designed to save the weareast mobiles within the cell which one typically located at the edge of cells. - For a given distance between the centre of polygon & it's farthest perimeter points, the hexagon has the largest area of three geomery. - Thus by using the hexagon geometry, fewest number of cells can cover a given region. & hexagon could closely approximate to circular area of radiation pattern which would accure for an omnidirectional base station antenna. **Hexagon model can be depicted as:** - **Centre excited cell** - Omnidirectional antennas used - **Edge excited cell** - Sectored directional antennas used - System design permits a base station to be positioned upto one-fourth the cell radius away from ideal location. **Designing concept →** - Consider cellular system with S duplex channels available for use. (K<S) S= Duplex channels available for use K = Allocated channels to each cell N= No. of cells S=KN N cells which comptet collectively use the complete cell set of available frequency is called a cluster. - If cluster is replicated 'm' times within the system the total no. of duplex channel, 'c' can be used to measure the capacity 'k' is given by E = MKN = Ms - The capacity of cellular system is directly proportional to number of times a cluster is replicated in a fixed service area. - The factor N is called cluster size & typically equal to 4,7,12 - If the cluster size N reduced while cell size is kept constant, more clusters are required to cover a given area t: more capacity is achieved. - Large cluster size indicates that ratio between inc cell radius & distance bet co-channel cells is small. - Conversely, a small cluster size indicate that cochannel cells are located much closer. - The value of N is a function of how much interference a base station can tolerate while maintaining a sufficient quality of communications. - The smallest possible value of N is desirable in order to maximise capacity over a given coverage area. - Frequency Reuse factor is 1/N of total available Channel. N can have values which satisfy the eq' N = P+Ji+j² i, j Non negative integer **How to find nearest co-channel hexagones:** - Move 'i' cells along any chain of hexagones - Turn 60° counterclockwise & move 'j' cells **A Spectrum of 30MHz is allocated to wireless FDD cellular. Uses two 25kHz simpler channel to prouvide full duplex voice & contol channel. Compute the number of channels available per cell if a system uses N = 4,7,12. If 1/10th of allocated spectrum is dedicated to control channels, determine equitable distribution of control channels & voice channels in each cells** - Total B.W = 30MHz - Two 25KHz Simplon channel - = 50Knt duplen channel - Total available channels 30×106 = 50×103 = 30×100×102 = 30×103 = 30100% = 600 50 50 50 50 :: 600 channels. - Total No. of available channels = 600 - N=4 600 = 150 channels. 4 - N = 7 600 = 85 channels 7 - N=12 600 = 50 channels 12 - 1MHz control channels means = 1X106 5Ox103 = 1000 20 50 - 20 control channels out of 600 channels. **Co-channel reuse ratio -** - Q = 1/√3N - R = Radius of cell - D = distance to nearest cochannel cell - N = No. of cells in cluster - en = N=4 - Q = √3/16 - R = 6.92 ≈ 7 - D = 7R **Channel Assignment Strategies -** - There is trade-off between increasing capacity and minimizing interference. - Channel assignment can be: - Fixed - Dynamic **Fixed Channel -** - Each cell is allocated a predetermined set of voice channels. - Any call attempt within the cell can only be served by the unused channels in that particular cell. - If all the channels in that cell are occupied the call is blocked & no service given to subscribers. - One of the strategies of fixed assignment is a borrowing strategy, where cell is allowed to borrow channels from a neighboring cell if all channels are occupied. MSC controls this process. **Dynamic Channel:** - Voice channels are not allocated to different cells permanently instead each time a call request is made - BSC requests MSC. - Switch allocates channel to req. call. Following algo is not precisely used in cell or any other cell: MSC only allocates a given frequency if that frequency which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference. **Handoff Strategies** - When a mobile moves into different cell while a conversation is in process, the MSC automatically transfers the call to a new channel belonging to new base station. - **Hand off operation involves -** - Identifying a new base station. - Voice & Control channel signals to be allocated to channels associated with the new base station. - Processing handoff is important task - many HO strategies prioritize handoff requests over call initiate requests when allocating unused channels in a cell cite. - **Hand off must be performed -** - Successfully - As infrequently as possible - Be imperceptable to the user. - In order to meet the handoff requirement system designer must specify an optimal signal level at which to Initiate a handoff. - Once a particular signal level is specified as the minimum Usable signal for acceptance voice quality at base station receiver [-godBm & - 100dBm] a slightly stronger signal level is used as a threshold at which hand off is made. - Δ = Prhandoff-Pr minimum usable. - Δ = Margine. - Too large - creates unnecessary handoff, can burden MSC - Too small - There may be insufficient time to complete a handoff before the call is lost due to weak signal conditions. **Drop call event can happen when there is an excessive delay by MSC in assigning a handoff when Δ is set too small for handoff firm or excessive delay can accure during high traffic condition.** **While deciding the hand off it is importent that - ** - To ensure that drop in measured sig level is not due to momentary fading - & Mobile is actually moving away from serving base station. - The length of time needed to decide if a handoff is necessary depends on speed at which the vehicle is moving - If the slop of short term received signal Average stay Rx sig level in a given time. - Interval is steep, the handoff should be made quickly Information about vehicle speed, which can be useful in hand off decisions, can also be computed from the statistics of received short term fading sig at base station. - The time over which a call may be maintained within cell, without handoff is called 'dwell time'. **Dwell time governing factors -** - Propagation - Interference - Distance between subscribers & base station - Time varying effects. **To improve the quality of service as perceived by users, various methods have been devised to prioritise handoff requests over call initiation.** **Prioritising Handoffs -** - **Guard channel concept ->** Fraction of the toal available channels in a cell is reserved exclusively for handoff requests from ongoing calls. **Limitations -** - Reduced total no. of carried traffic, as fewer channels are allocated to originating calls. - Queuing of handoff requests. - Queuing of handoff is possible due to the fact that there is finite time interval between time of RX sig level drops below threshold & the time the call terminated due to insufficient sig level. **Practical Handoff considerations -** - In practice, high speed vehicles pass through coverage region of cell very quickly while pedestrian users may never need a handoff during call. - By Using different antenna size heights & different power levels, it is possible to provide large and small cells which are co-located at a single locasión. This technique is called as Umbrella cell approach. - This will provide large area coverage to high speed users & small area coverage to low speed users. - **Cell Dragging:** It's occurs when pedestrian users that provide a very strong sig to base station. - In urban environment where there is LOS radio path beth sa users & BS, as the user travels avery slowly from base station, at very slow speed, the average sig strength doesnt decay rapielly when though travelled well beyond the designed range. - Because of this sig strength is above Ho threshold & Ho cannot made. This creates interference + traffic management problem. - To address tuis handoff threshold & radio coverage parameters must be adjusted carefully. **Interference & System Capacity** - Interference is the major limiting factor in the performance of cellular radio systems - **Sources of Interference** - Another mobile in same cell - A Call in a progress in a neighboring cell. - Other base stations operating at in the same frequency bond. **Any's non cellular system which inadvertently leaks energy into Cellular system. Two major system generated interference are:** - Co-channel interference. - Adjacent channel interference. **Co-channel Interference & System Capacity:** - Frequency reuse implies that in a given coverage area there are several calls that use the same set of frequencies. - These cells are called co-channel cells, & the interference between signals from these cells is called co-channel interference. - Cochannel interference cannot be combated by simply increasing carrier power of a transmitter. - To reduce co-channel interference, co-channel cells must be physically separated by minimum distence, to provide sufficient isolation due to propagation: - Co-channel interference problem become promincult in a cellular system when the system uses frequency reuse with a small cluster size. - By increasing the ratio of DIR, the spatial seperation between co-channel cells relative to the coverage distance of cell is increased. - Thus interference is reduced from improved Isolation of RF energy from the co-channel cell. - Q = cm-channel reuse ratio - Q = 1/√3N - Small Q = Multiple larger capacity since cluster size is small (N), whereas a larger value of Q improves transmission quality, due to smaller level of co-channel interference. - (N+1²+ij+j²) - Signal to Interference rabo(S11) for mobile receiver which monitors a forward Channel can be cmpressed as - S11 = S/I n= path luss exponent ΣJi CDIR - or S/1= (DIR)" (J3N)" **Adjacent channel Interference:** - Interference resulting from signals which are adjacent in frequency to the desired signal is called as adjacent channel interference. - Adjacent channel interference results from imperfect receiver filters which allow nearby prey.. to leak into pass hand. - Adjacent Channel interference can be minimised thro careful filtering & channel assignments - By keeping the frequency seperation between each channel in a given cell as large as possible, the adjarent channel interference may be reduce considerably. - In a higher density urban areas, cells are typically closer together & signal density is high. - Adjacent channel interference accure when signals from adjacents channels overlap interfere with each other - coreful channel assignment is crucial to minimize this type interference by ensuring that adjacent channels are not used in nearby cells, thereby preventing overlapping Frequencies. **Improving Coverage & Capacity in Cellular systems.** - As the demand for wireless service increases, the number of channels assigned to a cell eventually becomes insufficient to support the no. of Users. At this point cellular design tech are needed to provide more channels per unit coverage area. Techniques such as - - cell splitting - cell sectoring - Micro cell zone concept are used to increase the capacity. **Cell splitting** - Cell splitting is used to increase the capacity of a cellular niw by dividing lage cell into smaller Cells. each with its own base station & frequency set. - **Why cell splitting?** When user demand exceeds the capacity of existing cells. - In densely populated urban areas where signal strength weaken due to high user density. - **Key Chara:** - Smaller cells have shorter coverage distance. - Lower antinna weight - to suit the signal range to desired smaller area. - Lower Pensmission pouce - ensures that the sig does not interfere with neighboring cells. - Frequency reuse Improvement - smaller cells allow for closer placement of co-channel cells without interference. - **Advantages -** - Increased capacity - Accomodate more users in same geo-area without interference. - Enhanced frequnny reuse - Allow freq to be reused in adjacent channel. - Better Call quality - reduced no. of users/ base station leading to improved signal quality. - **Limitations -** - Infrastructue cost - Required additional bose station & resources - Smaller cells increase the risk of co-channel interference. - Handoff overhead - Most frequent hand off are needed due to smaller cell site. **Cell Sectoring** - Defination - Cell sectoring is a technique where a cell is divided into sectors, each served by a directional antenna that focuses signals within specified sector. - **Why sectoring?** - By using directional antennas, sectoring reduces co-channel & adjacent channel interference. - Capariny improvement - Allow more effective reuse of frequencies within the same geographic area. - **Types of sectoring -** - 3-sector configuration - Divided cells in three 120g - 6-sector confignans - Divides cells into six 60° scute - **Key Chara** - Directional antennas - Focus signals withing specific coverage area rather than broadcasting omnidireda - Improved signal strength - Reduces sig coverage outside sector. - **Advantages -** - Isolate sectors to prevent Overlap - Each sector can operate on seperate freg. sets - Enhanced sly clarity within sectors. - **Challenges -** - Requires careful attmna alignment & confiaban. - Improper design can lead to Sector boundary interference. - **Cell Splitting** - Capariny by creating smaller cell - Add new bose stations with small coverage - More frequent handoff due to smaller cells - **Cell sectoring** - Interfence & capacity ↑ using directional antenna composed multiple disoihanal antennas - Less Preyuurt handoff splitsing. - Sus existing bose station