Cellular Telephone Systems


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1 Cellular Telephone Systems by Lotis P. Patunob, M.Eng., PECE

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2 1940s MTSs (Mobile Telephone Systems or Manual Telephone systems): – all calls were handled by an operator, use FM use a single carrier frequency in the 35 MHz to 45 MHz range for both the mobile unit and base station half duplex operation, 120 kHz bandwidth per channel only one conversation could take place at a time could not be accessed directly through the PSTN with five digit long numbers. Cellular Telephone Service Cellular Telephone Systems

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3 1964 IMTS (Improved Mobile Telephone Systems): – use several carrier frequencies and could, therefore, handle several simultaneous mobile conversations at the same time - high output power between 13 W and 30 W and a range of 25 mile radius with assigned regular PSTN number so could be reach by dialing the PSTN directly, eliminating the need for an operator transmit power a channel bandwidth of 30 kHZ increasing the number of channels, . Cellular Telephone Systems

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4 1983 AMPS (Advanced Mobile Phone System) – the first U.S. cellular telephone system by AT&T with 666 30 kHz half-duplex mobile telephone channels, was based on analog radio technologies and has been phased out. Cellular Telephone Service Cellular Telephone Systems Disadvantages of early mobile telephone systems: High cost, limited availability, and narrow frequency allocation.

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5 AMPS Specifications Cellular Telephone Systems

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6 Coverage zone – a large geographic market area. Cells – the small sections of the large geographic area. It is defined by its physical size, and the size of its population. a. Macrocells – large cells typically have a radius 1 mile and 15 miles with base station transmit power between 1 W and 6 W. b. Microcells – the smallest cells typically have a radius of 1500 feet or less with base station transmit power between 0.1 W & 1 W Fundamental Concepts of Cellular Phone Cellular Telephone Systems

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7 Cellular Telephone Systems Honeycomb – the pattern formed by the hexagonal- shaped cells. Picocells – very small cells used indoor.

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9 Different locations of base station transmitters: 1. Center-excited cell Fundamental Concepts of Cellular Phone Cellular Telephone Systems 2. Corner-excited cell 3. Edge-excited cell

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10 800 – 900 MHz – original frequency assignment; previously occupied by UHF TV channels 68 through 83 2. 824 and 849 MHz – reserved for uplink 869 and 849 MHz – are for downlink Both 2) and 3) are divided into 832 channels with 30 kHz bandwidth. 4. 30 kHz, 200 kHz, 1.25 MHz – the different bandwidths used in different ways by different companies in different locations. 700 to 800 Mhz – abandoned UHF TV channels for digital high-definition TV in 2009. 6. 1700 to 1750 Mhz – from military 7. 1900 to 2300 Mhz – available for 3G. Frequency Allocation:

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11 Types: 1. Frequency reuse 2. FDMA - the spectrum is divided into many smaller channels. 3. TDMA – multiple users use different time slots 4. CDMA – with unique coding, up to 64 subscribers can share a 1.25 Mhz channel. 5. SDMA – it uses highly directional antennas to pinpoint users and reject others on the same frequency. Multiple Access – refers how the subscribers are allocated to the assigned frequency spectrum.

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12 Frequency reuse – the process in which the same set of frequencies (channels) can be allocated to more than one cell, provided the cells are separated by sufficient distance. Cluster – groups of cells Frequency Reuse Cellular Telephone Systems

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13 The number of channels available in a cluster, F: F = GN The total channel capacity in a given area, C: C = mF Where: G = # of channels in a cell N = # of cells in a cluster = 3, 7, or 12 m = # of clusters in a given area Cellular Telephone Systems Frequency Reuse

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14 If the cluster size is reduced and cell size held constant, more clusters are required to cover a given area, and the total channel capacity increases. Therefore the channel capacity is directly proportional to the number of times the cluster is duplicated. The frequency reuse factor of a cellular telephone system is inversely proportional to the number of cells in a cluster. Therefore, each cell within a cluster is assigned 1/Nth of the total available channels in a cluster. Note: Cellular Telephone Systems

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15 Determine the number of channels per cluster and the total channel capacity for a cellular telephone area comprised or 10 clusters with seven cells in each cluster and 10 channels in each cell. Example: Cellular Telephone Systems F = 10(7) channels per cluster C = 10(7)(10) total channels

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16 Cellular Telephone Systems Frequency Reuse Factor, FRF: - the number of subscribers who can use the same set of frequencies in nonadjacent cells at the same time in a small area like city is generally 4. FRF = N/C Where: N = total number of full-duplex channels in an area C = total number of full-duplex channels in a cell Frequency Reuse

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17 Cellular Telephone Systems Note: Splitting the cells (each with its own base station) effectively allows more calls to be handled by the system, provided the cells do not become too small. If < 1500 feet in diameter, interference will occur between adjacent cells. The relationship between frequency reuse and cluster size determines how cellular telephone systems can be rescaled when subscriber density increases.

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18 Cellular Telephone Systems Cells – use a hexagonal shape, which provides exactly six equidistant neighboring cell are separated by multiples of 60°. Therefore, a limited number of cluster sizes and cell layouts is possible. To connect cells without gaps in between (tesselate), the geometry of a hexagon is such that the # of cells per cluster can have only values that satisfy the equation Where: i and j = nonnegative integers Frequency Reuse

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19 Cellular Telephone Systems The process of finding the tier with the nearest co channel cells (first tier): 1. Move i cells through the center of successive cells. 2. Turn 60° in a counter-clockwise direction. 3. Move j cells forward through the center of successive cells Frequency Reuse

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20 Determine the number of cells in a cluster and locate the first tier co-channel cells for the following values: j = 2 and i = 3. Example: Cellular Telephone Systems There six nearest first-tier 1 co-channel cells for cell A.

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21 Cellular Telephone Systems Two major kinds of interferences produced within a cellular telephone system: 1. Co-channel interference - the interference that occurs between co-channel cells (two cells using the same set of frequencies). Note: To reduce co-channel interference, a certain minimum distance must separate co-channels. It can’t be reduce by simply increasing transmit powers. Interference

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22 Cellular Telephone Systems Note: Interference between cells is proportional not to the distance between the two cells but rather to the ratio of the distance to the cell’s radius. Interference

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23 Since a cell’s radius is proportional to transmit power, more radio channels can be added to a system by either: 1. Decreasing the transmit power per cell. 2. Making cells smaller. 3. Filling vacated coverage areas with new cells. Cellular Telephone Systems Note: In a cellular system where all cells are approximately the same size, co-channel interference is dependent on the radius (R) of the cells and the distance to the center of the nearest co-channel cell (D).

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24 Co-channel reuse ratio, Q = D/R – increasing the D/R ratio increases the spatial separation between co-channel cells relative to the coverage distance. Cellular Telephone Systems Note: The smaller the value of Q, the larger the channel capacity since the cluster size is also smaller. However, a large value of Q improves the co-channel interference and, thus, the overall transmission quality.

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25 2. Adjacent-channel interference – occurs when transmissions from adjacent channels (channels next to one another in the frequency domain) interfere with each other. - results from imperfect filters in receivers that allow nearby frequencies to enter the receiver. Near-far effect: Adjacent-channel interference is most prevalent when an adjacent channel is transmitting very close to a mobile unit’s receiver at the same time the mobile unit is trying to receive transmission from the base station on an adjacent frequency. - most prevalent when a mobile unit is receiving a weak signal from the base station.

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27 Cellular Telephone Systems Two methods of increasing the capacity of a cellular system: 1. Cell Splitting 2. Sectoring 1. Cell Splitting – when the area of a cell, or independent component coverage areas of a cellular system, is further divided, thus creating more cell areas. - occurs when traffic levels in a cell reach the point where channel availability is jeopardized. - the process of subdividing highly congested cells into smaller cells each with their own base station and set of channel frequencies.

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28 With cell splitting, a large number of low-power transmitters take over an area previously served by a single, higher-powered transmitter. Note: Splitting cell areas creates new cells, providing an increase in the degree of frequency reuse, thus increasing the channel capacity of a cellular network. If the radius of a cell is divided in half, four times as many smaller cells could be created to provide service to the same coverage area.

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29 Maximum traffic load – the point when a cell reaches maximum capacity occurs when the number of subscribers wishing to place a call at any given time equals the number of channels in the cell. Blocking - if a new call is initiated in an area where all the channels are in use. More Base station transfers – the major drawback of cell splitting, more handoffs per call and a higher processing load per subscriber. It has been proven that a reduction of a cell radius by a factor of 4 produces a 10-fold increase in the handoff rate per subscriber.

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30 Example: Determine a. The channel capacity for a cellular telephone area comprised of seven macrocells with 10 channels per cell. b. Channel capacity if each macrocell is split into four minicells. c. Channel capacity if each minicell is further split into four microcells. Solution:

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31 2. Sectoring – another means of increasing the channel capacity of a cellular telephone system is to decrease the D/R ratio while maintaining the same cell radius. - capacity improvement can be achieved by reducing the number of cells in a cluster, thus increasing the frequency reuse. To accomplish this, the relative interference must be reduced without decreasing transmit power. - co-channel interference can be decreased by replacing a single omnidirectional antenna with several directional antennas, each radiating within a smaller area.

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32 Sectors – the smaller areas. Sectoring - decreasing co-channel interference while increasing capacity by using directional antennas. Space diversity - placing two receive antennas one above the other. It improves reception by effectively providing a larger target for signals radiated from mobile units. Cellular Telephone Systems Terms:

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33 Note: The separation between the two receive antennas depends on the height of the antennas above the ground. 30 m above ground: require 8λ separation 50 m above ground: require 11λ separation Cellular Telephone Systems

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34 Cellular Telephone Systems Techniques incorporated when additional cells are required within the reuse distance: 1. Segmentation – divides a group of channels into smaller groupings or segments of mutually exclusive frequencies; cell sites, which are within the reuse distance are assigned their own segment of the channel group. - a means of avoiding co-channel interference, although it lowers the capacity of a cell by enabling reuse inside the reuse distance, which is normally prohibited.

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35 Cellular Telephone Systems Techniques incorporated when additional cells are required within the reuse distance: 2. Dualization – a means of avoiding full-cell splitting where the entire area would otherwise need to be segmented into smaller cells. - its major drawback is that it requires an extra base station in the middle of a cell. There are now two base stations in a cell; one a high-power station that covers the entire secondary cell and one a low-power station that covers the smaller primary cell.

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36 Cellular Systems Topology Terms: Radio network – is defined by a set of radio-frequency transceivers located within each cells Base stations – the locations of radio-frequency transceivers, consists of a low-power radio transceiver, power amplifiers, a control unit (computer), and other hardware, depending on the system configuration. It can improve transmission quality, but they cannot increase the channel capacity within the fixed bandwidth of the network. It serves as central control for all users within that cell. - are distributed over the area of system coverage.

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37 Cellular Systems Topology Terms: Cell-site controller – handles all cell-site control and switching functions. Mobile Telephone Switching Office (MTSO) – controls channel assignment, call processing, cal setup, and call termination which includes signaling, switching, supervision, and allocating radio-frequency channels. – provides a centralized administration and maintenance point for the entire network and interfaces with the PTN over wireline voice trunks to honor services from conventional wireline telephone subscribers.

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39 Cellular Telephone Systems Roaming and Handoffs Roaming - is when a mobile unit moves from one cell to another – possibly from one company’s service area into another company’s service area (requiring roaming agreements). Handoff / Handover – the transfer of a mobile unit from one base station’s control to another base station’s control. 4 stages of handoff: 1. Initiation – either the mobile unit of the network determines the need for a handoff and initiates the necessary network procedures.

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40 Cellular Telephone Systems 4 Stages of handoff: 2. Resource reservation – appropriate network procedures reserve the resources needed to support the handoff (i.e. a voice and a control channel). 3. Execution – the actual transfer of control from one base station to another base station takes place. 4. Completion – unnecessary network resources are relinquished and made available to another mobile units.

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41 Cellular Telephone Systems Types of Handoff: 1. Hard Handoff – a connection that is momentarily broken during the cell-to-cell transfer. It is a break-before-make process. - generally occur when a mobile unit is passed between disjointed systems with different frequency assignments, air interface characteristics, or technologies. 2. Soft Handoff – a flawless handoff, normally takes approximately 200ms, which is imperceptible to voice telephone users, although the delay may be disruptive when transmitting data

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43 6 Essential Components: 1. Electronic Switching Center – the heart of a cellular telephone system. It controls switching between the public wireline telephone network and the cell-site base stations for wireline-to-mobile... 2.Cell-site Controller– manage each of the radio channels at each site, supervise calls, tx/rx on off. 3. Radio transceivers - transmitter/receiver 4. System interconnections – used four-wire leased lines to connect switching centers to cell sites and to PTN. 5. Mobile telephone units 6. Common Communications Protocol - governs the way telephone calls are established and disconnected.

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44 Cellular Telephone Call Processing 1. User channel – the actual voice channel where mobile users communicate directly with other mobile and wireline subscribers through a base station. 2. Control channel – used for transferring control and diagnostic information between mobile users and a central cellular telephone switch through a base station. Note: Base stations transmit on the forward control channel and forward voice channel & receive on the reverse control & reverse voice channel.

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45 Cellular Telephone Systems 3 Types of Calls: 1. Cellular-to-PSTN 2. Cellular-to-Cellular 3. PSTN-to-Cellular Digital Cell Phone Systems - developed primarily to expand the capacity of the cell phone systems already in place. Advantages: - more reliable in a noisy environment - digital circuits can be made smaller and more power-efficient, and therefore handsets can be more compact and can operate for longer times.

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46 Cellular Telephone Systems Advantages: - digital cell phones greatly facilitate the transmission of data as well as voice so that data services such as e-mail and Internet access are possible. 2G Cell Phone Systems - most modern digital cell phones 1. GSM – Global System for Mobile Communications – uses TDMA, uses a compression scheme that allows eight telephone calls to be transmitted concurrently in a single 200 kHz wide channel; GMSK modulation.

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47 - basic data rate is 270 kbps in the 200 khz channel. - considerable error detection and correction coding is used to improve the reliability in the presence of noise, multipath fading, & interference. - also uses a frequency-hopping scheme to minimize inter-channel interference. 2. IS – 136 - the Telecommunications Industry Association standard that fully describes TDMA. - 7.4 kbps data rate; permits three subscribers to concurrently use a single 30 khz channel. - uses separate channels for simultaneous transmit and receive; QPSK modulation. 3. Spread Spectrum

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48 2.5G Cell Phone Systems - refers to a generation of cell phones between the original 2G digital phones and the newer 3G phones. - bring data transmission capability to 2G phones in addition to normal voice service. - permits subscribers to exchange e-mails and access the Internet by cell phone. - because of the small screen size and a small or very restricted keyboard, data transmission capability is limited but available to those who need it. Cellular Telephone Systems

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49 1. General Packet Radio Service, GPRS –uses one or more of the eight TDMA time slots in a GSM phone system to transmit data rather than digitized voice. - data rate is from 20 kbps up to 160 kbps - typical rate is about 40 kbps, which is more than enough for e-mail and short message service but poor for Internet access. - involves an automatic rate adjustment algorithm that adjusts the class and data rate to the robustness of the wireless channel. 2. Enhanced Data Rate for GSM Evolution, EDGE - based upon GPRS system but uses 8-PSK modulation to achieve even higher data rates up to 384 kbps, thereby tripling the rate.

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50 - uses the GPRS class concept whereby the data rate is a function of the encoding and the number of time slots used. - theoretical max. data rate is 473.6 kbps with all eight slots used; typical everyday rates are usually over 100 kbps but less than 200 kbps. - if implemented, EDGE needs linear power amplifiers at the base station and in the handset. 3. CDMA2000 – uses 1.25 MHz wide channels - packet-based; permits a data rate of 144 kbps - uses three 1.25 MHZ channels = 3.75 MHz - Evolution-Data Optimized, a recent version has higher rate approaching 3.1 Mbps downlink and an uplink rate up to 1.8 Mbps. These speeds qualify for 3G.

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51 3G Cell Phone Systems - are true packet data phones - feature enhanced digital voice and high-speed data transmission capability. - described by the term International Mobile Telecommunications 2000; CDMA 2000. - frequency range: 1800 to 2200 MHz. - can achieve a data rate up to 2.048 Mbps in a fixed position; 384 kbps in a slow-moving pedestrian environment, and 144 kbps in a fast mobile environment. - include fast e-mail and Internet access - permits the transmission of video - subscribers can watch a movie

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