COMP1706: MOBILE AND NETWORK TECHNOLOGIES Cellular technologies Dr. George Loukas University of Greenwich

Handheld mobile phones

mobile service was only provided by one high powered transmitter/receiver typically supported about 25 channels had a radius of about 80km Prior to cellular radio

1 st Gen. Cellular Networks  Divide the area into cells using multiple low power transmitters in each cell  tiling pattern to provide full coverage (each cell is km)  each with own antenna  each with own range of frequencies  served by a base station  consisting of transceiver (transmitter – receiver) and control unit  adjacent cells use different frequencies to avoid crosstalk  but cells sufficiently distant can use same frequency band

Cellular Geometries All area is covered nicely, BUT antennas (at the centres of the squares) are not equidistant 1 1 Equidistant BUT There are gaps (or overlaps) between the circles Equidistant No gaps 1 1 Squares Circles Hexagons

Cellular Geometries 1 1 Hexagons For the same reasons, hexagons are also very common in board and computer games Equidistant No gaps

Shape of cell Not really a perfect hexagon. It depends on geography (buildings, mountains etc.) transceiver

Operation of Cellular System A base station (BS) at centre of cell. Each BS has one or more antennas, a controller (handling the call process) and a number of transceivers (for communicating on the channels) Between the mobile unit and the base station: Control channels exchange information for setting up and maintaining calls and establishing a relationship between a mobile unit and the nearest BS. Traffic channels carry voice or data connection between users. Each BS is connected to a Mobile Telecommunications Switching Office (MTSO)

Call Stages Monitor for strongest signal Request connection Paging Call accepted Ongoing Call Handoff (aka handover) (As the mobile units move, they pass from cell to cell, which requires transferring of the call from one base transceiver to another.) MTSOMTSO

Other operations Call Blocking : If all traffic channels assigned to the nearest base station are busy, the mobile phone makes a preconfigured number of repeated attempts. If they all fail, it returns a busy tone to the user. Call termination : When one of the two users hangs up, the MTSO is informed and the traffic channels at the two base stations are released. Call drop : Due to interference or weak signal, a base station may not be able to maintain the minimum required signal strength for a certain period of time. The traffic channel to the user is dropped and the MTSO is informed

Propagation Effects signal strength  Depends on distance from BS and on environment  Signal between BS and mobile unit needs to be strong enough to maintain signal quality  but not too strong so as to create co-channel interference  and must handle variations in noise transceiver Types of transmission Impairment Attenuation Attenuation distortion Cross-talk noise Delay distortion Impulse noise Inter modulation noise Thermal noise

Propagation Effects: Fading Fading: Time variation of received signal caused by changes in transmission paths Even if signal strength is in effective range, signal propagation effects may disrupt the signal Fast Fading. Rapid changes in strength over half wavelength distances. Slow Fading. Slower changes due to user passing different height buildings, gaps in buildings etc. Flat Fading. Affects all frequencies in same proportion simultaneously. Selective Fading. Affects different frequency components differently. transceiver

Okumura-Hata model Based on measurements made in Tokyo in Estimates the mean path loss L dB. (the attenuation in dB) λ Wavelength f carrier frequency (Between 150 and 1500 MHz) r distance (between 1 and 10 km) h b base station antenna height (between 30 and 200 m) h m receiver antenna height (between 1 and 10 m) h 0 height of a building For a small city: L dB = *log 10 f c – 13.82*log 10 h b + (44.9 – 6.55 log 10 h b )*log 10 R + (1.56*log 10 fc – 0.8) - (1.1*log 10 fc – 0.7)*h m It is OK to forget this!

Propagation Effects: Multipath Propagation R eflection. A signal encounters a large surface. The reflected waves may interfere (positively or negatively). D iffraction. At the edge of a large impenetrable body. Helps receive signals even without line of sight. S cattering. If the size of the obstacle is on the order of the wavelength of the signal or less, it may scatter into several weaker signals. Lamp post

Error Compensation Mechanisms  Forward error correction (Typical ratio of total bits to data bits is 2-3:1)  Diversity (e.g. through spread spectrum)  by space, frequency or time Since different channels experience different fading:  send parts of a signal over different channels

INTERMISSION

Frequency Reuse  Power of Base Transceiver controlled  Allows communication within cell on given frequency  Limits power escaping to adjacent cells  Sharing cell frequencies with nearby (but not adjacent) cells without interfering with each other  Allows multiple simultaneous conversations  10 to 50 frequencies per cell transceiver

Frequency Reuse Patterns Typical parameters:  Reuse factor N = number of cells in a repetitious pattern (each cell in the pattern uses a unique band of frequencies)  R = radius of a cell  D = minimum distance between centers of cells that use the same band of frequencies

Frequency Reuse Patterns Typical parameters:  Reuse factor N = number of cells in a repetitious pattern (each cell in the pattern uses a unique band of frequencies)  R = radius of a cell  D = minimum distance between centers of cells that use the same band of frequencies Area A = 3√3R 2 /2 = 2.598*R 2 Number of channels in one cell = total number of channels / N D = R* √(3N)

Exercise Total area covered = 32 x 6.65 = 213 km 2 Total area covered = 133 x 1.66 = 221 km 2 Consider a geographical area A divided into (a) 32 hexagonal cells of 1.6 km radius or (b) 133 hexagonal cells of 0.8 km radius. The reuse factor is 7 and there are 336 channels in total. Calculate: i)the number of channels per cell ii)the maximum number of concurrent calls that can be handled in A iii)the total area covered 336 / 7 = 48 channels per cell (a) ii) Total channel capacity (number of concurrent calls that can be handled) = 48 x 32 = 1,536 channels (b) ii) Total channel capacity (number of concurrent calls that can be handled) = 48 x 133 = 6,384 channels (a) iii) 32 cells, each with radius R = 1.6 km Area A = 2.598*R2 = 6.65 km 2 (b) iii) 32 cells, each with radius R = 0.8 km Area A = 2.598*R2 = 1.66 km 2

Another exercise A certain city has an area of 1,289 square miles and is covered by a cellular system using a 7-cell reuse pattern. Each cell has a radius of 4 miles and the city is allocated 84 MHz of spectrum with a full duplex channel bandwidth of 60 kHz. A) What is the minimum distance between centres of cells that use the same band of frequencies? B) what is the number of cells in the city? C) What is the number of channels per cell? B) R = 4 miles Area of a cell A = 2.598*4 2 = square miles Total number of cells = 1,289 / = 31 C) Total number of channels = allocated spectrum / channel bandwidth = 84,000,000 / 60,000 = 1,400 Number of channels per cell = total number of channels / N = 1,400 / 7 = 200 A)D = R*√(3N) = 4 * √(3*7)= 4 * √21 = miles

And another A total of 33MHz of bandwidth is allocated to a particular cellular telephone system which uses two 25kHz simplex channels to provide full duplex voice and control channels. If 1 MHz of the allocated spectrum is dedicated to control channels, how many voice channels should be allocated in each cell if the re-use factor N is 4. Assume that all cells have the same number of voice channels. A total of 33MHz of bandwidth is allocated to a particular cellular telephone system which uses two 25kHz simplex channels to provide full duplex voice and control channels. If 1 MHz of the allocated spectrum is dedicated to control channels, how many voice channels should be allocated in each cell if the re-use factor N is 4. Assume that all cells have the same number of voice channels. Total bandwidth = 33 MHz Full duplex channel bandwidth = 2*25 = 50 KHz Total available channels = 33,000,000 / 50,000 = 660 channels 1 MHz for control channels means that there are 1,000,000/50,000 = 20 control channels So, the total number of voice channels is 660 – 20 = 640 In each cell, there are 640 / 4 = 160 voice channels

Increasing Capacity  add new channels  frequency borrowing congested cells take frequencies from adjacent cells assign frequencies dynamically  cell splitting use smaller cells in high use areas

Increasing Capacity: Cell Splitting Cells can be divided to provide more capacity. To use a smaller cell, the power level must be reduced to keep the signal within the cell. The smaller the cells, the more frequent the handoffs.

Increasing Capacity: Cell Sectoring Each sector is assigned a separate subset of the cell’s channels. This reduces transmission power and increases battery life Single omni-directional antenna Three directional antennas (120 o sectoring) Six directional antennas (60 o sectoring)

Cell sizes MACRO CELL 1 – 30 km (large areas) MACRO CELL 1 – 30 km (large areas) MICRO CELL 200 – 2,000 m (One block of an urban street grid) MICRO CELL 200 – 2,000 m (One block of an urban street grid) PICO CELL (Shopping malls, large office buildings) PICO CELL (Shopping malls, large office buildings) FEMTO CELL (small buildings, houses) FEMTO CELL (small buildings, houses)

Large Vs. Small cell sizes Decreasing the cell size, _______________ the capacity increases _______________ the number of handovers _______________ the mobile phones’ power consumption increases decreases

Design Factors When designing a mobile phone network, we need to take into account: Geography - Propagation effects (difficult to predict. Often using Okumura- Hata model for path loss) desired maximum transmit power level at BS and mobile units typical height of mobile unit antennas available height of the base station antenna Map of base stations around Greenwich from