MC-Mobile Communication Systems-Lecture Slides, Slides of Mobile Communication Systems

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C O N N E C T I V I T Y

MC

Mobile Communication Systems

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Radio Wave Propagation

• Important Facts  The waves from an antenna spreads out in a spherical fashion  The more the power of transmitter is the more farther the signal will travel  Radio signals are absorbed by buildings and vegetation  Radio signals are reflected from surroundings
• Signal attenuation  Distance related  Fast Fading  Slow Fading

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Degradation o f signal strength with small change in location-

Strength of signal at a MS (receiver) at walking speed, during 1 second

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Convenient unit for radio signal

• Measuring signal strength require efficient units to avoid large numbers
• Bel  Signal strength measured in log 10  Bel is not good for writing down small changes in signal strength
• Decibel  Bel multiplied by 10  DeciBel is a ratio , it measures the strength of a signal relative to other signal
• Example  A signal 10 dB stronger than other mean it is 10 times stronger  A signal 20 dB stronger than other mean it is 100 time stronger
• Commonly a 3 dB signal is twice stronger than the other
• Unit of signal power  Watt or KWatt
• Finally  DeciBelWatt (dBW)

Fading

• Slow Fading  Single strength varies slowly or signal weakens because of obstacles  e.g. behind large structure such as building
• Fast Fading  Single strength varies sharply  Common reason is multipath propagation

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Slow Fading, Fast Fading

• Already discussed
• Example of Fast Fading  Assume data rate is 100kbps (kilo bits per second)  Signal used is 100KHz (approximately)  Time to transmit one bit is 1/100K = 10 us (micro second)  If a signal is reflected and it arrives at MS after 10 us than it will arrive at the time when the next bit is at the MS (receiver)  Bits at MS (receiver) will be distorted and will not be distinguished properly  A delay of 10 us occurs if reflection is from an obstacle 1.5 km far from the MS (receiver) Frequency Distance of obstacle causing reflection for ISI 100 KHz 3 Km 1MHz 300 m 100 MHz 3 m 1 GHz 0.3 m

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Reflection, Diffraction, Scattering

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Effects of Multipath Propagation

• Signals may cancel out due to phase differences
• Intersymbol Interference (ISI)  Sending narrow pulse at given frequency between fixed antenna and mobile unit  Channel may deliver multiple copies at different times  Delayed pulses act as noise making recovery of bit information difficult  Timing changes as mobile unit moves - Harder to design signal processing to filter out multipath effects

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Equalizers and ISI

• With higher frequencies the distance of reflection for ISI reduces
• In practice the obstacles around a caller can cause ISI
• Special devices are used to cope with ISI problem, these devices keep a short history of previous data so can detect new or old bits  Commonly termed as equalizers

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Predicting Cellular Coverage

• How large a cell should be?  A digital map of the area is prepared  Specialized software is used to process the map - The software uses algorithms to determine the range of radio antenna - A suitable point is selected for mounting BTS in the area after extensive calculations

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Sectorization

• Congested Cell  Divide in sector  A sector is formed by replacing omni directional antennas with directional antennas
• Directional Antennas  Transmits in a narrow beam  Transmits in a particular angle range say 1200
• Typical Sectors  Three antennas covering 120^0 each [ 3 X 120 = 360 ]  Six antennas covering 60^0 each

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sectorization..

• Each directional antennas require a different set of frequencies for transmission  The actual frequency range of the cell is divided by number of antennas and each antenna is assigned an equal portion of the total frequency range
• Can the frequencies be reused in sectors?  Same frequency cannot be used in more than one sector of a cell
• Do we have a capacity gain using sectors?  No significant capacity gain is obtained - The frequencies are not reused in sectors - The number of channels in the cell will remain same
• What advantages are brought about by sectors?  Increase in signal strength  Increased signal range  Reduced path loss - LOS is available on three different side  Reduced reflection - The signals are propagated in a narrow beam not in all directions

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micro cells..

• Number of mobile units are relatively less in a micro cell this leads to availability of channels and room for more mobile units in that area
• What disadvantages are brought about by micro cells  Handover problems
• As the cells are small in size the user can quickly move from one micro cell to other, less time is available for handover
• A user moving in a vehicle might face problem in handovers  Micro cells cannot cover the whole city [the whole area]
• Micro cells are typically for narrow areas where signal are likely to be very weak or inaccessible
• If the whole city is divided into micro cells there will remain gaps where micro cell’s signal will be unavailable

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micro cells..

School

Hospital

Super Market

Residential Area

Offices