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Material Type: Notes; Class: Wireless Communications; Subject: Electrical & Computer Engr; University: Villanova University; Term: Unknown 1989;
Typology: Study notes
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Early Mobile Telephone System Low Capacity: Consider that the assigned frequency band can afford 100 simultaneous conversations (channels).
Early Mobile Telephone System
Low Capacity: Consider that the assigned frequency band can afford 100 simultaneous conversations (channels).
Cellular System High Capacity: Consider that the assigned frequency band is reused in 10 cells, the system may afford 1000 simultaneous conversations (channels).
Cellular System High Capacity: Consider that the assigned frequency band is reused in 10 cells, the system may afford 1000 simultaneous conversations (channels).
l The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity.
l It offered very high capacity in a limited spectrum allocation without any major technological changes.
l The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity.
l It offered very high capacity in a limited spectrum allocation without any major technological changes.
Cellular System However, neighboring cells will interfere to each other. Therefore, they shall use different frequency bands. A cell cluster is outlined in bold, and replicated over the coverage area. Cells with the same letter use the same set of frequencies.
Cellular System
However, neighboring cells will interfere to each other. Therefore, they shall use different frequency bands.
A cell cluster is outlined in bold, and replicated over the coverage area. Cells with the same letter use the same set of frequencies.
In this example, the cluster size is N =7, and the frequency reuse factor is 1/7, since each cell contains 1/7 of the total number of available channels.
In this example, the cluster size is N =7, and the frequency reuse factor is 1/7, since each cell contains 1/7 of the total number of available channels.
Actual cellular footprint is determined by the contour of a given transmitting antenna.
Consider a cellular system
In order to connect without gaps between adjacent cells
where i and j are non-negative integers.
Problem: If a particular FDD cellular telephone system has a total bandwidth of 33 MHz, and if the phone system uses two 25 KHz simplex channels to provide full duplex voice and control channels...
Problem: If a particular FDD cellular telephone system has a total bandwidth of 33 MHz,
and if the phone system uses two 25 KHz simplex channels to provide full duplex voice and control channels...
Solution: Total bandwidth = 33 MHz Channel bandwidth = 25 KHz x 2 = 50 KHz Total available channels = 33 MHz / 50 KHz = 660
Solution: Total bandwidth = 33 MHz Channel bandwidth = 25 KHz x 2 = 50 KHz Total available channels = 33 MHz / 50 KHz = 660
Handoff: 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.
When designing handoff, it is important to bear in mind that it requires certain time to complete the handoff.
Handoff:
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.
When designing handoff, it is important to bear in mind that it requires certain time to complete the handoff.
(a) Improper Handoff Situation (a) Improper Handoff Situation
Received signal levelReceived signal level
Level at point ALevel at point A
Handoff thresholdHandoff threshold
Minimum acceptable signalMinimum acceptable signal to maintain the callto maintain the call
TimeTime
Level at point BLevel at point B
Pr, handoff
Pr, min usable
call is terminatedcall is terminated call is terminated
(b) Proper Handoff Situation (b) Proper Handoff Situation
Level at which handoff is made Level at which handoff is made
TimeTime
Level at point BLevel at point B
Received signal level Received signal level^ call properly transferred to BS
call properly transferred to BS2call properly transferred to BS
In deciding when to handoff, it is important to ensure that the drop in the measured signal level is not due to momentary fading.
Each base station constantly monitors the signal strength of all its reverse voice channels to determine the relative location of each mobile user with respect to the base station tower.
Dwell Time: time over which a call may be maintained within a cell, without hand-off.
Dwell time of a particular user is governed by a number of factors, including propagation, interference, distance between the subscriber and the base station, and other time-varying effects.
Major limiting factor in performance of cellular radio systems:
Signal to interference ratio (SIR, or S/I ) for a mobile receiver is given by:
co-channel cell base station
=
= = 0
1
SIR (^) i
i
Ii
S I
S
