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Cellular Concept and Frequency Reuse, Channel Assignment and Handoffs
• • When a user/call moves to a new cell, then a new base station and new channel should be assigned (handoff)
• • Handoffs should be transparent to users, while their number should be kept to minimum
• A threshold in the received power (Pr, handoff process. This threshold value should be larger than the minimum acceptable received power (Pr, acceptable)
• Define: - Δ=Pr, handoff Pr, acceptable
–If D is large then too many handoffs .
–If D is small then insufficient time t.
• • In order to correctly determine the beginning of handoff, we need to determine that a drop in the signal strength is not due to the momentary (temporary) bad channel condition, but it is due to the fact that the mobile is moving away from BS.
• • Thus the BS needs to monitor the signal level for a certain period of time before initiating a handoff. The length of the time (running average measurements of signal) and handoff process depends on speed and moving pattern.
• • First generation systems typical time interval to make a handoff was 10 seconds (large Δ). Second generations and after typical time interval to make a handoff is 1-2 seconds (small Δ).
•First generation systems: handoff decision was made by BS by measuring the signal strength in reverse channels.
•Second generation and after: Mobile Assisted Hand-Off (MAHO).
Mobiles measure the signal strength from different neighboring BSs. Handoff is initiated if the signal strength from a neighboring BS is higher than the curr signal strength.
Cell Dwell Time
• It is the time over which a call maybe maintained within a cell (without handoff).
• It depends on: propagation, interference, distance between BS and MS, speed and moving pattern (direction), etc.
• Highway moving pattern: the cell dwell time is ar.v. with distribution highly concentrated around the mean.
• Other micro-cell moving patterns mix of different user types with large variations of dwell time (around the mean).
•Guard Channels: Fraction of total bandwidth in a cell is reserved for exclusive use of handoff calls. Therefore, total carried traffic is reduced if fixed channel assignment is used. However, if dynamic channel assignment is used the guard channel mechanisms may offer efficient spectrum utilization.
–Number of channels to be reserved: If it is low (under-reservation) the QoS on handoff call blocking probability can not be met. If reservation is high (over-reservation) may result in waste of resources and rejection of large number of new calls.
–Static and Dynamic schemes: Advantage of static scheme is its simplicity since no communication and computation overheads are involved.
· However problems of under- reservation and over reservations may occur if traffic does not conform to prior knowledge.
· Dynamic schemes may adjust better to changing traffic conditions.
•Queuing Handoffs: The objective is to decrease the probability of forced determination of a call due to lack of available channels. When a handoff call (and in some schemes a new call) can not be granted the required resources at the time of its arrival, the request is put in a queue waiting for its admitting conditions to be met.
–This is achieved because there is a finite time interval between the time that the signal of a call drops below the handoff threshold, and the time that the call is terminated due to low (unacceptable) signal level. Queuing and size of buffer depends on traffic and QoS. Queueing in wireless systems is possible because signaling is done on separate control channels (without affecting the data transmission channels).
• • According to the types of calls that are queued, queuing priority schemes are classified as: handoff call queuing, new call queuing and handoff/new call queuing (handoff calls are given non-preemptive priority over new calls).
Practical Issues (Capacity/Handoff)
• To increase capacity, use more cells (add extra sites).
• Using different antenna heights and powers, we can provide ―large‖ and ―small‖ cells co-located at a signal location (it is used especially to handle high speed users and low speed users simultaneously.
• Reuse partitioning (use of different reuse patterns)
• Cell splitting: Change cell radius R and keep co-channel reuse ratio (D/R) unchanged. If R‘=R/2 than the transmit power needs to be changed by (1/2)4 = 1/16.
• Another way is to keep cell radius R unchanged and decrease D/R ratio required (that is decrease the number of cells in a cluster). To do this it is required to decrease interference without decreasing transmit power.
• Sectoring: Use directional antennas (instead of omni-directional) and therefore you receive interference from only a fraction of the neighboring cells.
• Hard handoffs vs. soft handoffs: more than one BSs handle the call during handoff phase (used in CDMA systems)
Super audio tone (SAT): SAT is superimposed on the voice signal on both the forward and reverse link and is barely audible to the user
• • The particular frequency of the SAT denotes the particular base station location for a given channel and is assigned by the MSC for each call.
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