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).
Prioritizing Handoffs
•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.
Prioritizing Handoffs
•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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