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Handover and Channel Assignment in Mobile Cellular Networks
Sirin Tekinay and Bijan Jabbari
Commonly used handover strategies are described.
- Handoff: Handoff critical to an efficient system. Handoff
related system characteristics include probability of blocking
new traffic, probability of forced termination, delay in channel
assignment, and total carried traffic.
- Fixed schemes:
- Basic: A permanent set of channels is assigned
to each cell. "Full" cells imply blocked calls
and dropped calls during handoff.
- Simple Borrowing: If all permanent channels in a
cell are busy, unused channels can be borrowed from a
neighbour, assuming this doesn't cause interference. A
calculation of additional cells now prohibited from
borrowed frequency is performed. In heavy traffic,
this implies degraded channel utilization.
- Hybrid: Permanent channels of a cell are split
into two groups - one can only be used locally, a
second can be lent to other cells. Benefits of
borrowing but mitigates degradation by limiting extra
- Borrowing with Channel Ordering: Like hybrid, but
with dynamic variation of local-to-borrowable channel
ratio according to current traffic condition.
- Flexible (Scheduled): Each cell has assigned
permanent channels. The MSC (mobile switching center)
also has a pool of flexible channels that it can assign
to cells that need extra. The assignment is done on a
scheduled basis according to known traffic
distributions over time.
- Flexible (Predictive): Same as scheduled, but
assignment is done based on current measurements of
traffic intensity. This rquires the MSC to have
current information about traffic patterns.
- Dynamic (call-by-call): All handoffs and call attempts are
refered to the MSC, which assigns frequencies on a case-by-case
basis according to a minimum cost function. Again, the MSC must
have load information and radio channel measurements of MS's
- Handover decisions: Made by both the MS and BS by
monitoring channel quality, or by MS alone. The new BS is
determined by the MSC or MS. Measurements must discriminate
temporary fades, but still react quickly enough to avoid call
dropping or interruption. Policy may dictate that BS's accept
handovers with more priority than call initiation to avoid
irritating customers. "Guard" channels may be reserved
for handover traffic, although this may decrease spectrum
- Thresholds: The handover threshold is defined as the
time at which a new BS's signal strength is greater than the
current BS's. The receiver threshold is the time at which
the current BS's signal is too weak to be usable. A MS must
handover during the interval between these thresholds; handover
requests may be queued during this interval.
Handovers must be efficient and reliable in a cellular network; this paper
explores common handover strategies.
- The paper is far more descriptive than analytical - it seems to
be a literature survey.
- Nothing is said about multicast handoff strategies - perhaps the
article predates this idea.
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