Handover and Channel Assignment in Mobile Cellular Networks

One-line summary: Commonly used handover strategies are described.

Overview/Main Points

• 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 using the 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 frequency prohibitions.
  • Borrowing with Channel Ordering: Like hybrid, but with dynamic variation of local-to-borrowable channel ratio according to current traffic condition.
• Flexible:
  • 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 (mobile stations).
• 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 against 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 utilization.
• 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.

Relevance

Flaws

• 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.