Conventional Blocking Analysis for CDMA


Conventional Blocking formula and its Analysis for cdma.

In AMPS and TDMA systems, voice/traffic channels are assigned to users as long as they are available. Given the required offered traffic, the Erlang B model is used to determine the number of traffic channels required to provide a predetermined grade of service. The Erlang B model is based upon a model of serving without queuing. In other words, all blocked calls are cleared.

Traffic load is the product of call rate and call holding time. It is a dimensionless quantity
measured in Erlangs. One Erlang is the traffic intensity of a traffic channel which is  continuously occupied. Grade of service is a term used to quantify the extent to which congestion occurs in any trunking system and is typically expressed as the probability of finding blocking. Blocking in AMPS and TDMA is defined to occur when all voice frequencies (for AMPS) or time slots (for TDMA) have been assigned to other subscriber stations.

The values quoted for traffic load and grade of service for cellular systems are usually taken during the busy hour. Busy hour is defined as the continuous one-hour period in the day during which the highest average traffic density is experienced by the system.

The Erlang B formula is given by:

P blocking = (Ac/ci)/((cEk=0)*(Ak/Ki))

A is the offered traffic
C is the number of available servers

Assumptions of the Erlang “B” Model:

  1. The number of potential users is infinite
  2. Intervals between originations are random
  3. Call holding times are random
  4. Call set up time is negligible