A new call can only be connected by an unused channel. If all the channel are occupied than the new call is blocked in this system. There are however several dynamic radio-resource management schemes that can be combined with FCA. A simple form is traffic-adaptive handover threshold, implying that calls from cell phones situated in the overlap between two adjacent cells can be forced to make the handover to the cell with the lowest load for the moment.
If FCA is combined with spread spectrum, the maximum number of channels is not fixed in theory, but in practice a maximum limit is applied, since too many calls would cause too high co-channel interference level, causing the quality to be problematic.
Spread spectrum allows cell breathing to be applied, by allowing an overloaded cell to borrow capacity maximum number of simultaneous calls in the cell from a nearby cell that is sharing the same frequency. The access points automatically select frequency channels with low interference levels. The channel is allocated following an algorithm which accounts the following criteria: Future blocking probability in neighboring cells and Reuse distance Usage frequency of the candidate channel Average blocking probability of the overall system Instantaneous channel occupancy distribution It requires the MSC to collect real time data on channel occupancy, traffic distribution and Received Signal Strength Indications RSSI.
There are two channel assignment strategies in cellular system. Fixed channel assignment: In fixed channel assignment each cell is permanently allocated predetermined group of channels. Any call attempt within cell can only be served by unused channels in that particular cell. If all channels are occupied, the call is blocked and subscriber does not receive service. The major problem with channel borrowing is that when a cell borrows a channel from a neighboring cell, other nearby cells are prohibited from using the borrowed channel because of co-channel interference.
This can lead to increased call blocking over time. To reduce this call blocking penalty, algorithms are necessary to ensure that the channels are borrowed from the most available neighboring cells; i. Borrowing with Channel Locking was designed as an improvement over the simpler Channel Borrowing approach as described above [Elnoubi].
BCO systems have two distinctive characteristics [Elnoubi]: The ratio of fixed to dynamic channels varies with traffic load. Nominal channels are ordered such that the first nominal channel of a cell has the highest priority of being applied to a call within the cell. The last nominal channel is most likely to be borrowed by neighboring channels. Once a channel is borrowed, that channel is locked in the co-channel cells within the reuse distance of the cell in question.
To be "locked" means that a channel can not be used or borrowed. From a frequency reuse standpoint, in a BCO system, a channel may be borrowed only if it is free in the neighboring cochannel cells.
This criteria is often too strict. In Borrowing with Directional Channel Locking, borrowed channels are only locked in nearby cells that are affected by the borrowing. This differs from the BCO scheme in which a borrowed channel is locked in every cell within the reuse distance. The benefit of BDCL is that more channels are available in the presence of borrowing and subsequent call blocking is reduced.
A disadvantage of BDCL is that the statement "borrowed channels are only locked in nearby cells that are affected by the borrowing" requires a clear understanding of the term "affected.
Ideally, a system can be general enough that detailed analysis of specific propagation measurements is not necessary for implementation. This animation page also gives you the opportunity to try your luck as a network operator. A natural extension of channel borrowing is to set aside a portion of the channels in a system as dynamic channels with the remaining nominal channels being fixed to specified cells. If a cell requires an extra channel, instead of borrowing the channel from a neighboring cell, the channel is borrowed from the common "bank" of dynamic channels.
An important consideration in hybrid systems of this type is the ratio of dynamic channels to fixed channels. Analysis by Cox and Reudlink [Cox - ] showed that given ten channels per cell, an optimum ratio was 8 fixed channels and 2 dynamic channels.
In general, the optimum ratio depends upon the traffic load [Zhang]. Referred to as Locally Optimized Dynamic Assignment Strategy LODA , this method is best described as a purely dynamic channel allocation procedure as opposed to a hybrid method.
In this strategy there are no nominal channels; all channels are dynamic. When a given cell needs to accommodate a call, it chooses from among the bank of available channels according to some cost criteria.
The channel with minimum cost is assigned. In a general sense, the cost is a measure of the future blocking probability in the vicinity of the cell given that the candidate channel is assigned. A more detailed description of the cost function will be addressed below. Whereas a DCA scheme allocates a channel to an initial call or handover , a DCR system switches a cell's channel that is currently being used to another channel which is closer to the optimum according to frequency reuse or other cost criteria.
In comparing performance, typical system metrics include blocking probability of new calls and blocking probability of handover calls.
These metrics are written as functions of offered traffic where the traffic may be written in a variety of forms. It is generally assumed that a blocked new call is preferred over a blocked hand-off call.
The idea being that with a blocked hand-off, users are forced to terminate communication in the middle of their session. If this blocking happens at a particularly inopportune time, the results could be disastrous e. In the case of a blocked new call, at least the business negotiation hasn't started and the involved parties aren't interrupted. This is consistent with the assumption of small offered traffic loads. Cox and Reudink were the first researchers to present published comparisons of different channel allocation schemes.
Their comparison was based on simulation of an outdoor vehicular wireless communication system [Cox - , Cox - , Jakes]. The simulation divided a region into a grid of square cells. The movement of vehicles had a two dimensional normal distribution with 0 mean and 30 mph standard deviation in each of the two orthogonal directions. Poisson arrivals were assumed for the rate of calls per vehicle and call durations were assume to have a truncated normal distribution truncated on the left at zero with a "mean" 90 seconds true mean ofThe communication is allocated following an algorithm which accounts the following criteria: Future blocking probability in neighboring cells schemes could handle more calls than fixed channel allocation Average blocking probability of the mobile system Instantaneous channel occupancy assignment It requires the MSC to collect channel. Combinatorial Optimization  Aarts, E. For high system spectrum efficiencyDCA should be fixed Carbon nano tube synthesis protein schemes. These metrics are written as functions of offered traffic station including borrowing ensuring that borrowing does not interfere with any call in progress in donor cell.
Referred to as Locally Optimized Dynamic Assignment Strategy LODA , this method is best described as a purely dynamic channel allocation procedure as opposed to a hybrid method. Any call attempt within cell can only be served by unused channels in that particular cell. For efficient operation, FCA systems typically allocate channels in a manner that maximizes frequency reuse. This frees the dynamic channel for future use and ensures that a large number of channels being used are the optimally-spaced, fixed channels.
The access points automatically select frequency channels with low interference levels.
This differs from the BCO scheme in which a borrowed channel is locked in every cell within the reuse distance.
From a frequency reuse standpoint, in a BCO system, a channel may be borrowed only if it is free in the neighboring cochannel cells. In Borrowing with Directional Channel Locking, borrowed channels are only locked in nearby cells that are affected by the borrowing.
The virtue of the cellular system is that different cells can use the same channel given that the cells are separated by a minimum distance according to the system propagation characteristics; otherwise, intercellular or cochannel interference occurs. To reduce this call blocking penalty, algorithms are necessary to ensure that the channels are borrowed from the most available neighboring cells; i. This is consistent with the assumption of small offered traffic loads. Several methods have been presented that fall within this category and in addition, a great deal of comparison has been made with corresponding simulations and analyses [Cox, Elnoubi, Jiang, Katzela, Yue, Zhang].
In the case of a blocked new call, at least the business negotiation hasn't started and the involved parties aren't interrupted. If a cell needs a channel in excess of the channels previously assigned to it, that cell may borrow a channel from one of its neighboring cells given that a channel is available and use of this channel won't violate frequency reuse requirements.