Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-09-04
2001-04-17
Urban, Edward F. (Department: 2746)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S444000, C455S062000, C455S063300
Reexamination Certificate
active
06219554
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to cellular networks and, more particularly, to a system and method of allocating radio frequency channels in a cellular network environment.
BACKGROUND OF THE INVENTION
Currently, cellular radio frequency (“RF”) plans generally assign radio channels, or frequencies, to cells within an RF network based on forecasts and ongoing studies of traffic patterns. Unfortunately, this scheme, referred to as “Fixed Channel Allocation” or “FCA”, does not take into account the dynamic nature of traffic in certain portions of a coverage area. For example, in a high-density business corridor, the channel requirements in a subcell A during a given time period may be higher than the capacity of subcell A and the result in call blockage. At the same time, a neighboring subcell B may have idle channels due to less-than-capacity call rates.
One solution to this problem is to increase the number of channels assigned to subcell A to handle the higher call volumes at the expense of other cells and possibly violating existing reuse patterns. However, this does not account for the mobile nature of traffic, because at another period in time, subcell B may be experiencing call blockage, while channels in subcell A remain idle.
Another problem with FCA is that cell sites are shrinking as demand for capacity increases. Due to both irregularities in propagation and traffic distribution in these small cells, pre-assignment of channels becomes difficult.
Implementation of a centralized intelligence to monitor channel usage in various cells and assigning channels based on need is not practical, due to the large amount of overhead processing and messaging necessary to keep both the centralized intelligence and the individual cells apprised of channel usage. Additionally, inter-mobile telephone exchange (“MTX”) messaging would involve changes to IS-41 messaging.
A number of papers have been published describing a scheme known as “Adaptive Channel Allocation” or “ACA”. Most of these proposals deal with “Dynamic Channel Assignment” (“DCA”), which comprises the ability to dynamically allocate the entire spectrum based on need, without any frequency scanning per cell.
Other relevant channel assignment schemes include “Hybrid Channel Assignment” (“HCA”) and “Borrowing Channel Assignment” (“BCA”). In HCA, a combination of FCA and DCA, a portion of the total frequency channels uses FCA and the rest use DCA. In BCA, when all the fixed channels of a cell (“acceptor cell”) are occupied, the acceptor cell borrows free channels from a neighboring cell (donor cell). In generally, BCAs use some form of central control to lock out other cells from using the channel(s) borrowed by the acceptor cell. While BCA is effective under light to moderate traffic conditions, especially when compared to FCA, under heavy traffic, channel borrowing may be high enough to cause channel usage efficiency to drop drastically and increase blocking probability due to channel locking.
Segregation is described in the literature as a self-organizing dynamic channel allocation scheme. Channels are assigned probabilities of being clear based upon successful use of that channel for a call. When a call arrives at a cell, the determination of which channel to use to service the call is based on the current probabilities of the channels. Carriers sense (using received signal strength measurement) is performed on the selected channel (i.e., the channel with the highest probability) to ensure that it is clear to use. If not, the probability of that channel is decreased and the next channel is tried. If that channel is clear, the call is assigned to that channel and its probability is increased.
While segregation offers certain advantages over previously-described methods, it, too, suffers from certain deficiencies, including the fact that the time for convergence to optimal allocation is very long. In addition, the determination of carrier sense only at channel request is shown to cause extensive delay and if delay is limited to a finite wait time, performance of the system degrades rapidly.
Therefore, what is needed is an improved system and method of allocating radio frequency channels in a cellular network environment which take advantage of the best qualities of FCA and distributed channel borrowing utilizing a segregation scheme.
SUMMARY OF THE INVENTION
One embodiment of the invention, accordingly, is a Dynamic Frequency Association (“DFA”) technique comprising FCA and distributed channel borrowing techniques using a segregation scheme. The DFA technique described herein can be used autonomously to dynamically determine the best channels for a cell cluster. Additionally, a novel method of minimizing search delays at channel assignment by employing a channel usage history is disclosed.
In one aspect, as in FCA, each cell is assigned its nominal channels, if any, from the available frequency spectrum, with a fixed radio assigned to each of these frequencies, respectively. Additionally, in accordance with the teachings of the present invention, each cell is equipped with one or more radios designated as “DFA radios”. In operation, idle DFA radios scan channels that may be borrowed in order to build a probability matrix. This enables the cell to determine which channels to use for traffic prior to actual channel request by a mobile. This reduces the search delay experienced in classic segregation schemes.
The algorithm for implementing the DFA techniques of the present invention operates as follows. During low traffic periods, as previously indicated, FCA channels and DFA radios are assigned. Next, a scan list is determined to identify the channels that can potentially be used by a DFA radio in a subcell during its traffic peaks. The identified channels are placed in a scan list. Next, each individual cell uses its DFA radio(s) and Mobile Assisted Channel Allocation (IS-136 MACA) to cyclically scan each channel that may be borrowed to obtain reverse and forward RF information to determine whether the channel is clear. This functionality, referred to as “Scanning Mode”, is performed while fixed radios, and possibly other DFA radios, serve offered traffic.
Probabilities of being clear are then assigned to channels, with weight given to consecutive clear scans. A scan is determined to be clear if the RF signal strength of the channel does not exceed a predefine threshold, which is determined by the network in question. The channels are arranged in an updated, ordered “clear list.”
Before an overload condition occurs in which an acceptor cell exhausts all its available channels to service calls, that cell will attempt to borrow channels designated in the ordered clear list, using the DFA radios that were used to construct the ordered clear list. Only those channels whose probability is higher than a predefined minimum threshold may be selected for use. A DFA radio of the acceptor cell is then associated with, or tuned to, the selected channel, at which point the DFA radio is said to be in “Selection Mode.” Received signal strength on the channel is continuously monitored to ensure that it is below a predefined threshold. Because of the behavior of the DFA radio during the low traffic periods, the delay in performing this step is substantially decreased, expediting call service at the highest traffic periods.
New calls or handoffs may now be serviced by both fixed radios, as well as DFA radios that are in Selection Mode. If interference is detected on the borrowed channel, the DFA radio is returned to the Scanning Mode. However, if the DFA radio is still needed to service calls, it determines another channel that is free of interference, as described above, and returns to Selection Mode. The donor cell (i.e., the cell from which a channel is borrowed) detects the usage of one of its nominal channels by an acceptor cell is close enough, and uses that channel only as a last resort. This eliminates the central control problem of channel locking by allowing that channel to be used if necessary in the donor cell. Fin
Carter Daniel Thomas
Eswara Srinivas
McCarthy Michael John
Periyalwar Shalini
Wilson James Eric
Haynes and Boone LLP
Nortel Networks Limited
Smith Sheila
Urban Edward F.
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