In a wireless telecommunications system, such as a wireless local area network (LAN), telecommunications data is transported between a base station (i.e., an access point) and one or more wireless terminals located in a broadcast region associated with the base station. The telecommunications data that is transported between the base station and the one or more wireless terminals located in the broadcast region associated with the base station is, of course, transmitted in accordance with a particular telecommunications data protocol. One relatively well-known telecommunications protocol is the medium access control (MAC) protocol. The MAC protocol, as will be described in greater detail below, provides a time division multiple access, time division duplex (TDMA/TDD) frame structure, which comprises a downlink portion and an uplink portion. The downlink and uplink portions are, in turn, subdivided into a number of physical channels or time slots. By assigning each of the one or more wireless terminals associated with a base station to a particular time slot, in the downlink and to a particular time slot in the uplink, the base station can communicate with each of the one or more wireless terminals over a single frequency carrier that has been assigned to that base station. The use of a MAC protocol in a wireless LAN is also known. For instance, the European Telecommunications Standardization Institute (ETSI) has decided to employ a MAC protocol as the standard for the High Performance Local Area Network Type Two (Hiperlan Type 2).
In a wireless LAN, assigning and/or reassigning a frequency to each of the base stations in the network is a very important process. It is of particular importance that the frequencies be assigned in such a manner that the interference being experienced is at an acceptable level The process of assigning frequencies is especially complicated, however, with indoor networks such as Hiperlan 2, where the propagation environment is somewhat unpredictable. Further complicating matters is the fact that interference levels tend to increase dramatically as new base stations and wireless terminals are added to an expanding network. These and other complexities associated with the assignment of frequencies in indoor networks, such as Hiperlan 2, have led to the development of automatic frequency assignment techniques.
A significant amount of work has, in fact, been done with developing automatic frequency assignment algorithms. See, for example, Almgren et al., "Slow Adaptive Channel Allocation for Automatic Frequency Planning", Proceedings of the IEEE 5.sup.th ICUPC, 1996. In accordance with conventional automatic frequency assignment algorithms, it is necessary to first characterize the propagation environment, for instance, periodically measure the radio interference levels associated with the various frequencies that may be assigned to a base station. A typical algorithm then uses the interference measurement information to assign and/or reassign a frequency to the various base stations in the network in such a way that interference is minimized and signal quality is maximized.
There are a number of problems associated with making interference level measurements, particularly in wireless LANs. For example, in a wireless LAN such as a Hiperlan 2 based network, the wireless terminals are receiving information from their corresponding base station (e.g., downloading internet information) far more than they are transmitting information up to the base station. Accordingly, a vast majority of the information and data traffic is concentrated over the downlink portion of each MAC frame. Therefore, the remaining portion of each MAC frame, which is set aside for uplink purposes, is relatively small, and it is, in general, an insufficient period of time to make the necessary interference measurements. One solution to this problem has been to use a receive antenna which is exclusively dedicated to making interference measurements. However, as one skilled in the art will appreciate, a dedicated receive antenna has to be physically separated from the antenna being used to transmit and receive ordinary telecommunications traffic between the base station and the wireless terminals in the corresponding broadcast region by as much as 1 meter. Such an option is generally unattractive both from a physical and a cost point of view.
In addition to the problems associated with making interference measurements, there are problems associated with synchronizing the base station with the one or more wireless terminals during an automatic frequency exchange (i.e., an automatic frequency reassignment procedure). When a base station exchanges its present frequency carrier for another frequency carrier, presumably one experiencing less interference, there is always a risk that the wireless terminals operating in the corresponding broadcast region will not receive and/or process the synchronization and control information that precedes the exchange. Consequently, the communications link between the base station and the wireless terminals may be severed. Conventional wireless networks attempt to avoid this problem by accomplishing automatic frequency assignments when the broadcast region is "empty", that is, when the base station is not communicating with any wireless terminals. The problem with waiting until the broadcast region is empty is that it may take a long period of time before a new frequency can be assigned. During this period, the bases station and the corresponding wireless terminals may have to operate over an undesirable frequency channel that is subject to an unacceptably high level of interference.