Microcellular communications system using macrodiversity

Claims

I claim:

1. A communications system having a plurality of electromagnetic coverage areas created by electromagnetic radiators each radiator controlled by an area controller and serving a plurality of remote stations within each coverage area, comprising:

a first radiator transmitting a first electromagnetic signal during at least a first one of a plurality of time slots on a first electromagnetic frequency into at least a first portion of the electromagnetic coverage area;

a second radiator transmitting a second electromagnetic signal during at least a second one of said plurality of time slots on said first electromagnetic frequency into at least a first portion of the electromagnetic coverage area;

means at a remote station for selecting between said first and second electromagnetic signals and communicating said selection to the area controller; and

means at the area controller for selecting, in response to said communication of said selection between said first and second electromagnetic signals, a third one of said plurality of time slots on said first electromagnetic frequency for transmission of at least a portion of a first message to said remote station.

2. A communications system in accordance with claim 1 further comprising means, responsive to said remote station electromagnetic signal selection, for enabling during said third time slot the radiator transmitting said selected electromagnetic signal.

3. A communications system in accordance with claim 1 further comprising means at said remote station for transmitting at least a portion of a second message to a receiver means associated with the radiator transmitting said selected electromagnetic signal, said at least a portion of said second message transmitted in a time slot on a second electromagnetic frequency associated with said third time slot on said first electromagnetic frequency.

4. A communications system in accordance with claim 3 further comprising means at the area controller for measuring signal quality received in said time slot on said second electromagnetic frequency.

5. A communications system in accordance with claim 4 further comprising means at the area controller for determining that said measured signal quality indicates that said remote station should be handed off to another area controller in another electromagnetic coverage area.

6. A communications system in accordance with claim 1 further comprising means for cyclically repeating said plurality of time slots including said first, second, and third time slots.

7. A communications system in accordance with claim 6 wherein said means at a remote station for selecting further comprises means for determining signal quality of said first and second electromagnetic signals.

8. A communications system in accordance with claim 7 wherein said means at a remote station for selecting further comprises:

means for determining, after transmission during said third time slot, that said electromagnetic signal not selected has a better signal quality than said selected electromagnetic signal;

means for newly selecting said electromagnetic signal previously not selected; and

means for communicating said new selection to the area controller.

9. A communications system in accordance with claim 8 further comprising:

means at the area controller for enabling the radiator of said newly selected electromagnetic signal for transmitting at least a portion of said first message to said remote station in said third time slot; and

means for disabling during said third time slot the radiator of electromagnetic signals not newly selected.

10. A remote station for a communications system having a plurality of electromagentic coverage areas with fixed electromagnetic transceivers controlled by an area controller in each coverage area, comprising:

means for receiving a first electromagnetic signal from a first transceiver during a first one of a plurality of time slots on a first electromagnetic frequency and for receiving a second electromagnetic signal from a second transceiver during a second one of said plurality of time slots on said first electromagnetic frequency;

means for selecting between said first and second electromagnetic signals;

means for communicating said selection to the area controller; and

means for receiving from the area controller said selected electromagnetic signal which conveys at least a portion of a message to the remote station during a third one of said plurality of time slots, said third one of said plurality of time slots selected by the area controller.

11. A remote station in accordance with claim 10 further comprising means for transmitting at least a portion of a second message on a second electromagnetic frequency to the transceiver transmitting said selected electromagnetic signal in a time slot associated with said selected third time slot.

12. A remote station in accordance with claim 10 wherein said means for selecting further comprises means for determining signal quality of said first and second electromagnetic signals.

13. A remote station in accordance with claim 12 wherein said means for selecting further comprises:

means for determining, after transmission of said selected third time slot, that said electromagnetic signal not selected has a better signal quality than said selected electromagnetic signal;

means for newly selecting said electromagnetic signal previously not selected; and

means for communicating said new selection to the area controller.

14. Fixed site control and transceiving apparatus for a communications system having a plurality of electromagnetic coverage areas serving a plurality of remote stations having transceivers within each coverage area, comprising:

means for transmitting a first electromagnetic signal during at least a first one of a plurality of time slots on a first electromagnetic frequency into at least a first portion of the electromagnetic coverage area;

means for transmitting a second electromagnetic signal during at least a second one of said plurality of time slots on said first electromagnetic frequency into at least a second portion of the electromagnetic coverage area;

means for receiving a transmission from a remote station which identifies a selected one of said first and second electromagnetic signals; and

means, responsive to said means for receiving, for selecting a third one of said plurality of time slots on said first electromagnetic frequency for transmission of at least a portion of a first message to said remote station by said selected one of said first and second electromagnetic signals.

15. Fixed site control and transceiving apparatus in accordance with claim 14 further comprising means for measuring signal quality of at least a portion of a second message received from said remote station in a time slot on a second electromagnetic frequency.

16. Fixed site control and transceiving apparatus in accordance with claim 15 further comprising means for determining that said measured signal quality indicates that said remote station should be handed off to another fixed site control and transceiving apparatus in another electromagnetic coverage area.

17. A method of communications channel selection in a communications system havng a plurality of electromagnetic coverage areas created by electromagnetic radiators each radiator controlled by an area controller and transmitting to a plurality of remote stations within each coverage area, comprisng the steps of:

transmitting a first electromagnetic signal during at least a first one of a plurality of time slots on a first electromagnetic frequency into at least a first portion of the electromagnetic coverage area;

transmitting a second electromagnetic signal during at least a second one of said plurality of time slots on said first electromagnetic frequency into at least a second portion of the electromagnetic coverage area;

selecting, at a remote station, between said first and second electromagnetic signals;

communicating said selection to the area controller; and

selecting, at the area controller, a third one of said plurality of time slots on said first electromagnetic frequency in response to said communicating said selection between said first and second electromagnetic signals for transmission of at least a portion of a first message to said remote station.

18. A method of communications channel selection in accordance with the method of claim 17 further comprising the step of enabling during said third time slot the radiator which transmitted said selected electromagnetic signal, in response to said remote station selection.

19. A method of communications channel selection in accordance with the method of claim 18 further comprising the step of transmitting, at said remote station, at least a portion of a second message in a time slot on a second electromagnetic frequency associated with said third time slot on said first electromagnetic frequency.

20. A method in accordance with the method of claim 19 further comprising the step of measuring signal quality received in said time slot on said second electromagnetic frequency.

21. A method in accordance with the method of claim 20 further comprising the step of determining, at the area controller, that said measured signal quality indicates that said remote station should be handed off to another area controller in another electromagnetic coverage area.

22. A method of communications channel selection in accordance with the method of claim 18 further comprising the step of cyclically repeating at least said first, second, and third time slots.

23. A method of communications channel selection in accordance with claim 22 wherein said step of selecting, at a remote station, further comprises the step of determining signal quality of said first and second electromagnetic signals.

24. A method of communications channel selection in accordance with claim 23 wherein said step of selecting, at a remote station, further comprises the steps of:

determining, after transmission during said third time slot, that said electromagnetic signal not selected has a better signal quality than said selected electromagnetic signal;

newly selecting said electromagnetic signal previously not selected; and

communicating said new selection to the area controller.

25. A method of communications channel selection in accordance with the method of claim 24 further comprising the steps of:

enabling, at the area controller, the radiator of said newly selected electromagnetic signal for transmitting at least a portion of said first message to said remote station in said third time slot; and

disabling that radiator of electromagnetic signals not newly selected during said third time slot.

26. A method of communications channel selection in a remote station for a communications system having a plurality of electromagentic coverage areas with fixed electromagnetic transceivers controlled by an area controller in each coverage area, comprising the steps of:

receiving a first electromagnetic signal from a first transceiver during a first one of a plurality of time slots on a first electromagnetic frequency;

receiving a second electromagnetic signal from a second transceiver during a second one of said plurality of time slots on said first electromagnetic frequency;

selecting between said first and second electromagnetic signals;

communicating said selection to the area controller; and

receiving from the area controller said selected electromagnetic signal which conveys at least a portion of a first message to the remote station during a third one of said plurality of time slots, said third one of said plurality of time slots selected by the area controller.

27. A method of communications channel selection in accordance with the method of claim 26 further comprising the step of transmitting at least a portion of a second message on a second electromagnetic frequency to the transceiver transmitting said selected electromagnetic signal in a time slot associated with said selected third time slot.

28. A method in accordance with the method of claim 27 further comprising the step of measuring signal quality received in said time slot on said second electromagnetic frequency.

29. A method in accordance with the method of claim 28 further comprising the step of determining, at the area controller, that said measured signal quality indicates that said remote station should be handed off to another area controller in another electromagnetic coverage area.

30. A method of communications channel selection in accordance with the method of claim 26 wherein said step of selecting further comprises the step of determining signal quality of said first and second electromagnetic signals.

31. A method of communications channel selection in accordance with the method of claim 30 wherein said step of selecting further comprises the steps of:

determining, after reception of said at least a portion of a first message during said third time slot, that said electromagnetic signal not selected has a better signal quality than said selected electromagnetic signal;

newly selecting said electromagnetic signal previously not selected; and

communicating said new selection to the area controller.

32. A method of communications channel selection for fixed site control and transceiving apparatus for a communications system having a plurality of electromagnetic coverage areas serving a plurality of remote stations having transceivers within each coverage area, comprising the steps of:

transmitting a first electromagnetic signal during at least a first one of a plurality of time slots on a first electromagnetic frequency into at least a first portion of the electromagnetic coverage area;

transmitting a second electromagnetic signal during at least a second one of said plurality of time slots on said first electromagnetic frequency into at least a second portion of the electromagnetic coverage area;

receiving a transmission from a remote station which identifies a selected one of said first and second electromagnetic signals; and

selecting, in response to said receiving step, a third one of said plurality of time slots on said first electromagnetic frequency for transmission of at least a portion of a first message to said remote station by said selected one of said first and second electromagnetic signals.

33. A method in accordance with the method of claim 32 further comprising the step of measuring signal quality of at least a portion of a second message received from said remote unit in a time slot on a second electromagnetic frequency.

34. A method in accordance with the method of claim 33 further comprising the step of determining that said measured signal quality indicates that said remote station should be handed off to another fixed site control and transceiving apparatus in another electromagnetic coverage area.

35. A cellular telephone system having a plurality of electromagnetic coverage areas created by electromagnetic radiators each radiator controlled by an area controller and serving a plurality of remote stations within each coverage area, comprising:

a first radiator transmitting a first electromagnetic signal during at least a first one of a plurality of time slots on a first electromagnetic frequency into at least a first portion of the electromagnetic coverage area;

a second radiator transmitting a second electromagnetic signal during at least a second one of said plurality of time slots on said first electromagnetic frequency into at least a second portion of the electromagnetic coverage area;

means at a remote station for selecting between said first and second electromagnetic signals based on electromagnetic signal quality to establish a selected signal and an unselected signal, and communicating said selection to the area controller;

means at the area controller for selecting, in response to said remote station selection, a third one of said plurality of time slots on said first electromagnetic frequency for transmission of at least a portion of a first message to said remote station during said third time slot;

means for determining, after transmission during said third time slot, that said unselected signal has a better signal quality than said selected signal;

means for newly selecting said unselected signal and for communicating said new selection to the area controller; and

means at the area controller, responsive to said communication of said new selection, for enabling the radiator of said newly selected unselected signal for transmitting at least a portion of said first message to said remote station in said third one of said plurality of time slots on said first electromagnetic frequency.

36. A cellular telephone system in accordance with claim 35 further comprising means, responsive to said communication of said new selection, for disabling during said third time slot the radiator of said selected signal.

37. A cellular telephone system in accordance with claim 35 further comprising means at said remote station for transmitting at least a portion of a second message to the radiator associated with said selected signal, said at least a portion of a second message transmitted in a time slot on a second electromagnetic frequency associated with said third one of a plurality of time slots on said first electromagnetic frequency.

38. A remote station for a cellular telephone system having a plurality of electromagnetic coverage areas with fixed electromagnetic transceivers controlled by an area controller in each coverage area, comprising:

means for receiving a first electromagnetic signal from a first transceiver during a first one of a plurality of time slots on a first electromagnetic frequency and for receiving a second electromagnetic signal from a second transceiver during a second one of said plurality of time slots on said first electromagnetic frequency;

means for selecting between said first and second electromagnetic signals based on electromagnetic signal quality;

means for communicating said selection to the area controller;

means for receiving from the area controller said selected electromagnetic signal which conveys at least a portion of a message to the remote station during a thrd one of said plurality of time slots;

means for transmitting at least a portion of a second message on a second electromagnetic frequency to the transceiver transmitting said selected electromagnetic signal in a fourth time slot associated with said selected third time slot;

means for determining, after transmission during said selected third time slot, that said electromagnetic signal not selected has a better signal quality than said selected electromagnetic signal;

means for newly selecting said electromagnetic signal previously not selected; and

means for communicating said new selection to the area controller.

39. A cellular communications system having a plurality of coverage areas each illuminated from an electromagnetic energy radiator, each radiator controlled by an area controller and serving a plurality of remote stations within each coverage area, the system comprising:

means, at a first radiator, for transmitting at least a first information signal during a first one of a plurality of time slots on a first electromagnetic frequency into at least a first portion of the electromagnetic coverage area;

means, at a second radiator, for transmitting at least a second information signal during a second one of said plurality of time slots on said first electromagnetic frequency into at least a second portion of the electromagnetic coverage area;

means at a remote station for selecting between said first and second signals and communicating said selection to the area controller; and

means at the area controller for selecting, in response to said communication of said selection between said first and second signals, a third one of said plurality of time slots on said first electromagnetic frequency for transmission of a message comprising control and message information data bits to said remote station.

BACKGROUND OF THE INVENTION

This invention relates generally to high density radio communications systems and more particularly to cellular radiotelephone systems employing digital communications techniques to increase the number of channels available in a fixed radio frequency bandwidth and geographic area.

Radio communications systems which employ controlled transmission and reception parameters to realize a plurality of non-interfering defined coverage areas are well known in the art as cellular radiotelephone systems. Variations in design, direction of radio signal illumination, and techniques of system growth have been the subject of several U.S. Pat. Nos.: 3,663,762--Joel, Jr.--"Mobile Communication System"; 3,819,872--Hamrick--"Mobile Telephone Cellular Switching System"; 3,906,166--Cooper et al.--"Radio Telephone System"; 4,128,740--Graziano--"Antenna Array for a Cellular RF Communications System"; and 4,144,411--Frenkiel--"Cellular Radiotelephone System Structured for Flexible Use of Different Cell Sizes". Cellular systems may further be characterized as being capable of automatically and unobtrusively maintaining radio communications between fixed stations and remote stations as the remote stations move across the borders of the cells.

Some of these traditional systems anticipate an increasing number of users with passing time and have developed graceful methods of subdividing and shrinking cell areas to enable multiple reuse of radio frequencies in a fixed geographic area. Generally, each system grows by making the cells smaller and maintaining the pattern of frequency allocation to each of the cells. There are, however, at least two factors which place a limit on the minimum size to which a cell can be shrunk. These factors are the rate at which remote stations move through the cells, and the non-uniformity of the electromagnetic field in the cell. Both factors relate to the time required to determine the relative location of the remote station and to process a handoff of the remote station from the fixed station of one cell to the fixed station of another cell, where the remote station is currently located.

Determination of the location of a remote station is typically performed by measuring the signal strength or quality of the radio signal as received at the fixed station. Because the electromagnetic field in non-uniform, the measurement of signal strength (or quality) is made a plurality of times or averaged over a period of time. The time required becomes longer as the turbulence of the electromagnetic field increases or as the necessary accuracy of the signal strength measurement increases. Thus, there is a finite amount of time which must be spent in determining the location of the remote station. When the density of remote stations becomes large, dedicated equipment is employed at the fixed stations full time in making signal measurements.

Once the measurement is made, a decision must be made whether a handoff of the remote station to another cell is required. If a handoff is required, one or more candidate cells must be queried for their idle channel status and for a verification of the remote station's signal strength in that candidate cell. Processing of the decision, status, and verification usually requires the intervention of higher level system control functions in addition to the control functions in the serving and candidate cells. Additionally, the remote station must be instructed to tune to a frequency available in the candidate cell and verification of its presence after the handoff must be made by the candidate cell. Thus, a significant amount of time is used for handoff processing.

Digital radio transmission techniques, such as would be used with point-to-point systems, have been considered for high capacity cellular systems but previously have not found practical application due to the cost and complexity of digital equipment required to mitigate the effects of intersymbol interference caused by the multiplicity of signal arrival times at the receivers in the system.

SUMMARY OF THE INVENTION

Therefore, it is one object of the present invention to provide means for implementing a very small cell (microcell) in a cellular system.

It is another object of the present invention to employ a digital burst technique to communicate information between a fixed station and a remote station in a cellular system.

It is another object of the present invention to enable the remote station to select the best of several radiators within a particular cell.

Accordingly, these and other objects are realized in the present invention which encompasses a radio system having a plurality of distinct electromagnetic radiation coverage areas, each of which are controlled by an area controller and which serves a plurality of remote stations within a coverage area. The system includes a radiator for transmitting a first electromagnetic signal which is separated into a plurality of time slots on a predetermined frequency into a first portion of a coverage area. The system also includes a radiator for transmitting a second electromagnetic signal, which is separated into a plurality of time slots on the same frequency, into a second portion of the coverage area. A remote station selects between the first and second electromagnetic signals and communicates the selection to the area controller. The area controller then selects a time slot in the electromagnetic signal selected by the remote station to transmit a portion of a message to the remote station.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a geographic area covered by a stylized hexagonal radio coverage area pattern with radio transmission and reception occurring from fixed stations located at the vertices of the hexagonal patterns, as is conventional for one type of the cellular radiotelephone system.

FIG. 2 is an illustration of a geographic area covered by a stylized hexagonal radio coverage area pattern with radio transmission and reception occurring from fixed stations located at the center of the hexagonal patterns and further subdividing the pattern into sectors, as is conventional for a second type of cellular radiotelephone system.

FIG. 3 is an illustration of a geographic area covered by a stylized hexagonal cell in which an obstruction to radio signals is shown.

FIG. 4 is an illustration of a small area having an irregular shape and several obstructions and which may be considered to be single cell.

FIG. 5 is a block diagram showing the interconnection between the fixed equipment which may be employed in the present invention.

FIG. 6 is a timing diagram illustrating the time slots which may be employed in the present invention.

FIG. 7 is a timing diagram of a cycle of the time slots of FIG. 6.

FIG. 8 is a geographic illustration indicating the relationship between the fixed equipment and the remote unit as exists in the present invention.

FIG. 9 is a timing diagram relating the activities of fixed radiators and a remote unit over several cycles of data time slot transmission which might occur in the present invention.

FIG. 10 is a block diagram of a remote unit which may be employed in the present invention.

FIG. 11 is a block diagram of a cell area controller which may be employed in the present invention.

FIG. 12 is a block diagram of a fixed equipment radiator which may be employed in the present invention.

FIG. 13 is a flowchart of the channel access process employed by a remote unit of the present invention.

FIG. 14 is a flowchart of the channel access process employed by a cell area controller of the present invention.

FIG. 15 is a flowchart of the valid input determination process of FIG. 14.

FIG. 16 is a flowchart of the radiator change process employed by a remote unit of the present invention.

FIG. 17 is a flowchart of the radiator change process employed by a cell area controller of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Generally cellular systems are conceptualized as a packing of hexagonal geographic areas, or cells, and having definite and sharp boundaries between the cells. Each cell can be illuminated with radio signals from the conceptual vertices of the hexagonal cells as shown in FIG. 1 or from the center outward, in segments, such as that shown in FIG. 2. These conceptual patterns enable a cellular system designer to study and plan cellular systems without the perturbations of problems encountered in field implementation of the systems. Non-uniform electromagnetic fields are encountered due to reflections and obstructions such as that diagramed in FIG. 3.

In FIG. 3 a remote station may drive or be carried behind an obstruction such that radio signals to and from the remote station will be blocked or greatly attenuated. Known or discovered obstructions are typically cured by proper layout of the cellular pattern so that as the remote station moves into the attenuated area, it is handed off to another cell which can provide radio coverage into the shadow area.

As described earlier, when the cells become small the number of handoffs between one cell and another become large and the time required to process the handoff can become objectionable. It is conceivable that a cell's size may be shrunk to the dimensions of a city block or a single floor of an office building. In situations such as these, a hand-held remote station can be carried into locations where large and abrupt changes in signal strength can be realized in a matter of a few steps. A single cell, then can be conceptualized as a corridor with sharp corners and/or a series of rooms electromagnetically shielded from each other.

One of the virtually limitless conceptualizations is diagramed in FIG. 4. Here there are two remote stations 401 and 403 which can be moved or carried throughout the cell 405. A plurality of radiators of electromagnetic energy (407, 409, and 411), which may be radio transmitters and receivers or infrared transmitters and receivers, are placed at optimum locations within cell 405. Such non-colocation of radiators and sites of reception constitutes a macrodiversity system. A cell area controller 413 can be placed at a convenient location such that interconnection with radiators 407, 409 and 411 may be made.

A block diagram illustrating the interconnection between the controller 413 and the radiators within cell 405 is shown in FIG. 5. Additionally, connections to other cells, such as cell 501, may be made from controller 413 via a cellular switch and system controller 503. Two configurations are illustrated: a cell site (405) employing radiators and antennas remote from the cell area controller 413 and utilizing a radiator site switcher 505 to convey channel and control information to the remote radiators (407, 409, and 411); and a cell site (501) employing radiators 506 and 507 colocated with the cell area controller 509 and utilizing remote antennas selected by an antenna switcher 511. Other configurations of hardware are possible and the present invention need not be limited to a particular cell hardware configuration. Such cellular switches and system controllers may employ conventional cellular radiotelephone hardware.

With a configuration such as that shown in FIG. 5, it is possible for a remote station to be handed off between cell 405 and cell 501 in a conventional fashion. If cells 405 and 501 are, for example, the fourth and fifth floor of an office building, the radiators 407, 409, and 411 would operate at one frequency of electromagnetic radiation and radiators 506 and 507 would operate at a second frequency of electromagnetic radiation. An area of electromagnetic energy overlap would be provided, for example in a stairwell, such that a conventional frequency change handoff between cell area controller 413 and cell area controller 509 and their associated radiators or radiator antennas can take place. The cellular switch and system controller 503 mediates the conventional handoffs between cells and further interconnects the messages received from the remote stations to telephone trunks coupled to the switched telephone network.

The operation of the system of the present invention can best be apprehended by again considering FIG. 4. The remote station 401 can obtain service from the electromagnetic radiations of radiator 409 or radiator 407. In conventional cellular systems, a decision as to which radiator would serve remote station 401 would be made by the area controller 413. This decision, of course, would entail the time consuming handoff process described previously. In the microcell of the present invention, remote station 401 decides which radiator is providing the best signal strength or the best signal quality. (Radio signal strength measurement is well-known in the art and can be measured by comparison of the amplitude of the electromagnetic signal from radiator 409 to the amplitude of the electromagnetic signal received from radiator 407. A signal quality measurement may be made by comparison of the signal level above received noise or by measurement of data bit error rate, as conventionally known). Methods and apparatus to realize signal strength measurements have been described in U.S. Pat. Nos. 4,549,311; 4,704,734; and 4,696,027. As the remote station 401 moves toward radiator 409, it will encounter a point at which transmissions from all three radiators in the present example can be received. Upon turning the corner toward radiator 411, remote station 401 will quickly lose the electromagnetic signal from radiator 407. This is, the signal strength will go from a perfectly usable signal to a totally unusable signal in a matter of a few steps. If this dramatic drop in received signal strength were to occur in a conventional cellular system, it is likely that the call would be lost completely. By providing the remote unit itself with the capability of measuring and deciding upon the best radiator, the remote station itself will quickly select the best radiator without the attendant delay of a conventional handoff. Furthermore, the remote station can be assured that its inbound signal, containing the information about the radiator of choice, will be heard by one of the radiators and communicated to the area controller. Thus, in the example of FIG. 4, remote station 401 would initially select radiator 407 using signal strength or signal quality measurements. Then as it moved toward radiator 409, it would select radiator 409 and subsequently select radiator 411 as the remote station 401 moved closer to radiator 411.

If the type of system selected for this microcell employed frequency division multiplex, the remote station would be forced to have at least two receivers. This is because the remote station must now operate on at least two radio frequency channels simultaneously in order to measure the quality of the various radiator signals: one being the frequency it is using for communications, one being the frequency it is testing. The alternative is to steal time from the communications frequency for the measurement of a candidate frequency, causing a loss of data at the output of the remote station at an unacceptably rapid rate. If the radiators operated at a single radio frequency channel for each connection to a remote station, that is, simulcast, the traditional problems of frequency stability and coverage area overlap signal cancellation would have to be solved for each radio channel in the cell. It is primarily for these reasons that the preferred embodiment of the present invention employs a form of burst digital modulation commonly known as time division multiple access (TDMA), where a single remote receiver may be used to perform a quality measurement on a number of radiators without loss of received information.

TDMA is a well-known technique of sharing a limited channel resource among a large number of users. In the preferred embodiment, a radio channel or other segment of the electromagnetic spectrum having a bandwidth of approximately 250 KHz is divided into frames of time which, in turn, are divided into a plurality of slots. In the preferred embodiment there are 12 slots per frame as shown in FIG. 6. Each radiator is assigned a frame during which the radiator transmitter may transmit radiator slot information in one slot (R1) and transmit message information designated to a particular remote station in one of the remaining 11 slots (such as shown for U3). Furthermore, within each radiator slot (R1) there is a frame-defining pattern and a radiator identification sequence prior to information directed to the remote units. This information may be used to assign remote stations requesting service to a particular unused time slot so that a conversation or other message may be communicated. Within the remote station message slot there is found control and identification bits preceding the message information bits which contain the message to the remote unit.

Referring now to FIG. 7, the transmissions on a single electromagnetic frequency from 3 radiators are shown. Starting in time at the beginning of frame 1, the transmitter of radiator 1 turns on and transmits radiator slot information in the first time slot (R1) as shown. At the end of the first time slot, radiator 1 turns off thereby allowing any of the three radiators to transmit in the second time slot. In this example, there are no transmissions in the second or third time slot; the first transmission to a remote unit is in the fourth time slot (to remote un