Demodulation element assignment in a system capable of receiving multiple signals

We claim:

1. In a receiver having a plurality of demodulation elements, a method of assigning said plurality of demodulation elements to a set of available signals, comprising the steps of:

measuring said available signals and creating electronic representations of a list of survey paths comprised of an arrival time, a signal strength, and a transmitter index of each of said available signals;

matching a list of demodulation paths comprised of an arrival time, a signal strength, and a transmitter index corresponding to signals being demodulated by said receiver to said list of survey paths; and

assigning an un-assigned demodulation element, if said un-assigned demodulation element exists, to a particular survey path having a corresponding transmitter index that is different than every transmitter index in said list of demodulation paths.

2. The method of claim 1 wherein, in said step of assigning said un-assigned demodulation element, said particular survey path corresponds to the survey path having strongest signal strength of any survey path having said corresponding transmitter index.

3. The method of claim 1 further comprising the steps of:

re-assigning a particular demodulation element to said particular survey path if no un-assigned demodulation element exists comprising the steps:

un-assigning said particular demodulation element; and

assigning said particular demodulation element to said particular survey path.

4. The method of claim 3 wherein said particular demodulation element corresponds to the demodulation path having a signal strength that is weaker than the signal strength of any demodulation element.

5. The method of claim 3 wherein said particular demodulation element corresponds to a demodulation path having a signal strength that is weaker than the signal strength of said particular survey path.

6. The method of claim 3 wherein said particular demodulation element is assigned to a demodulation path having a signal strength that is at least a certain ratio weaker than the signal strength of said particular survey path.

7. The method of claim 3 wherein said particular demodulation element is assigned to a demodulation path having a signal strength that is 3 dB weaker than the signal strength of said particular survey path.

8. The method of claim 3 wherein said particular demodulation element is assigned to a particular demodulation path having a transmitter index which is the same as the transmitter index of at least one other demodulation path.

9. The method of claim 8 wherein said particular demodulation path has the weakest signal strength of any such path in said list of demodulation paths.

10. The method of claim 1 wherein said transmitter index represents a base station.

11. The method of claim 1 wherein said transmitter index represents a sector of a base station.

12. The method of claim 1 wherein each transmitter transmits a spread spectrum signal using pseudo noise modulation and wherein said transmitter index represents the code offset of the transmitted spread spectrum signal.

13. The method of claim 1 wherein said step of matching said list of demodulation paths to said list of survey paths causes each entry in said list of demodulation paths to correspond to an entry in said list of survey paths.

14. The method of claim 1 wherein said step of matching said list of demodulation paths to said list of survey paths comprises the steps of:

matching a first demodulation path having a first signal strength to a first survey path;

matching a second demodulation path having a second signal strength to said first survey path wherein said second signal strength is greater than said first signal strength; and

un-assigning a first demodulation element corresponding to said first demodulation path.

15. The method of claim 1, wherein each demodulation element of said plurality of demodulation elements that is assigned to a demodulation path indicates a state of successful or jeopardized demodulation, further comprising the step of un-assigning a particular demodulation element if said particular demodulation element indicates jeopardized demodulation.

16. The method of claim 1, wherein said receiver has at least one searcher element, and wherein said step of conducting a survey of available signals and creating a list of survey paths comprises the steps of:

receiving from said at least one searcher element a local maximum data point having an arrival time, a signal strength, and a transmitter index;

adding said local maximum data point to said list of survey paths if said signal strength of said local maximum data point exceeds a predetermined level.

17. The method of claim 1, wherein said receiver has at least one searcher element, and wherein said step of conducting a survey of available signals and creating a list of survey paths comprises the steps of:

receiving from said at least one searcher element a set of local maximum data points each having an arrival time, a signal strength, and a common transmitter index; and

adding a limited number of said local maximum data points to said list of survey paths if signal strength each of said added limited number of local maximum data points exceeds a predetermined level.

18. The method of claim 17, wherein said limited number of said local maximum data points is equal to the number of demodulation elements of said plurality of demodulation elements.

19. The method of claim 1, wherein said step of matching said list of demodulation paths to said list of survey paths further comprises the steps of:

finding a demodulation path that does not match any survey path of said list of survey paths; and

adding an entry in said list of survey paths corresponding to said unmatched demodulation path.

20. The method of claim 1, wherein each transmitter index in said list of survey paths is the same as a transmitter index in said list of demodulation paths, further comprising the step of assigning an un-assigned demodulation element, if said un-assigned demodulation element exists, to a second particular survey path.

21. The method of claim 20 wherein, in said step of assigning said un-assigned demodulation element, said second particular survey path does not have the same arrival time and transmitter index of any demodulation path on said list of demodulation paths.

22. The method of claim 21 wherein said second particular survey path has the strongest signal strength of any such path.

23. The method of claim 1 further comprising the steps of:

re-assigning a particular demodulation element comprising the steps of:

un-assigning said particular demodulation element assigned to a particular demodulation path; and

assigning said particular demodulation element to a second particular survey path.

24. The method of claim 23 wherein said particular demodulation path has the same transmitter index as said second particular survey path.

25. The method of claim 24 wherein said particular demodulation path has a signal strength that is weaker than the signal strength of said second particular survey path.

26. The method of claim 24 wherein said particular demodulation path has a signal strength that is at least a certain ratio weaker than the signal strength of said second particular survey path.

27. The method of claim 24 wherein said particular demodulation path has a signal strength that is 3 dB weaker than the signal strength of said second particular survey path.

28. The method of claim 23 wherein said particular demodulation path has the same transmitter index as at least one other entry on said list of demodulation paths.

29. The method of claim 23 wherein said particular demodulation path has the weakest signal strength of any demodulation path having the same transmitter index as said particular demodulation path.

30. The method of claim 1 wherein said step of matching said list of demodulation paths to said list of survey paths further comprises the step of matching said arrival time of each demodulation path in said list of demodulation paths to an arrival time of a corresponding entry on said survey paths.

31. The method of claim 1 wherein said step of matching said list of demodulation paths to said list of survey paths further comprises the step of matching within a predetermine time offset said arrival time of each demodulation path in said list of demodulation paths to an arrival time of a corresponding entry on said survey paths.

32. The method of claim 1 wherein each transmitter transmits a pseudo noise modulated signal using a pseudo noise code comprised of a sequence of code values and wherein said arrival time corresponds to a code value offset of said pseudo noise code.

33. The method of claim 1 wherein each transmitter transmits a pseudo noise modulated signal using a pseudo noise code comprised of a sequence of code values and wherein said arrival time corresponds to a window of time around a code value offset of said pseudo noise code.

34. The method of claim 33 wherein each code value of said sequence of code values has a duration and wherein said window of time is one half of said duration.

35. The method of claim 1 wherein each transmitter transmits a common pseudo noise modulated signal using a common pseudo noise code comprised of a sequence of code values and each transmitter transmits at a different time offset than every other of said transmitters and wherein said transmitter index corresponds to said different time offset.

36. The method of claim 1 wherein said step of conducting a survey of said available signals is systematically repeated over time.

37. In a receiver system comprised of a set of receivers, a method of assigning said receivers to a set of existing signals from at least one source:

creating a list of said existing signals, each existing signal of said list of existing signals having a signal strength indication, a time indication, and a corresponding source indication;

comparing said list of said existing signals to a list of signals currently assigned to said receivers, each signal of said list of signals currently assigned to said receivers having a signal strength indication, a time indication, and a corresponding source indication; and

assigning said receivers to said existing signals such that a maximum number of different corresponding source indications is present on said list of signals currently assigned to said receivers.

38. The method of claim 37 further comprises the step of re-assigning a receiver such that said maximum number of different corresponding source indications is present on said list of signals currently assigned to said receivers comprising the steps of:

un-assigning a particular receiver; and

assigning said particular receiver to one of said existing signals of said list of existing signals.

39. The method of claim 37 wherein the rate of occurrence of said steps of re-assigning is limited over time.

40. The method of claim 37 wherein no more than a predetermined number of said steps of re-assigning occurs for each of said step of creating said list of said existing signals.

41. The method of claim 37 wherein said step of creating said list of said existing signals is systematically repeated over time.

42. The method of claim 37 wherein said step of creating said list of said existing signals is systematically repeated over time.

43. The method of claim 37 further comprising the step of making available a first receiver assigned to a first signal wherein a signal strength indication of said first signal is below a predetermined level.

44. The method of claim 43 wherein said available receiver becomes idle.

45. The method of claim 43 wherein said available receiver continues to receive said first signal.

46. The method of claim 43 wherein said available receiver may be assigned a particular existing signal in said step of assigning said receivers to existing signals.

47. The method of claim 37 further comprising the step of making available a first receiver assigned to a first signal wherein the corresponding signal strength indication of said first signal is below a predetermined level for a predetermined period of time.

48. The method of claim 47 wherein said available receiver is idle.

49. The method of claim 47 wherein said available receiver continues to receive said first signal.

50. The method of claim 47 wherein said available receiver may be assigned a particular existing signal in said step of assigning said receivers to existing signals.

51. The method of claim 37 wherein said signal strength indication of said signals assigned to said receivers is generated from an RSSI output on said receivers.

52. The method of claim 51 wherein said RSSI output on said receivers is measured repetitively.

53. The method of claim 37 further comprising the step of making available a first receiver assigned to a first signal wherein the corresponding signal strength indication of said first signal is below a predetermined level for a predetermined number of said measurements.

54. An apparatus for demodulating a signal comprising:

a set of demodulation elements for demodulating a first set of instances of said signal arriving at said apparatus via a set of survey paths, each instance of said signal having an arrival time, signal strength, and transmitter index;

a searcher for determining a second set of instances of said signal arriving at said apparatus, and a corresponding arrival time, signal strength, and transmitter index for each instance;

a control system for matching said first set of instances of said signal with said second set of instances of said signal, and for assigning an un-assigned demodulation element, if said un-assigned demodulation element exists, to a particular instance of said signal from said second set of instances of said signal, said particular instance having a corresponding transmitter index that is different than every transmitter index from said first set of instances of said of signal.

55. The apparatus of claim 54 wherein said signal strength of said particular instance of said signal is greater than any signal strength of any remaining instances of said signal from said first and second set of instances of said signal.

56. The apparatus of claim 55 wherein said control system:

un-assigns a particular demodulation element if no un-assigned demodulation element exists; and

assigns said particular demodulation element to said particular instance of said signal.

57. The method of claim 56 wherein said particular demodulation element was assigned to an instance of said signal having a signal strength that is weaker than the signal strength of any other instance of said signal from said first set of instances of said signal.

58. The method of claim 57 wherein said particular demodulation element corresponds to an instance of said signal having a signal strength that is weaker than said signal strength of said particular instance of said signal.

59. A method for processing a signal during a code division multiple access forward link transmission comprising the steps of:

(a) measuring a series of energy levels of said signal;

(b) generating a series of electronic representations of said signal based on said series of energy levels;

(c) searching said series of electronic representations for a set of forward link signals;

(d) determining a corresponding arrival time, a corresponding signal strength, and a corresponding transmitter for each forward link signal from said set of forward link signals; and

(e) demodulating a sub-set of forward link signals that contains at least one forward link signal for each transmitter, said at least one forward link signal having a greater signal strength than a remaining set of all other forward link signals also associated with said transmitter.

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BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communication systems, particularly to a demodulation element assignment method for a communication system capable of receiving multiple signals.

II. Description of the Related Art

In a code division multiple access (CDMA) cellular telephone system, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneously communication between a mobile station and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high speed pseudonoise (PN) code. The high speed PN code is used to modulate signals transmitted from the base stations and the mobile stations. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station. The high speed PN modulation also allows the receiving station to receive a signal from a single transmitting station where the signal has traveled over several distinct propagation paths.

A signal having traveled several distinct propagation paths is generated by the multipath characteristics of the cellular channel. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.

In the mobile radio channel, the multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars, and people. In general the mobile radio channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses would change in time location, attenuation, and phase as a function of the time that the ideal impulse was transmitted.

The multipath characteristic of a channel can result in signal fading. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors are added destructively, yielding a received signal that is smaller than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of .delta. with a phase shift of .THETA. radians, and the second path has an attenuation factor of X dB, a time delay of .delta. with a phase shift of .THETA.+.pi. radians, no signal would be received at the output of the channel.

In narrow band modulation systems such as the analog FM modulation employed by conventional radio telephone systems, the existence of multiple path in the radio channel results in severe multipath fading. As noted above with a wideband CDMA, however, the different paths may be discriminated in the demodulation process. This discrimination not only greatly reduces the severity of multipath fading but provides an advantage to the CDMA system.

The deleterious effects of fading can be mitigated by controlling transmitter power in the CDMA system. A system for base station and mobile station power control is disclosed in U.S. Pat. No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM," issued Oct. 8, 1991, assigned to the Assignee of the present invention. Furthermore the effect of multipath fading can be reduced communication with multiple base stations using a soft handoff process. A handoff process is disclosed in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," issued Oct. 8, 1991, and assigned to the Assignee of the present invention.

In a cellular telephone system, maximizing the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is extremely important. System capacity in a spread spectrum system can be maximized if the transmitter power of each mobile station is controlled such that each transmitted signal arrives at the base station receiver at the same level. In an actual system, each mobile station may transmit the minimum signal level that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal transmitted by a mobile station arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to interference from the other mobile stations. On the other hand, if the mobile station transmitted signal is at a power level that is too high when received at the base station, communication with this particular mobile station is acceptable but this high power signal acts as interference to other mobile stations. This interference may adversely affect communications with other mobile stations.

Therefore to maximize capacity in an exemplary CDMA spread spectrum system, the transmit power of each mobile station within the coverage area of a base station is controlled by the base station to produce the same nominal received signal power at the base station. In the ideal case, the total signal power received at the base station is equal to the nominal power received from each mobile station multiplied by the number of mobile stations transmitting within the coverage area of the base station plus the power received at the base station from mobile stations in the coverage area of neighboring base stations.

The path loss in the mobile radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the mobile station, operates on a different frequency than the reverse link, from the mobile station to the base station. However because the forward link and reverse link frequencies are within the same frequency band, a significant correlation between the average path loss of the two links exists. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time.

In an exemplary CDMA system, each mobile station estimates the path loss of forward link based on the total power at the input to the mobile station. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the mobile station. From the estimate of the forward link path loss averaged over time, the mobile station sets the transmit level of the reverse link signal. Should the reverse link channel for one mobile station suddenly improve compared to the forward link channel for the same mobile station due to independent fading of the two channels, the signal as received at the base station from this mobile station would increase in power. This increase in power causes additional interference to all signals sharing the same frequency assignment. Thus a rapid response of the mobile station transmit power to the sudden improvement in the channel would improve system performance.

Mobile station transmit power is also controlled by one or more base stations. Each base station with which the mobile unit is in communication measures the received signal strength from the mobile unit. The measured signal strength is compared to a desired signal strength level for that particular mobile station. A power adjustment command is generated by each base station and sent to the mobile unit on the forward link. In response to the base station power adjustment command, the mobile unit increases or decreases the mobile unit transmit power by a predetermined amount. By this method, a rapid response to a change in the channel is effected and the average system performance is improved.

When a mobile station is in communication with more than one base station, power adjustment commands are provided from each base station. The mobile station acts upon these multiple base station power control commands to avoid transmit power levels that may adversely interfere with other mobile station communications and yet provide sufficient power to support communication from the mobile station to at least one of the base stations. This power control mechanism is accomplished by having the mobile station increase its transmit signal level only if every base station with which the mobile station is in communication requests an increase in power level. The mobile station decreases its transmit signal level if any base station with which the mobile station is in communication requests that the power be decreased.

The existence of multipath can provide path diversity to a wideband spread spectrum system. A spread spectrum system generates a spread information signal by modulating an information signal with a pseudonoise (PN) code. Generally the PN code runs at many times the rate of the information signal. The rate that the PN code is generated is called the chip rate and the duration of one data bit of the PN code is called the chip time. If two or more paths are available with greater than chip time differential path delay, two or more processing elements, called demodulation elements, can be employed to separately demodulate these signals. These signals typically exhibit independence in multipath fading, i.e., they do not usually fade together. Therefore the output of the two or more demodulation elements can be combined to obtain path diversity. A loss of signal occurs only when the signals from all demodulation elements experience a fade at the same time. In an ideal system, both the base station and the mobile station employ multiple demodulation elements.

As a mobile station moves through the physical environment, the number of signal paths and the strength of the signals on these paths vary constantly, both as received at the mobile station and as received at the base station. Therefore, a receiver incorporating the present invention uses a special processing element, called a searcher element, that continually scans the channel in the time domain to determine the existence, time offset, and the signal strength of signals in the multiple path environment. The output of the searcher element provides the information for ensuring that the demodulation elements are tracking the most advantageous paths. The present invention provides a method of assigning the multiple demodulation elements to the multiple received signals based on the searcher element information.

In an exemplary CDMA cellular telephone system, each base station transmits a spread spectrum "pilot" reference signal. This pilot signal is used by the mobile stations to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of the base station transmitted signals. The pilot signal transmitted by each base station in a system may use the same PN code but with a different code phase offset meaning that the PN codes transmitted by neighboring base stations are identical but skewed in time with respect to one another. Phase offset allows the pilot signals to be distinguished from one another according to the base station from which they originate. The mobile station's searcher element continues to scan the received signal at the code offsets corresponding to neighboring base station's transmitted pilot signals while in the call inactive mode. When a call is initiated, a PN code address is determined for use during this call. The code address may be either assigned by the base station or be determined by prearrangement based upon the identity of the mobile station. After a call is initiated the mobile station's searcher element continues to scan the pilot signal transmitted by neighboring base stations. When the pilot signal transmitted by a neighboring base station becomes strong enough to establish communication, the mobile station generates and transmits a control message to the base station currently servicing the call. The current base station provides the control message to the cellular system controller.

The cellular system controller begins the base station diversity or so-called "soft handoff" process. The cellular system controller begins by assigning a modem located in the new base station to the call. This modem is given the PN address associated with the call between the mobile station and the current base station modem. The new base station modem assigned to service the call searches for and finds the mobile station transmitted signal. The new base station modem also begins transmitting a forward link signal to the mobile station. The mobile station's searcher element searches for this forward link signal according to the signal information provided by the old base station. When the mobile station acquires the new base station modem transmitted signal, the mobile station may continue to communicate through the two base stations. Another base station could be added in the same manner as the first new base station above. In this case the mobile station may continue to communicate through three base stations. This process can continue until the mobile station is in communication with one base station for each demodulation element that the mobile station contains and beyond.

Diversity combining in the mobile station significantly advances the quality and reliability of communications in a cellular telephone system. A form of maximal ratio combining may be used to increase the benefit in which the signal-to-noise ratio is determined for each path. Each path may then be combined with the contributions from the other paths weighted according to the signal-to-noise ratio. Combining may be coherent because pilot signal demodulation allows the phase of each path to be determined.

In the path from the mobile station to the base station, path diversity reception is obtained in a similar manner. A base station may contain an analogous set of processing elements as the mobile station in that a searcher element may provide data to assign a plurality of demodulation elements. The present invention defines a method for assigning the demodulation elements to the multipath signals in the base station.

During communication with an end user, the demodulated data signals of a base stations are forwarded to the cellular system controller along with an indication of signal quality. The cellular system controller relays these signals to the end user. When a mobile station is in a base station diversity mode with two independent base stations, the demodulated data signals of both base stations are forwarded to the cellular system controller along with an indication of signal quality. The cellular system controller then combines the two versions of the mobile station signal or selects the signal with the best quality indication. An alternative system configuration may transmit the undecoded or even the undemodulated signals to the cellular system controller to allow a better diversity combining process to be used.

A typical base station configuration may contain multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas. When a mobile station is in base station diversity mode and communicating with two sectors of the same base station, the demodulated data signals of both sectors are available for combination within the base station before the signals are passed to the cellular system controller. In fact, within a multi-sector base station, a system may be configured such that each demodulation element may be assigned to any arriving signal regardless of the sector that signal was received from. This system configuration allows a process called softer handoff and the present invention defines a method for assigning the demodulation elements for this configuration.

It is therefore the object of the present invention to provide a method of assigning multiple demodulation elements in a mobile station.

It is another object of the present invention to provide a method of assigning multiple demodulation elements in a base station.

SUMMARY OF THE INVENTION

The present invention defines a method for assigning multiple demodulation elements in a spread spectrum system. In the present invention within the mobile station, a searcher element performs a survey in which it scans a window of time offsets around nominal arrival time of each signal of each base station with which active communication is established. Each survey yields a list of survey paths that comprises pilot signal strengths, time offsets, and corresponding base station pilot offset. The searcher element passes the information to a controller. The controller attempts to match the time offset of each survey path to the time offset of paths currently being demodulated by the demodulation elements. If there are multiple demodulation paths that match one survey path, all demodulation elements assigned to that path, except the demodulation element having the strongest signal strength indication, are labeled "free." If a demodulation path exists that does not correspond to a survey path, a survey path entry based on the demodulation path information is added to the list of survey paths.

Next the controller considers the survey paths in order of signal strength with the strongest signal strength survey path being first. If there is no demodulation element assigned to any path in the corresponding sector of the survey path under consideration, the controller attempts to assign a demodulation element to the survey path in the following order. If there is an unassigned or labeled "free" demodulation element, the demodulation element is assigned to the survey path. If no demodulation element is free, the demodulation element having the weakest path that is not the only demodulation path from its base station sector, if any, is re-assigned to the survey path. Finally if the first two cases fail to assign a demodulation element to the survey path, a demodulation element assigned to the weakest path is re-assigned to the survey path if the survey path's signal strength is stronger than the signal strength of the weakest demodulation path. This process continues until one re-assignment occurs or until the last criteria fails to re-assign a demodulation element to the survey path under consideration.

If none of the above rules re-assign a demodulation element for the present survey, the controller considers the survey paths again in order of signal strength with the strongest signal strength survey path being first. If the survey path is not currently assigned to a demodulation element, the controller may assign any unassigned or labeled "free" demodulation element to the survey path under consideration. If there are no unassigned or labeled "free" demodulation elements, the controller may also re-assign a demodulation element that is assigned to the same base station sector as a survey path if the survey path is stronger than the demodulation path. The controller may also re-assign the weakest demodulation element that is assigned to any base station sector having two or more assigned demodulation elements if the survey path is stronger than the demodulation path. Once either of the two above rules causes a re-assignment or both of the above rules for re-assignment fail for the survey path under consideration, the process begins again.

The present invention uses these steps to ensure base station and sector diversity. Each time a demodulation element is re-assigned, a finite time lapses in which no data is demodulated. Therefore, the present invention limits the number of demodulation element re-assignments per survey. Comparison ratios are used to create hysteresis in the assignments and thus reduce excessive re-assignment of demodulation elements.

The base station uses a similar but less complicated method to assign the demodulation elements. Because each base station sector receives the same information from a single mobile station, there is no need to sacrifice the maximum signal level paths to promote diversity. Thus the base station method is based more strictly on signal level while limiting the number of re-assignments per survey similar to the mobile station method. The base station also uses ratios similar to the mobile station to create hysteresis to reduce excessive re-assignment of demodulation elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters correspond throughout and wherein:

FIG. 1 is an illustration of an exemplary mobile station comprising multiple independent demodulation elements;

FIG. 2 is a detailed block diagram of an exemplary mobile station demodulation element of FIG. 1;

FIGS. 3A-3C illustrate the pilot signal strength versus time for three different base stations or base station sectors;

FIG. 4 is a summary of the demodulation element assignment method for a mobile station according to the present invention;

FIGS. 5A-5D are a detailed example of the demodulation element assignment method for a mobile station according to the present invention;

FIG. 6 is an illustration of an exemplary base station comprising