HK1065216A - Combined searching and page monitoring using offline sample storage - Google Patents

Description

Combined search and page monitoring using offline sample storage

The application is a divisional application of an invention patent application with the application number of 99814319.9 and the international application date of 10/12 in 1999 and the title of "combined search and page monitoring using offline sample storage".

Technical Field

The present invention relates to wireless communications. More particularly, the present invention relates to a novel and improved searcher for detecting paging messages in spread spectrum communications.

Background

Searching for a detected spread spectrum signal is described in U.S. patent application No. 08/316,177 (the' 177 application), entitled "multipath search processor for spread spectrum multiple access communication systems". This search is particularly useful for identifying pilot signals transmitted within a CDMA system in a CDMA signal based cellular telephone system. Once the pilot channel is identified, the phone or "subscriber unit" uses the relevant timing information to perform functions such as monitoring for paging messages and conducting communications.

The' 177 searcher typically works in conjunction with a set of finger elements (finger elements) and a decoder provided on a single integrated circuit. In addition, these elements perform the processing required for CDMA communication and page monitoring. For example, to receive a CDMA signal, the searcher performs pilot channel searches at various time offsets. Upon detection of the pilot channel, the finger is enabled to process an associated data channel, such as a paging channel or a traffic channel. For searching and signal processing, the searcher and finger elements receive samples (samples) generated in response to RF signals received by the subscriber unit. Typically, the samples are generated by an RF/IF unit within the mobile phone or subscriber unit.

In general, it is desirable to reduce the power consumption of the subscriber unit to reduce battery size and weight. In addition, it is desirable to increase the reliability with which paging and other messages are received and processed by the subscriber unit. The present invention has been made for this purpose as well as other purposes.

Disclosure of Invention

In accordance with the present invention, an apparatus for receiving a page is provided. The apparatus comprises the steps of: means for storing a first received sample; means for searching for a pilot channel in the first received sample to generate a set of detected pilot channels; means for demodulating the first receive samples for a quick page message based on the detected pilot channel group; means for generating a second receive sample when the quick page message is detected; means for demodulating the second received samples for a second paging channel.

In one embodiment of the invention, a searcher is used to detect spread spectrum signals. Samples of the received RF signal are stored in a sample buffer. In the standby state, samples are collected during the paging slots assigned to the mobile station. A set of searches is performed on the samples and, if a pilot signal is detected, additional demodulation is performed to detect for paging messages. The resulting set of demodulated data can be combined to increase detection. After detecting the paging message, additional demodulation resources may be activated to process a more complete paging message or other information channel. In one embodiment of the invention, the searcher includes a demodulator to perform quick page detection without finger elements, thereby reducing idle mode power consumption.

Drawings

The features, objects, and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings in which like reference characters identify correspondingly throughout:

FIG. 1 is a cellular telephone system constructed in accordance with an embodiment of the present invention;

FIG. 2 is a block diagram of a subscriber unit constructed in accordance with an embodiment of the invention;

FIG. 3 is a flow diagram illustrating a process performed within a subscriber unit when performed in accordance with one embodiment of the present invention;

fig. 4 is a block diagram of a search when constructed in accordance with an embodiment of the invention.

Detailed Description

Novel and improved methods and apparatus for detecting paging messages are described. The exemplary embodiments described herein are presented in the context of a digital cellular telephone system. Although advantageous for use in this context, different embodiments of the present invention may be incorporated in different environments or configurations. In general, the various systems described herein may be formed using software-controlled processors, integrated circuits, or discrete logic, however, it is preferred that they be implemented on an integrated circuit. It may be advantageous to represent data, instructions, commands, information, signals, symbols, and chips that may be referenced throughout the application by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or a combination thereof. Furthermore, the blocks shown in each block diagram may represent hardware or method steps.

Fig. 1 is a highly simplified block diagram of a cellular telephone system constructed in accordance with the application of the present invention. Mobile telephones and other communication systems (subscriber units) 10 are located between base stations 12, with the base stations 12 being coupled to Base Station Controllers (BSCs) 14. The mobile switching center MSC16 connects the BSC14 to the Public Switched Telephone Network (PSTN) 18. During operation, by connecting with the base station 12, some mobile telephones make telephone calls while, in the event that they monitor for paging messages, other mobile telephones are in an idle or standby mode in which they monitor for paging messages.

Pursuant to some CDMA communication protocol applications, subscriber unit 10 may be simultaneously connected to two base stations in soft handoff. Systems and methods for operating a cellular telephone using CDMA techniques are described in U.S. patent No. 5,103,459 entitled "system and method for generating signal waveforms in a CDMA cellular telephone system," which is assigned to the assignee of the present invention and incorporated herein by reference (the' 459 patent). Essentially, the system of the' 459 patent IS constructed in accordance with the use of IS-95 in the air interface standard.

Further, in one embodiment of the present invention, paging of subscriber units 10 is performed substantially in accordance with the paging methods described in numerous U.S. patent application nos. 08/865,650 and 08/890,355, both of which are entitled "dual channel slotted paging" and are assigned to the assignee of the present invention and incorporated herein by reference (dual channel paging application). In those patent applications, the application of quick paging messages (quick pages) sent on a reduced code channel is described. One or more quick pages are sent prior to a full page message (full page) to allow the subscriber unit to reduce page monitoring time and therefore standby power consumption. If the subscriber unit does not receive a positive quick page, it does not monitor for a full page, thereby reducing idle mode power consumption.

Fig. 2 is a block diagram of a demodulator that processes CDMA signals according to one embodiment of the present invention. The RF/IF system 190 and the antenna system 192 generate receive (Rx) samples, where the system receives an RF signal, filters, downconverts, and digitizes the RF signal to baseband. The samples are supplied to a multiplexer 202 and a sample RAM 204. The output of multiplexer 202 is provided to a searcher unit 206 and finger elements 208 coupled to a control unit 210. Typically, the control unit 210 is a microprocessor controlled by software and may be located on the same integrated circuit or on a separate integrated circuit.

During operation, received samples (samples) are stored in the sample RAM200 and provided to the multiplexer 202. Multiplexer 202 provides either real-time samples or stored samples to searcher unit 206 and finger elements 208. Control unit 210 constitutes finger elements 208 to perform demodulation at different time offsets based on the search results from searcher unit 208. The results of the demodulation are combined and passed to a decoder 214 which outputs the data.

In general, the search performed by searcher 208 tests the timing hypotheses corresponding to each sector with non-coherent demodulation of the pilot channel, while the demodulation performed by finger elements 208 is performed by coherent demodulation of the data channel. Non-coherent demodulation does not require carrier phase information, but detects signal energy rather than data (for a certain type of waveform) contained in the signal. Coherent demodulation requires phase information and therefore more information about the signal, but can determine the data sent on the signal. In this application, the term demodulation refers to coherent demodulation only, and searching refers to noncoherent demodulation. In one embodiment of the invention, despreading is performed by multiplying the received samples with the complex conjugate of the PN and assigned walsh function under a single timing hypothesis, and digitally filtering the resulting samples (typically implemented with an integrate and dump accumulator circuit).

In one embodiment of the present invention, an enhanced searcher is provided that performs pilot channel searching and demodulates the paging channel on samples stored in a sample RAM. Demodulation and searching may be performed at various data offsets, and the demodulation results combined to determine whether to receive a paging message. Preferably, the paging channel demodulated by the searcher is similar to the quick paging channel described in the dual channel paging application referenced above. Just as short for the quick page message duration (104 or 208 microseconds at 128 or 256PN chips of 1.2288 Mcps) and the required anti-distortion is small, the required received samples can be easily buffered (approximately 100-.

Fig. 3 is a flowchart of the operation of the demodulator of fig. 2 in idle mode, according to an embodiment of the present invention. Idle mode is a state in which the user unit is powered up (power up) but not making a call. During idle mode, the subscriber unit detects a paging message addressed to it. The paging message may represent an incoming communication or a telephone call. As described above, the present invention is described in the context of a two channel paging system as described in the dual channel paging application.

In step 300, the subscriber unit collects and stores received samples during the quick paging slots assigned to it in step 302. In one embodiment, the search is performed by energizing the RF/IF cell 190, storing the sample in the sample RAM, and then de-energizing the RF/IF system 190. Typically, the subscriber unit collects samples for a longer duration than a single quick paging slot, thereby storing multiple time offset signals in the received sample set.

In step 304, searcher unit 206 (fig. 2) performs a pilot search on the stored samples at each time offset. Further, pilot searching may be performed for different signals. For example, a search may be performed for signals from different base stations employing pilot codes of different or different offsets. When a local maximum (localmaxima) is detected above a certain threshold and the combining function is enabled for a particular search window, the resulting hypotheses are demodulated and combined. This step is completed once all the hypotheses in the search table are completed.

In one embodiment of the present invention, it is preferable to make sample RAM302 large enough to cover the time offsets of a set of multipath signals. Thus, different pilot signals can be detected by simply searching the same set of samples at different offsets. Similarly, the same set of samples may be demodulated at different offsets to handle quick paging. Although a quick paging channel designed for coherent signaling provides better performance and is preferred in most cases. And a quick paging system may be designed for non-coherent signaling.

In step 306, searcher 206 is switched to a demodulation mode and the paging channel associated with each signal detected during the search mode is demodulated to determine whether a quick page is received. Quick pages are processed by performing coherent demodulation on a set of paging channels corresponding to a set of pilot channels detected during the search. Thus, in one embodiment of the invention, the quick paging channel is demodulated within the searcher after the search is performed. Each demodulation is performed at a particular offset within a sample, and the resulting diversity combination demodulates the soft decision data set using an accumulator within searcher 206.

In step 308, the combined demodulated data is examined to determine if a positive quick page has been received (i.e., indicating that the following full page message can be sent to the subscriber unit 10). If not, the subscriber unit returns to step 300. IF so, finger elements 208, decoder 214 and RF/IF unit 190 are activated at step 3lO and the full page is processed at step 312. In another embodiment of the invention, the subscriber unit continues to search samples for other pilots to find a new signal to process when the next paging slot occurs. In addition, if the quick paging channel is not received with sufficient quality, step 310 is performed to keep the full paging message from being lost.

By performing the search and quick page processing within searcher unit 206, the quick page channel can be monitored without having to activate finger elements until a positive quick page is received. Typically, most quick page messages are negative, indicating that no call or message is pending. Thus, the time to activate finger elements 208 and other circuitry is greatly reduced. Thus, reducing the circuitry used to perform quick paging channel monitoring increases the standby time of the subscriber unit 10.

This circuit reduction is accomplished by taking advantage of the reduced coding level quick paging channel and quick paging message and storing the samples received for processing. This reduced encoding allows demodulation of the quick paging channel to be performed with a limited amount of demodulation functionality and a limited amount of additional complexity within the searcher. Furthermore, the use of sample RAM204 allows a single demodulation engine utilized within searcher 206 to perform multiple time offset demodulation, which greatly reduces the circuitry required to monitor for paging messages.

Additional power savings are realized by employing stored samples to perform search and paging channel monitoring. In one embodiment, the quick paging channel is uncoded BPSK or OOK bits that are transmitted one or two times. In particular, by storing samples as they are generated, the time during which the RF/IF unit 190 operates during each paging cycle is reduced. Once the samples are stored, the subscriber unit deactivates the RF/IF to maintain power and only repeats the search for samples with digital circuitry at different offsets or different pilot signals or both.

As described above, performing different searches on the same sample allows the RF unit to be turned off once the initial set of samples is collected. Turning off the RF unit reduces power consumption of the mobile station in idle mode. Conversely, if the samples are not stored, additional samples must be accumulated as long as each pilot signal and time offset needs to be searched. This continuous aggregation of pilot data requires the RF unit to continue for a longer period of time and thus consumes power, which reduces the standby time of the subscriber unit 10.

The described embodiments of the present invention provide performance enhancements as well as improved idle mode power consumption. In particular, by performing demodulation and searching on the same set of samples, the performance of the demodulation is improved. This is because the best signal determined by the pilot channel search will be the best signal for paging channel demodulation because the sample sets are the same. In other systems, a search is performed on a first set of samples and the search results are used to determine how to demodulate the paging channel in a second set of samples. While consistency (correlation) between the search results and the paging channel quality is generally reasonable if the time span between the two events is small, any channel differences between search and demodulation are virtually eliminated as compared to the fading channel decorrelation time, where the elimination is achieved by performing search and demodulation on the same samples.

Fig. 4 is a block diagram of searcher 206 when constructed in accordance with an embodiment of the invention. The in-phase and quadrature-phase samples are read from sample RAM302 (fig. 2) and despread by QPSK despreader 402 with a PN code from PN code generator 404, where the PN code includes an in-phase Portion (PNI) and a quadrature-phase Portion (PNQ). The resulting in-phase and quadrature-phase components from QPSK despreader 402 are used in multipliers 406 a-d. The processing after the sample RAM may occur at any clock frequency, such as 19MHz, regardless of the original chip rate.

In the search mode, walsh code generators 408 and 410 generate pilot channel walsh codes that are applied to multipliers 406a-406 d. Multipliers 406a-406d and accumulators 408a-408d operate together to despread samples with the pilot walsh code from pilot walsh code generator 408. The QPSK despreader and WALSH multiplication may occur sequentially or combined as a single operation to achieve equivalent results.

The decovered pilot samples from accumulators 408a and 408b are applied twice to multipliers 420 and 422: once directly and once through the multiplexer 422. The result is to square the decovered pilot samples and add the squared outputs with adder 412. Thus, in search mode, the dot product (dot product) of the decovered pilot data is calculated, as well as the correlation energy of the pilot channel at the current offset.

Similarly, the decovered pilot samples from accumulators 408c and 408d are applied to a squaring circuit (square circuit)410, the outputs of which are summed by summer 412. The squaring circuit 410 and summer 412 are then used to calculate the dot product of the decovered pilot data and itself, as well as the correlation energy of the pilot channel at the current offset.

The dot products from adders 412 and 422 are received by local maximum calculator 414. The local maximum calculator 414 determines the most likely offset or offsets from a set of offsets (or hypotheses) attempted by the searcher based on the correlation energy. For example, the local maximum calculator 414 may save the local maximum energy in a set of additional sample correlation energies to isolate the samples closest to the true offset. Multipliers 406a and 406b and accumulators 408a and 408b operate together to despread samples with the quick page walsh code from the quick page walsh code generator.

As the timing of the PN code, an offset group is generated, and the walsh code is adjusted with respect to the samples. In an example search, the PN and walsh codes are adjusted in small increments of a particular search region. Typically, the code generator is formed by a control system that also defines the search area with a start offset and an end offset. The control system may be a microprocessor or digital signal processor controlled by software stored in a memory.

An N-max (N-max) tracker 416 collects a set of N-max correlation energies for different search regions. N is an integer, preferably in the range of 4 to 6. Other criteria for correcting the search results, such as signal source diversity, are consistent with the use of the present invention. The resulting set of correlation energies and correlation offsets (search results) are reported to the control system.

In an exemplary embodiment of the invention, once the search operation is performed, the control system configures the searcher to demodulate a group of pages on the paging channel and offset according to the search results. To perform demodulation of the paging channel (preferably the quick paging channel), a walsh generator 410 is constructed to generate the paging channel walsh codes, and a multiplexer 423 is constructed to apply the outputs of accumulators 408e and 408d to a multiplier 420. In addition, accumulators 408a and 408b are constructed to integrate precisely within a bit duration.

For each signal to be demodulated, the control system forms a PN generator and a walsh generator at a particular offset, and demodulates the samples again. The quick paging channel decover samples from accumulators 408a and 408b are applied to multiplier 420. In addition, the pilot channel decovered samples are applied to a multiplier 420 through a multiplexer 423.

To perform the dot product of pilot and paging data, the output of multiplier 420 is summed by summer 422 and resulting expected (projected) quick paging channel soft decision data is received by latch 424. Various other methods of adjusting the carrier phase are apparent, including the use of cross product operations or other phase rotation methods. The dot product decovers and weights the data in phase with the pilot for combining. Combiner accumulator 426 then receives the output of latch 424. For each signal demodulated, the accumulator 426 adds the demodulation result. Upon demodulating the signal group, the combined quick paging data is output to a control system which anticipates the transmitted data by making a hard determination based on the accumulated soft determination data. Based on the hard determination, it is determined whether to send a quick page.

Further, in one embodiment of the invention, the energy of the decovered pilot channel data is again calculated by performing a dot product operation, and the accumulator 426 accumulates the resulting pilot energy for each signal. The accumulated pilot energy is forwarded to the control system.

In one embodiment of the invention, the control system determines whether to rely on quick paging data based on accumulated pilot energy. If the accumulated pilot energy is above a certain threshold, then the quick paging channel result is relied upon. Otherwise, the next quick paging slot is processed or the full paging channel is processed. As described above, processing the pilot and paging channels with the same samples ensures that the channels are the same for both processes, which improves demodulation performance.

Thus, systems and methods for performing paging monitoring are described. The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (5)

1. An apparatus for receiving a page, comprising:

means for storing a first received sample;

means for searching for a pilot channel in the first received sample to generate a set of detected pilot channels;

means for demodulating the first receive samples for a quick page message based on the detected pilot channel group;

means for generating a second receive sample when the quick page message is detected;

means for demodulating the second received samples for a second paging channel.

2. The apparatus of claim 1, further comprising means for searching on a series of pilot channels from potentially neighboring base stations.

3. The apparatus of claim 1, further comprising means for energizing a demodulation element to demodulate the second received samples.

4. The apparatus of claim 3, further comprising means for energizing an RF unit to produce the second received sample.

5. The apparatus of claim 4, further comprising means for deactivating the RF unit after storing the first receive sample.