MXPA01007057A - Pilot filtering in the presence of phase discontinuities in a cdma receiver - Google Patents

Abstract

A novel and improved method and apparatus for coherent demodulation in the presence of phase discontinuities is described. The pilot signal is prepared for optimal coherent demodulation by the use of two filters:one capable of withstanding the effects of phase discontinuity;a second providing superior filtering performance than the first so long as phase discontinuities are not present. Both filters are simultaneously operated. In the exemplary embodiment ofthe present invention, a sliding window filter (410) is employed for the superior performing filter absent phase discontinuity and a block filter (440) is employed for use when phase discontinuities are present.

Description

PILOT FILTRATION IN THE PRESENCE OF PHASE DISCONTINUITIES IN A CDMA RECEIVER FIELD OF THE INVENTION The present invention relates to communications. More particularly, the present invention relates to a novel and improved method for demodulating coherent data in the presence of phase discontinuities.

BACKGROUND OF THE INVENTION Wireless communication systems that include cellular, satellite and point-to-point communications systems use a wireless link comprised of a modulated radio frequency (RF) signal to transmit data between two systems. The use of a wireless link is desirable for a variety of reasons including increased mobility and reduced infrastructure requirements compared to cable communication systems. A disadvantage of using a wireless link is the limited amount of communication capacity that results from the limited amount of available RF bandwidth.

This limited communication capability contrasts with cable-based communications systems where additional capacity can be added when installing additional cable connections. Recognizing the limited nature of RF bandwidth, various signal processing techniques have been developed to increase the efficiency with which wireless communication systems use RF bandwidth. A widely accepted example of such efficient bandwidth signal processing technique is the air interface standard IS-95 and its derivatives such as IS-95-A and ANSI J-STD-008 (hereinafter referred to collectively as the IS-95 standard) promulgated by the Association of the Telecommunications Industry (TIA) and used mainly in cellular telecommunication systems. The IS-95 incorporates code division multiple access (CDMA) signal modulation techniques to carry out multiple communications simultaneously over the same RF bandwidth. When combined with global power control, carrying out multiple communications over the same bandwidth increases the total number of calls and other communications can be carried out in a wireless communication system, inter alia, by increasing the reuse of frequency compared to other wireless telecommunication technologies. The use of CDMA techniques in a multiple access communication system is described in the U.S. Patent. No. 4,901,307, entitled "SPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", and the US Patent. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", both of which are assigned to the assignee of the present invention and incorporated herein by reference. Figure 1 provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During the operation, a set of subscribing units lOa-d carries out wireless communications by establishing one or more RF interfaces with one or more base stations 12a-d using modulated CDMA RF signals. Each RF interface between a base station 12 and a subscriber unit 10 is comprised of a forward link signal transmitted from the base station 12, and a reverse link signal transmitted from the subscriber unit. Using these RF interfaces, a communication is carried out with another user generally by means of a mobile telephone switching office (MTSO) 14 and by a public switched telephony network (PSTN) 16. The links between the base stations 12, MTSO 14 and PSTN 16 are typically formed through cable connections, although the use of additional RF or microwave link is also known. According to IS-95, each subscriber unit 10 transmits the data to the user through a reverse link signal, not coherent, a channel, at a maximum data rate of 9.6 or 14.4 kbits / sec depending on what rate is selected from a set of rates. A non-coherent link is one in which the phase information is not used by the received system. A coherent link is one in which the receiver exploits the knowledge of the phase of the carrier signals during processing. The phase information typically takes the form of a pilot signal, but can also be estimated from the transmitted data. A coherent reverse link CDMA system is described in copending application 08 / 654,443, entitled "HIGH DATA RATE CDMA WIRELESS COMMUN I CAT ION SYSTEM", filed on May 28, 1996, (hereinafter application 43) assigned to the assignee of the present invention and incorporated for reference herein. In this system, a set of individually adjusted gain subscriber channels is formed through the use of a set of orthogonal subchannel codes having a small number of PN broadcast chips each orthogonal waveform period. In a preferred embodiment of this system, the pilot data is transmitted through a first transmission channel and the power control data is transmitted through a second transmission channel. The two remaining transmission channels are used to transmit unspecified digital data that includes user data or signaling data, or both. The pilot channel carries a pilot signal which is used to determine phase information which allows the demodulation of the data channels. Prior to its use in demodulation, the pilot signal must be filtered to extract as much distortion introduced by the transmission as possible. Typically a low pass filter is used in the pilot signal. Sliding window filters are also known, which deliver superior performance to a blocking filter under some circumstances. An important circumstance for the superior performance of sliding window filters is the lack of phase discontinuity. Because the pilot signal exists to provide phase information, ideally there will be no phase discontinuity in the signal. However, as a practical matter, the use of cost-effective power amplifiers in the subscriber unit will introduce such phase discontinuities. A typical power amplifier of this type can be a linear amplifier in sections, which will produce a discontinuity each time the polarization point is switched. Therefore, there is a need to design demodulators that are capable of demodulating efficiently in the presence of phase discontinuities.

BRIEF DESCRIPTION OF THE INVENTION A novel and improved method and apparatus for coherent demodulation in the presence of phase discontinuities is described. In the exemplary embodiment of this invention, they are known a prior! the times when phase discontinuities occur in the receiver in which the demodulation is carried out. In an alternative mode, the discontinuity location is signaled to the receiver in advance by the transmitter that generates the signals to be demodulated. The pilot signal is prepared for optimal coherent demodulation by using two filters: one capable of withstanding the effects of phase discontinuity; a second that provides a filtration performance superior to the first as long as no phase discontinuities are present. Both filters are operated simultaneously. However, the upper performance filter is selected for use in the determination wherever possible. In the exemplary embodiment of the present invention, a sliding window filter employed for the phase discontinuity absent from the upper performance filter and a blocking filter is employed for use when phase discontinuities are present. Only when the receiver detects that the sliding window filter will be integrated over a phase discontinuity will the receiver select the output of the blocking filter instead of the slip window filter output for use as the pilot signal used in the demodulation. coherent. This technique can be used any time the receiver can know the times when phase discontinuities are likely to occur. They can happen periodically and so the receiver can calculate when they will arrive. Alternatively a transmitter can signal when a phase discontinuity will occur. Similarly, if a transmitter signals subsequent to a phase discontinuity that such an event has occurred, a receiver may compensate for this by delaying the demodulation for such a time as necessary to take into account the incoming phase discontinuity information.

BRIEF DESCRIPTION OF THE INVENTION 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 similar reference characters identify correspondingly throughout. and in which: Figure 1 is a block diagram of a cellular telephone system; Figure 2 is a block diagram of a transmitter and a receiver configured in accordance with the exemplary embodiment of the invention; Figure 3 is a block diagram of a pilot filter configured in accordance with the exemplary embodiment of the invention; Figure 4 is a more detailed block diagram of a pilot filter configured in accordance with the exemplary embodiment of the invention; and Figure 5 is a flow diagram detailing the steps for realizing the present invention; DETAILED DESCRIPTION OF THE INVENTION A novel and improved method and apparatus for coherent demodulation in the presence of phase discontinuities in the context of the reverse link transmission portion of a cellular telecommunications system is described. While the invention is particularly adapted for use in multipoint-to-point reverse link transmission of a cellular telephone system, the present invention is equally applicable to forward link transmissions. In addition, many other wireless communication systems will benefit from the incorporation of the invention, which includes satellite-based wireless communications systems, point-to-point wireless communications systems, and systems that transmit radio frequency signals through the use of coaxial or network cable. other broadband cables. Figure 2 is a block diagram of reception and transmission systems configured as a subscriber unit, or transmitter 100, and a base station, or receiver 200, according to one embodiment of the invention. A pilot signal and the related coherent data are retransmitted through the antenna 101 to the receiver 200 where they are received through the antenna 201. During the typical operation, multiple subscribers with a single base station will be in communication. The modulation format of the signals in the exemplary embodiment of the invention is that described in the aforementioned M43 application. The following description of the receiver 200 is applicable to any coherent modulation system, as will be apparent to those skilled in the art. The signals received from the antenna 201 are processed in the analog receiver 210 chain. A typical analog receiver chain will have subverters, filters and perhaps IF mixers which will subvert the modulated signals to the baseband. Frequently the resulting signal will be subverted to its digital form, and some portion of the subconver s ion can be completed using digital signal processing techniques. Various techniques for performing this subconver sion process are known in the art and can be used in implementing the present invention. The coherent demodulator 220 and the pilot filter 240 are shown as separate blocks in Figure 2. Each is shown to receive an output from an analog receiver string 210. The output from the analog receiver chain 210 which is directed to pilot filter 240 is designated raw pilot signal 280. Pilot filter 240 produces an output signal designated as filtered pilot signal 230, which is connected as an input to coherent demodulator 220. This configuration does not mean that suggests that the present invention be limited to discrete and separate blocks, but is found as shown for the purpose of illustration of the present invention. Typically in practice, the functions of the coherent demodulator 220 and the pilot filter 240 may be incorporated in an entity known as a demodulator or other similar term known in the art. It will be apparent that these and other similar configurations are simply embodiments of the present invention. In addition, the unprocessed pilot signal 280 does not need to be a signal other than that signal that is sent to the coherent demodulator 220 for demodulation. The configuration shows the general nature of the present invention. It may be convenient to separate a pilot from its data in the analog receiver chain 210, but that is not a requirement. In a typical CDMA spread spectrum system, pilot signals are only one of the signal components that must be separated in the demodulator.

In the preferred embodiment, described in the aforementioned application M43, the output of the analog receiver chain 210 will consist of the in-phase and quadrature components of the received baseband signal in digital form. Each sample of these components is known as IQ pairs, and the same digital flow of IQ pairs is sent to both the pilot filter 240 and the coherent demodulator 220. Also shown in Figure 2 are microprocessor 260 and memory 270, which they are connected Typically, a communications system, which includes a receiver or a transmitter, will have such a microprocessor to perform a number of functions for the operation of the communication system. The microprocessor 260, as noted, can be used for the sole purpose required by the present invention, but more likely will also be used for other functions. A similar argument applies for memory 270 as well. For clarity, microprocessor 260 and memory 270 will be described only as necessary to understand the present invention, but techniques that use one or more microprocessors and one or more memories for a variety of tasks in a communications system are well known in the art. matter. The microprocessor 260 acts together with the memory 270 to perform the functions or subroutines programs therein. These functions and subroutines will be described more fully in the following paragraphs. Alternatively, other special-purpose hardware may be implemented to perform the functions described as performed by the microprocessor 260, as will be apparent to those skilled in the art, and the result will simply be an alternative embodiment of the present invention. In the present invention, the microprocessor 260 produces an output, designated "discontinuous boundary 250". This signal indicates the presence of a current phase discontinuity, or it may alternatively be used to signal when a phase discontinuity is permissible. In the example mode, this signal is generated periodically twice per frame at the half-frame boundaries. The receiver does not know if there is in fact a phase discontinuity in each half-frame boundary. But, these are the occasions when it is permissible for the transmitter to change the power amplifier bias points (as defined in the specification), the result of which is prone to producing a phase discontinuity. Other algorithms can be easily employed in the present invention which use a different period, different boundaries, or other calculations appropriate to the type of coherent modulation to be employed. These adaptations are within the scope of the present invention and can be calculated by the microprocessor 260, perhaps in conjunction with the memory 270, or in equivalent special purpose hardware as described above. The alternate embodiments employ other diverse techniques to generate discontinuous signal boundaries 250. In one embodiment, the transmitter 100 signals in advance to a receiver 200 (through common signaling techniques not shown but readily available in the art) that is imminent. a phase discontinuity and the location in time of the discontinuity. In another mode, the transmitter may not be able to transmit the location of a discontinuity until one has occurred. A system employing the present invention can be designed to take this into account when using such techniques as buffering to delay the processing of incoming data, providing time to prepare the discontinuity, culminating in the activation of the signal designated "discontinuous boundary". 250". Both of these techniques have in common that discontinuous boundary 250 will be activated only when there really is a discontinuity. Other similar variations of these will be readily apparent to someone skilled in the art. These techniques are useful when additional optimization through the maximized use of a filter sensitive to discontinuities delivers substantial benefits. In the modality as an example, discontinuous boundary 250 may be activated during some half-frame boundaries which do not include a phase discontinuity, but the fraction of total time that adds to the use of a filter insensitive to non-optimal discontinuities is not significant. The pilot filter 240 acts on an unprocessed pilot signal 280 in conjunction with the dashed border 250 to produce the filtered pilot signal 230. The filtered pilot signal 230 is connected as an input to the coherent demodulator 220. It is used to coherently demodulate the signal of baseband provided by the analog receiver chain 210. In a coherent demodulation system, the distortion in the pilot signal can lead to deleterious demodulation effects. The details of the pilot filter 240 and its configuration in the present invention to combat those harmful effects are described below. Note that, as set forth above, the microprocessor 260 may be in control of other functions of the receiver, including demodulation. Although no connections are shown in this embodiment, it is implicit that if alternate phase discontinuity signaling techniques are employed, the information necessary to create the discontinuous signal boundary 250 can be transmitted to the microprocessor 260 (or some special equivalent circuitry used in its place). Figure 3 provides more details of the pilot filter 240. The raw pilot signal 280 is input to the pilot filter 240 and connected to the inlet of two different filters, designated sensitive filter 300 and insensitive filter 310. The meaning of sensitive and insensitive in this context is the sensitivity of the filter for phase discontinuities. The present invention achieves an increase in performance when the sensitive filter is a filter selected such that it provides pilot filtration performance higher than that of the insensitive filter at any time that phase discontinuities are not present. Of course, as a corollary, the insensitive filter will provide superior pilot filtration performance to the sensitive filter when phase discontinuities are present, but otherwise will provide lower performance. In the exemplary embodiment, the sensitive filter 300 is selected to be a sliding window filter and the insensitive filter 310 is selected to be a blocking filter. The outputs of both the sensitive filter 300 and the insensitive filter 310 are connected as inputs to the multiplexer 320. The multiplexer 320 is controlled by the dashed border 250 to select the output of the sensitive filter 300 to supply the filtered filtered output signal 230 at any time that discontinuous border 250 is inactive. When discontinuous boundary 250 is active, which means that a phase discontinuity is present or allowed to be present, the output of the insensitive filter 310 is selected to be delivered as output by the multiplexer 320 for the output in the pilot signal filtered output 230. In the exemplary mode, the sliding window filter will not work well when performing integration over a discontinuity. Therefore, discontinuous boundary 250 will always be activated to select the blocking filter output at any time the integration occurs over a discontinuity. On other occasions, the output of the sliding window filter will be selected. An alternative filtration technique is to employ a sliding window filter with variable window size, such that the window size can be reduced as it approaches a discontinuity and increased following its occurrence to allow the use of the window filter of sliding for a greater fraction of time. It will be clear to those skilled in the art that a certain number of filters greater than two may be employed within the scope of the present invention. This will be useful any time there are more than two filters, each of which provides optimal pilot filtering under a unique set of possible circumstances. Next, the dashed signal boundary 250 will contain sufficient information to determine which of the available filters is optimal under the circumstances.

In a digital system, this means that a multiple bit signal is used instead of a single bit signal as the selection input of multiplexer 320. Other similar control mechanisms which are known in the art will also fall within the scope of the present invention. Figure 4 shows the exemplary embodiment of the pilot filter 240. As stated above, the sensitive phase discontinuity filter employed is a sliding window filter, and the insensitive phase discontinuity filter employed is a filter of blocking. These two filters operate in parallel, as will be described. The raw pilot signal 280 enters as an IQ pair, designated Pilot_I and Pilot_Q as described above. The accumulators 400 and 405 are optional. They are used to integrate and sub-sample the incoming unprocessed pilot samples. In the modality as an example, the integration is about 256 chips. Other integration periods could be selected, for example, an integration over 64 chips would deliver a sample for each data symbol. The memory requirements for the slip filter are affected by the choice of sub-sample. The sliding window filter 410 in the exemplary embodiment uses a window with constant amplitude. This makes the filtering calculation a simple integration function. Other window shapes may be selected, as is known in the art, but will require multiplication as well as addition. The memory is displayed as two stage shift registers M 412 and 414, one for the in-phase path and one for the quadrature phase path, respectively. The size of this memory is dependent on the sampling rate described above. A sliding window filter integrates over a period of time with each new filter output consisting of the integration of the previous result added to a new sample and from that result the oldest sample must be eliminated. In this way, the window slides through the samples one sample at a time. This filter will continuously operate its sliding functionality, even when the integration includes data with a phase discontinuity in it. Obviously, the results will be quite suboptimal during these periods. For example, a phase variation of 180 degrees in the middle part of a pilot flow can deliver an output of zero. The present invention avoids these harmful effects by ignoring the output of this sliding window filter during those times when the data are unreliable due to the presence of phase discontinuities. In the adders 416 and 418, the oldest samples coming from the stage shift registers M 412 and 414 are subtracted from the new samples coming from the accumulators 400 and 405. The current filter accumulations for the pilot trajectories in phase and quadrature are stored in registers 424 and 426 respectively. These results are updated through the adders 420 and 422 by adding the current accumulation values with the difference calculated in the adders 416 and 418. In many embodiments, such as this, it may be desirable to saturate and truncate values to ranges and widths of specific bits for specific reasons of implementation. This is optional, and is shown in saturated and truncated blocks 428 and 430. The I and Q outputs from the sliding window are shown and designated as I_PILOT__SWN and Q_PILOT_SWN, respectively. The blocking filter 440 uses the accumulators I and Q 442 and 444. In the exemplary mode, these accumulators operate on borders of 1.25 ms. Each 1.25 ms border includes a power control group, and a whole number of power control groups set within the half-frame boundaries. As previously established, phase discontinuities are allowed to occur only at half-frame boundaries. A blocking filter operates when integrating on a data block, and that data block must also be selected in order not to integrate over a discontinuity, for the reasons described. The careful selection of periods for integration ensures that this will not happen. It will be clear to one skilled in the art that the selected periods are only a subset of the possible periods that could be employed in the present invention. Many permutations of the same invention will be clear. For example, integrated blocks on a blocking filter do not need to be periodic. In addition, a filter other than the simple integrating filter can be easily used. Shifters 446 and 448 are optional. They are used in the modality as an example to move to the left by k bits to provide demultiplication. The demultiplication factor k in this mode is the proportion of the section of the sliding window divided by the section of the blocking window. The periods used are not mandatory. In this example, the sliding window period is 2.5 ms and the blocking filter period is 1.25 ms. Outputs I and Q are designated I_PILOT_BLK and Q_PILOT_BLK, respectively. The outputs of the sliding window filter, I_PILOT_SWN and Q_PILOT_SWN, and the outputs of the blocking filter, I PILOT_BLK and Q_PILOT_BLK, are delivered as input to the multiplexers 450 and 460, as shown. A discontinuous signal boundary, as described above, is used to select between the window outputs of the sliding window filter and the outputs of the blocking filter. The results are presented to entries in registers 480 and 490, as shown. These registers are designated by the output of the multiplexer 470, which selects the authorization signal at the appropriate frequency for the sliding window filter, SWN_EN, or the authorization signal at the appropriate frequency for the blocking filter, BLK_EN. The selection is controlled by the dashed signal boundary 250. The outputs of the registers 480 and 490 provide the filtered pilot signal 230, designated in this mode as I_PIL0T and Q_PIL0T. The records as shown in this configuration are illustrative only. Those skilled in the art will be able to configure the present invention in a variety of circuit implementations that interface with the pilot filter. Figure 5 is a flow chart depicting the steps for carrying out the present invention. Block 500 defines the initial state where the sensitive filter output is selected. In practice, at the beginning of a communications session it may be required to follow an initialization sequence where the unresponsive filter is used to initialize the session. When this initialization is completed, the steady state, the most optimal selection will be the selection of the sensitive filter and that is where the flow chart begins. It proceeds from the initial state to block 510 which is shown containing alternate blocks including block 510A, where the next phase discontinuity boundary is calculated, and block 510B, where boundary information from the transmitter is received. In the exemplary embodiment, block 510A is employed as block 510, but it is envisaged that either 510A, 510B, or a combination of both in block 510 may be employed. Once the boundary is known, it is made the determination in block 520 if the discontinuity is currently close to interfering with the operation of the sensitive filter, which means that it will no longer be the optimal selection among the available filters. If this condition is not met yet, the flow will continuously cycle back to block 520 until such time is met. Then proceed to block 530 and select the insensitive filter. Proceed to block 540. Determine if the phase discontinuity continues to interfere with the sensitive filter in such a way as to force it to remain suboptimal compared to the insensitive filter. As long as this condition persists, cycles back to block 540. Once this condition is no longer true, it selects the sensitive and most optimal filter once more and returns to block 510 to wait and / or calculate the next phase discontinuity. Accordingly, a method and apparatus for the coherent demodulation of phase discontinuities has been described. The description is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these modalities will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other modalities without the use of the inventive faculty. Accordingly, the present invention is not intended to be limited to the embodiments shown herein but is in accordance with the broadest scope consistent with the principles and novel features described herein.

Claims (24)

1. CLAIMS Having described the invention as an antecedent, the content of the following claims is claimed as property: 1. A communication system for use in the presence of phase discontinuities in the pilot signal, characterized in that it comprises: a transmitter for transmitting a signal that has a pilot signal; a receiver for receiving the signal, the receiver having: a first filter for filtering the pilot signal; a second filter for filtering the pilot signal in which the second filter provides a higher performance than the first filter when phase discontinuities are present in the pilot signal; and a selector for selecting the output of the second filter when phase discontinuities are not present in the pilot signal, which selects the output of the first filter when phase discontinuities are present in the pilot signal, and which provides the selected output for its use in demodulation.
2. 2. The communication system according to claim 1, characterized in that the location of the phase discontinuities in the pilot signal are limited to their occurrence at periodic boundaries which are known a priori by the receiver.
3. The communications system according to claim 1, characterized in that the locations of the phase discontinuities are transmitted from the transmitter to the receiver.
4. 4. The communication system according to claim 1, characterized in that the first filter is a blocking filter and the second filter is a window filter of desi i zamient.
5. 5. The communication system according to claim 2, characterized in that the first filter is a blocking filter and the second filter is a window filter of desi i zamient.
6. The communication system according to claim 3, characterized in that the first filter is a blocking filter and the second filter is a sliding window filter.
7. 7. A transmitter that transmits a pilot signal in which phase discontinuities in the pilot signal are allowed only at predetermined boundaries.
8. 8. A transmitter that transmits a pilot signal and the location of any phase discontinuity that occurs in the pilot signal.
9. 9. A receiver for coherently receiving a pilot signal characterized in that it comprises: a first filter for filtering the pilot signal; a second filter for filtering the pilot signal in which the second filter provides a higher performance than the first filter when phase discontinuities are present in the pilot signal; a selector to select the output of the second filter when no phase discontinuities are present in the pilot signal, which selects the output of the first filter when the phase discontinuities are present in the pilot signal, and which provides the selected output for its use in demodulation.
10. 10. The receiver according to the rei indication 9, characterized in that the first filter further comprises a processor for determining the location of the phase discontinuities and controlling the output of the selector.
11. 11. The receiver according to the claim 9, characterized in that the first filter is a blocking filter and the second filter is a sliding window filter.
12. The receiver according to claim 10, characterized in that the first filter is a blocking filter and the second filter is a sliding window filter.
13. The receiver according to claim 9, characterized in that it also comprises a demodulator to receive the selected output.
14. 14. A pilot filter characterized in that it comprises: a first filter for filtering a pilot signal; a second filter for processing the pilot signal in parallel with the first filter, in which the second filter provides a performance superior to that of the first filter when the pilot signal contains phase discontinuities; and a multiplexer for outputting the output of the second filter in response to a selection signal indicating the lack of phase discontinuities in the pilot filter and for outputting the output of the first filter in response to the selection signal that indicates the presence of phase discontinuities in the pilot signal.
15. 15. The pilot filter according to claim 14, characterized in that the first filter is a blocking filter and the second filter is a filter window filter.
16. 16. The pilot filter according to claim 15, characterized in that the blocking filter is an integrator and the sliding window filter is a slide integrator.
17. The pilot filter according to claim 16, characterized in that the sliding integrator comprises: a shift register for storing a sample window section when storing each incoming sample; and an accumulator to accumulate the incoming sample and subtract the oldest sample in the displacement record.
18. 18. A method for filtering a pilot signal in a communication system characterized in that it comprises the steps to: a) simultaneously filter a pilot signal with a first filter and a second filter, wherein the second filter provides a superior performance to the first in presence of phase discontinuities in the pilot signal; b calculating a phase discontinuity location and producing a selection signal which indicates when the discontinuity may interfere with the performance of the second filter; c select for the output of the pilot filter the output of the second filter when the selection signal is secured; and selecting the output of the first filter for the output of the pilot filter when the selection signal is not ensured; 19.
19. A method for filtering a pilot signal in a communication system characterized in that it comprises the steps to: a) simultaneously filter a pilot signal with a first filter and a second filter, wherein the second filter provides a higher performance than the first in presence of phase discontinuities in the pilot signal; b) receiving a phase discontinuity location and producing a selection signal which indicates when the discontinuity may interfere with the performance of the second filter; select for the output of the - 3; pilot filter the output of the second filter when the selection signal is secured; and selecting the output of the first filter for the output of the pilot filter when the selection signal is not ensured; 20.
20. A communication system for use in the presence of phase discontinuities in the pilot signal characterized in that it comprises: a transmitter for transmitting a signal having a pilot signal; a receiver for receiving the signal, the receiver having: a first filter for filtering the pilot signal; a second filter for filtering the pilot signal in which the second filter provides a higher performance than the first filter when phase discontinuities are present in the pilot signal; and a selector for selecting the output of the second filter and the output of the first filter and providing the selected output for use in demodulation.
21. The communications system according to claim 20, characterized in that the selector selects the output of the second filter when the phase discontinuities are not present in the pilot signal and selects the output of the first filter when the phase discontinuities can be present in the the pilot signal.
22. 22. The communication system according to claim 21, characterized in that the possible phase discontinuities occur at predetermined times.
23. 23. The communication system according to claim 22, characterized in that the possible phase discontinuities occur periodically. The communications system according to claim 21, characterized in that the locations of the possible phase discontinuities are transmitted from the transmitter to the receiver.