MXPA99007197A - Compensating for fading in analog am radio signals - Google Patents

Abstract

A fading compensation apparatus and method are provided in which blocks of the analog AM signal (e. g., voice) are time-compressed at a transmitter (10). Gaps in the time-compressed signal (15) are filled with pilot symbols (17) (or similar complex waveforms) having known characteristics, such as predetermined amplitudes. The time-compressed analog AM signal (e.g., SSB or VSB) is transmitted over the fading channel. At an analog AM receiver (20), an estimate (24) of the fading that occurred in the transmission channel is made by observing the amplitude of the received pilot symbols at discrete points in time, and interpolating (26) to determine the amount of fading that occurred across the total transmitted signal. A fading compensator (30) compares the received faded time-compressed signal (28) with the interpolation results (27), and adjusts the amplitude and phase of the received signal accordingly to compensate for the fading that occurred in the transmission channel. The compensated analog AM signal is time expanded (32) and output to a user.

Description

DISARMANCE COMPENSATION IN ANALOGUE AM RADIO SIGNALS BACKGROUND OF THE INVENTION Technical Field of the Invention The present invention relates generally to the field of radio communications, and more particularly, to an apparatus and method that compensates for fading in radio signals of Modulated amplitude (AM) analog. Description of the Related Art A major problem encountered in the transmission of radio signals to mobile receivers (or from mobile transmitters) is referred to as "fading." Fading in a transmitted radio signal can occur in two ways: 1) the transmitted signal is absorbed or blocked by obstacles (for example, buildings, metal towers, etc.) located between the radio transmitter and the receiver or (2) the transmitted signal is reflected and scattered through various obstacles located along the transmission path The type of fade absorption or blockage is known as * shading "or" normal log "fade.The type of reflection or scattering is known as 'multipath' fade or is it? Rayleigh. "In the Rayleigh fading, the reflected signals follow several paths to the receiver and therefore arrive out of phase.This means that the received signal is the sum of the reflected signals, which differ in phase (and to some extent in amplitude). These signals with phase difference are added as vectors, so that at certain moments, the vector sum of the signals can be almost zero, therefore, the strength of the signal received at this moment is also almost zero. Analog signals are transmitted on radio channels that use AM or frequency modulation (FM), for example, analogue voice signals are transmitted using AM in a mobile telephone system, such as the air telephone system (mobile phone). air to ground) produced by GTE Commercial and amateur fixed radio broadcast systems also transmit analog voice signals using AM. Analog oz can include conventional AM, single sideband (SSB), double sideband (DSB), or residual sideband (VSB) transmissions. Frequency modulation is typically used for many cellular mobile transmissions. An advantage of using FM in comparison to AM for cellular transmissions is that FM is much less sensitive to fading than AM, because the voice information carried is decoded as phase changes in the FM transmission but the modulation envelope remains constant. Since no information is present in the amplitude of the signal, the information can be reconstructed even if the fading causes a loss of fitness information. On the other hand, an advantage of using AM compared to FM is that AM transmissions occupy a smaller portion of the frequency spectrum than FM transmissions. However, SSB and VSB AM transmissions are highly sensitive to fading. Because the voice information is carried both in the phase and in the amplitude of the AM signal transmitted. Several methods have been employed to make AM SSB and VSB signals less sensitive to Rayleigh fading. One of these methods inserts a "pilot tone" that has a predetermined magnitude in the AM carrier signal transmitted In the AM receiver, the magnitude and phase of the received pilot tone is detected and used to estimate and compensate the effects of the channel However, pilot tones cause relatively large fluctuations in carrier signal and broaden signal bandwidth requirements Pilot symbols (as opposed to tones) inserted in digital signal transmissions are used to compensate for Rayleigh fading A technique used to insert pilot symbols into digital signal transmissions to compensate for Rayleigh fading is presented in U.S. Patent No. 5.Marchetto et al (^ Marchetto "), Marchetto presents the use of pilot symbols that are appended to a plurality of data symbols to form successive frames that are modulated in a transmitter. data and pilot signal The data signal consists of data symbols, and the pilot signal consists of pilot symbols, and channel impulse response estimates are made with successive blocks of pilot symbols, these estimates are interpolated and applied to successive symbols of data in the data signal (delayed), which allows the receiver to decode the data and compensate for fading effects, however, Marchetto's use of pilot symbols for fading compensation is presented for digital signals only and not for signals analogies of the invention Therefore there is a need in the industry for mobile radio A technique that can be used to compensate for the fading of an analog AM signal. In accordance with the present invention, an apparatus and fade compensation method are provided wherein blocks of analog AM signals (eg voice) are compressed in time on a transmitter. Spaces in a time compressed signal are filled with pilot symbols or similar complex waveforms having known characteristics, such as predetermined amplitudes and phases. The analog AM signal compressed in time (for example SSB or VSB) is transmitted in the fading channel. In an analog AM receiver, an estimate is made of the fade that occurred in the transmission channel by the amplitude of the pilot symbols received in discrete times of time, and by interpolation to determine the amount of fading that occurred in the transmitted signal total. A fading compensator compares the compressed signal by fade time received with the results of the interpolation and adjusts the phase amplitude of the received signal to compensate for the fade that occurred in the transmission channel. The compensated analog AM signal is expanded in time and sent to a user. An important technical advantage of the present invention is that the use of pilot symbols in comparison with pilot tones to compensate fading in an analog AM signal is that the pilot symbols cause significantly lower transmitted signal envelope fluctuations and require a width of transmission band lower than pilot tones. Another important technical advantage of the present invention is that the quality of the analog AM voice signals perceived in the receiver is significantly improved compared to the received signals that are not compensated by fading using pilot symbols. Another important technical advantage of the present invention is that the use of pilot symbols for fading compensation of analog AM signals instead of pilot tones allows the use of signal coding schemes that require a common time base between the transmitter and the receiver employees. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the method and apparatus of the present invention can be obtained with reference to the following detailed description when taken in conjunction with the accompanying drawings where: Figure 1 is a schematic block diagram of a transmitter analog AM that employs pilot symbols to compensate for fading, in accordance with a preferred embodiment of the present invention; and Figure 2 is a schematic block diagram of an analog AM receiver employing pilot symbols to compensate for fading, in accordance with the preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED MODALITY The preferred embodiment of the present invention and its advantages are better understood with reference to figures 1-2 of the drawings, where the same numbers are used for similar and corresponding parts of the various drawings. Figure 1 is a schematic block diagram of an analog AM transmitter employing pilot symbols to compensate for fading, in accordance with a preferred embodiment of the present invention. With reference to Figure 1, an analog AM transmitter 10 is shown. For this illustrative embodiment, a transmitter 10 includes a signal processor unit 12, said unit processes an incoming analog signal, such as, for example, analog speech signals from a microphone (not explicitly illustrated) and produces a signal of SSB or VSB. However, the basic concept of the present invention is not limited to this, and may include fading compensation using pilot symbols for AM or analog DSB radio signals, for example, or the analog AM (video) portion of a television broadcast. The output of the signal processor 12 is connected through the line 13 (for example, coaxial transmission cable) to an input of a compression unit at time 14, which compresses the processed analog signal over time. Accordingly, spaces in time are created between successive blocks of an analog signal compressed in time. The signal is compressed in time primarily to make room for the introduction of pilot symbols. The output of the compression unit at time 14 is connected via line 15 (for example, coaxial cable) to an input of a signal multiplexer 16. An output of a pilot symbol generator 17 is connected through from line 18 (for example, coaxial cable) to a second input of a signal multiplexer 16, which multiplexes the signal blocks compressed in time with one or more of a plurality of pilot input symbols (or complex waveforms) that perform a similar function). The pilot symbols are inserted in this way in the analog signals to be transmitted. An output of the signal multiplexer 16 is connected by a conventional transmission line to a conventional output power section (not explicitly illustrated) of analog AM transmitter 10. The multiplexed analog SSB or VSB signals containing the pilot symbols are then transmitted from a conventional antenna. In an exemplary application, instead of connecting the compressed signal in time from the compression unit 14 directly to an input of a signal multiplexer 16, the signal compressed in time can be sampled, processed, and filtered to produce an analog signal which can then be connected to the input of a signal multiplexer 16 and transmitted together with the pilot symbols. These signal samples can then be arranged, for example, in the form of signal coding. Figure 2 is a schematic block diagram of an analog AM receiver employing pilot symbols to compensate for fading, in accordance with the preferred embodiment of the present invention. With reference to Figure 2, an analog AM receiver 20 is shown. For this illustrative embodiment, a receiver section 20 includes a demultiplexer 22 that demultiplexes the analog SSB or VSB signals received from the front end portion of the receiver (not explicitly illustrated), and thereby separates the received signals into one or several pilot symbols in one output, and an analog signal compressed in time in a second output. The pilot symbol (s) output of the demultiplexer 22 is connected by means of a transmission line 23 (e.g., coaxial cable) to an input of a fading estimation unit 24. The analog signal output (s) of the demultiplexer unit 22 is connected by means of a transmission line 28 to an input of a compensation unit. fading 30. The output of a fading estimation unit 30 is connected by means of a transmission line 25 to an input of an interpolation unit 26. The output of the interpolation unit 26 is connected through a line of transmission 27 to a second fading compensation unit input 28. The output of a fading compensation unit 28 is connected to an input of an expansion unit at time 30. The output of the expansion unit in the time 30 is connected to the output section (not explicitly illustrated) of the receiver 20. In operation, the analog input signal (typically voice) to the t Transmitter 20 is processed to produce an analog signal SSB or VBS. The signal SSB or VSB is preferably compressed in time, which creates successive blocks of signals. Pilot symbols from a pilot symbol generator 18 are inserted into the analog signal blocks in the multiplexer 16, thereby multiplexing the compressed time signals and the pilot symbols. The resulting compressed SSB or VSB analog signal blocks containing pilot symbols (or complex waveforms carrying out an equivalent function) are then transmitted. Upon receiving the multiplexed time-compressed SSB or VSB analog signals, a demultiplexer 22 extracts the pilot symbols (or equivalent waveforms) by demultiplexing the received analog signals. A fading estimation unit 24 analyzes the extracted pilot symbols to determine fading at discrete points of time. An interpolation unit 26 interpolates between the discrete values by calculating a fade estimate that occurred for all time. This interpolation can be achieved due to the fact that the fading process has a relatively low bandwidth. However, the rate of occurrence of the fading analyzed could be more than twice the maximum fading frequency that occurred. It should be noted that, in order to improve the fade estimate, the amplitude of the inserted pilot symbols must be, maximum of an amplitude level allowed for the received analog signal. By comparing the interpolated fading estimate that occurred with the compressed signal in the fading time, the fading compensation unit 30 adjusts the amplitude and phase of the received signal accordingly to compensate for the fading that occurred. The signal compensated by fading, compressed in time, is then expanded in time and sent to a user. In order for the fading compensation apparatus described above to function efficiently, transmitter and receiver operations must be synchronized in such a manner that the receiver knows at what point of time each pilot symbol was inserted into the analog signal. Pilot symbols (or complex waveforms) can be used to fine-tune this synchronization.

A primary advantage of using pilot symbols to compensate for fading in an analog AM voice transmission is that the quality of the perceived speech information is significantly better compared to previous techniques. Also, since the transmitter and the receiver are synchronized, certain coding schemes can be employed for the analog signal, which are based on the fact that the transmitter and the receiver have a common time base. Accordingly, the present invention allows the use of coding schemes based on changing keys, which can not be used in the case of systems using pilot tones for fading compensation. An example of such a key-based coding scheme is what is known as the "RSA algorithm", which can be used to create public key / private key pairs for use in encoding transmitted messages. Knowing the private key in the pair, any message encoded with the public key can be decoded. Alternatively, instead of using the RSA algorithm to create a public key / private key pair, you can use what is known as an "exponential key exchange" algorithm of Diffie-Hellman in order to let the transmitter and the receiver are agreed to in a secret session key, which is used to encode the messages transmitted. Since the use of pilot symbols allows synchronization of transmitter and receiver operations, these types of coding schemes can be effectively employed. Although a preferred embodiment of the method and apparatus of the present invention is illustrated in the accompanying drawings and has been described in the Detailed Description, it will be understood that the invention is not limited to the embodiment presented, but can be lent to numerous arrangements, modifications and substitutions without departing from the spirit of the invention in accordance with what is presented and defined in the following claims.

Claims (23)

1. CLAIMS A system for use to ensate fading of an analog AM signal, rising: an analog AM transmitter, said transmitter includes: a pilot symbol generator; a signal time ressor; and a signal multiplexer, an output of said pilot symbol generator and said signal time ressor are connected to a respective input of said signal multiplexer; and an analog AM receiver, said receiver includes: a signal demultiplexer; a fade estimation apparatus; an interpolation apparatus; and a fade ensation apparatus, an output of said fade estimation apparatus is connected to an input of said interpolation apparatus, a first output of said signal demultiplexer is connected to an input of said fade estimation apparatus. , a second output of said signal demultiplexer which is connected to a first input of said fading ensation apparatus, and an output of said interpolation apparatus is connected to a second input of said fading ensation apparatus.
2. The system according to claim 1, wherein said analog AM transmitter further includes a signal processor for the conversion of an analog input signal into an analog AM signal, an output of said signal processor is connected to an input. of said ressor in the signal time.
3. The system according to claim 1, wherein said analog input signal rises a speech signal.
4. The system according to claim 2, wherein said analog AM signal rises an SSB signal.
5. The system according to claim 2, wherein said analog AM signal rises a signal of VSB.
6. The system according to claim 1, wherein said analog AM receiver further includes an expansion apparatus in the time of signals, an output of said fading ensation apparatus is connected to an input of said time expansion apparatus. signal.
7. The system according to claim 1, wherein said pilot symbol generator is operated to generate a plurality of lex waveforms.
8. The system according to claim 1, wherein said analog AM signal rises a coded signal.
9. The system according to claim 8, wherein said encoded signal is based on type of changing key coding scheme.
10. The system according to claim 8, wherein said encoded signal is derived from an RSA algorithm.
11. The system according to claim 8, wherein said encoded signal is derived from a Diffie-Hellman exponential key exchange algorithm.
12. An analog AM signal transmitter for use in fading ensation, rising: a pilot symbol generator; a signal time ressor; and a signal multiplexer, an output of said pilot symbol generator and said signal time ressor are coupled to a respective input of said signal multiplexer.
13. The analog AM transmitter, according to claim 12, wherein said pilot symbol generator can operate to generate a plurality of pilot symbols, each of said pilot symbols has a predetermined amplitude and phase.
14. The analog AM signal transmitter according to claim 12, wherein said signal time ressor can operate to ress an analog AM signal over time, such that gaps occur in said analog AM input signal.
15. The analog AM signal transmitter according to claim 14, wherein said pilot symbol generator can operate to generate a plurality of pilot symbols, and said signal multiplexer can operate to insert at least one of the plurality of pilot symbols. in at least one of these spaces.
16. 16. The analog AM signal transmitter according to claim 12, which further comprises a signal processor for converting an analog input signal into an analog AM signal, an output of said signal processor is connected to an input of said compressor in the signal time.
17. 17. An analog AM signal receiver for use in fading compensation, comprising: a signal demultiplexer; a fade estimation apparatus; an interpolation apparatus; and a fading compensation apparatus, an output of said fading estimation apparatus is connected to an input of said interpolation apparatus, a first output of said signal demultiplexer is connected to an input of said fading estimation apparatus. , a second output of said signal demultiplexer is connected to a first input of said fading compensation apparatus, and an output of said interpolation apparatus is connected to a second input of said fading compensation apparatus.
18. 18. The analog AM signal receiver according to claim 17, further comprising a signal time expansion apparatus, an output of said fading compensation apparatus is connected to an input of said time-of-time expansion apparatus. signs
19. 19. The analog AM signal receiver according to claim 17, wherein said signal demultiplexer operates to separate an analog AM input signal received in a first signal comprising a plurality of faded pilot symbols and a second signal comprising a plurality of vanishing analog AM input symbols, and said first signal is connected to said fading estimation apparatus and said second signal is connected to said fading compensation apparatus.
20. 20. A method for compensating fading of an analog AM signal, comprising the steps of: compressing said analog AM signal over time; inserting a plurality of pilot symbols in said analog AM signal compressed in time to form a composite signal, and transmitting said first compound; receiving said transmitted composite signal; preparing said received signal -in a first faded signal comprising said plurality of pilot symbols, and a second faded signal comprising said analog AM signal compressed in time; and compensating the fading using said first fading signal and said second fading signal to form a signal compensated by fading.
21. The method according to claim 20, wherein said compensation step further comprises the steps of: estimating the fading of said analog AM signal using said first fading signal; interpolate said estimate of said fading; and varying an amplitude and phase of said second faded signal in response to a result of said interpolation step.
22. 22. The method according to claim 20, further comprising the step of expanding said fading compensated signal over time.
23. 23. The method according to claim 20, wherein said analog AM signal comprises a speech signal. The method according to claim 20, wherein said analog AM signal comprises an SSB signal. The method according to claim 20, wherein said analog AM signal comprises a signal of VSB. The method according to claim 20, wherein said analog AM signal comprises a coded signal. A method for compensating fading of an analog AM signal, comprising the steps of: displaying the analog AM signal to produce a plurality of signal samples; processing said plurality of signal samples to produce a plurality of processed signal samples; filtering said plurality of samples of processed signals to produce a filtered signal; mixing a plurality of pilot symbols with said filtered signal to form a composite signal, and transmitting said composite signal; receiving said transmitted composite signal; separating said received composite signal into a first faded signal comprising said plurality of pilot symbols, and a second faded signal comprising said filtered signal; and compensating the fading using said first fading signal and said second fading signal to form a fading compensated signal. The method according to claim 27, wherein said plurality of signal samples comprise a coded signal.