DUAL-MODE RECEIVER FOR RECEIVING SATELLITE AND TERRESTRIAL SIGNALS IN A DIGITAL BROADCAST SYSTEM

Abstract: A dual-mode receiver is provided to receive QPSK-modulated satellite signals and multicarrier modulated (MCM) terrestrial signals. The dual-mode receiver has a combined architecture comprising a minimal number of filters selected to facilitate both QPSK and MCM demodulation.

Description: DUAL-MODE RECEIVER FOR RECEIVING SATELLITE AND TERRESTRIAL SIGNALS IN A DIGITAL BROADCAST SYSTEM BACKGROUND AND SUMMARY OF THE INVENTION The invention relates to a receiver for use in a digital broadcast system, such as the proposed digital audio radio service (DARS), which has a combined architecture for receiving both satellite signals and terrestrial signals. The broadcast system overcomes obstructions to line of sight (LOS) satellite signal reception at fixed and mobile radio receivers by employing one or more terrestrial repeaters. The terrestrial repeaters receive a QPSK-modulated, time-division multiplexed (TDM) satellite signal, perform baseband processing of the satellite signal, and retransmit the satellite signal via multicarrier modulation (MCM). A digital filter is employed which reduces the complexity of the receiver by fulfilling the requirements of QPSK satellite signal reception, as well as operating as a downsampling filter before MCM demodulation of the terrestrial signal. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 depicts a digital broadcast system for transmitting satellite signals and terrestrial signals; Fig. 2 is a block diagram illustrating a broadcast segment and a terrestrial repeater segment of a digital broadcast system in accordance with a preferred embodiment of the present invention; Fig. 3 illustrates a frequency plan for satellite signals and terrestrial signals in a full-diversity broadcast system in accordance with a preferred embodiment of the present invention; Fig. 4 is a block diagram of a receiver for satellite signals and terrestrial signals constructed in accordance with a preferred embodiment of the present invention; - 2 - Fig. 5 is a block diagram of a receiver arm for quadrature phase shift keyed (QPSK) satellite signals constructed in accordance with a preferred embodiment of the present invention; Fig. 6 illustrates the frequency response of a surface acoustic wave (SAW) filter for satellite signals and terrestrial signals in accordance with a preferred embodiment of the present invention; Fig. 7 illustrates the frequency response of a digital filter for satellite signals in accordance with a preferred embodiment of the present invention; Fig. 8 is a block diagram of a receiver arm for multicarrier-modulated (MCM) terrestrial signals constructed in accordance with a preferred embodiment of the present invention; Fig. 9 illustrates the frequency response of an MCM terrestrial signal following surface acoustic wave filtering in accordance with a preferred embodiment of the present invention; Fig. 10 illustrates the frequency response of an MCM terrestrial signal following digital filtering in accordance with a preferred embodiment of the present invention; Fig. 11 is a block diagram of a receiver configured to receive both QPSK satellite signals and MCM terrestrial signals; and Fig. 12 is a block diagram of a receiver having a combined architecture for receiving and demodulating QPSK satellite signals and MCM terrestrial signals in accordance with a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 depicts a digital broadcast system 10 comprising at least one geostationary satellite 12 for line of sight (LOS) satellite signal reception at radio receivers indicated generally at 14. Another geostationary satellite 16 at a different orbital position can be provided for time and/or spatial diversity purposes as discussed below in connection with Fig. 3. The system 10 further comprises at least one terrestrial repeater 18 for retransmission of satellite signals in geographic areas 20 where LOS reception is - 3 - obscured by tall buildings, hills and other obstructions. The radio receiver 14 is preferably configured for dual-mode operation to receive both satellite signals and terrestrial signals and to select one of the signals as the receiver output. The broadcast segment 22 and the terrestrial repeater segment 24 of the system 10 will now be described with reference to Fig. 2. The broadcast segment preferably includes encoding of a broadcast channel into a 3.68 Megabits per second (Mbps) time division multiplex (TDM) bit stream, as indicated in block 26. The TDM bit stream comprises 96 16 kilobits per second (kbps) prime rate channels and additional information for synchronization, demultiplexing, broadcast channel control and services. Broadcast channel encoding preferably involves MPEG audio coding, forward error correction (FEC) and multiplexing. The resulting TDM bit stream is modulated using quadrature phase shift keying (QPSK) modulation, as shown in block 28, prior to transmission via a satellite uplink 30. With continued reference to Fig. 2, the terrestrial repeater segment comprises a satellite downlink 32 and a demodulator 34 for performing QPSK demodulation to obtain the baseband TDM bit stream. A puncturing and delay block 36 reduces the TDM bit rate from 3.68 Mbps to 3.067 Mbps by removing selected bits which can be re-inserted at the radio receiver, and also delays the entire TDM bit stream by the amount of time- diversity delay (if any) between the transmissions from the satellites 12 and 16. The delayed, reduced rate bit stream is then subjected to multicarrier modulation in block 38 prior to being amplified by amplifier 39 and transmitted from a terrestrial repeater tower 40. Multicarrier modulation preferably involves dividing the 3.067 Mbps TDM bit stream in the time domain into 432 parallel paths, each carrying 7100 bits per second. The bits are paired into two-bit symbols identified as the imaginary (I) component and real (Q) component, respeαively, of a complex number. Thus, the complex symbol rate is 3550 symbols per second. The 432 parallel complex number numbers are provided as frequency coefficient inputs to a discrete inverse Fourier transform converter that is preferably implemented using a 512-coefficient inverse fast Fourier transform (IFFT) operating with 2 n inputs and outputs, where n=9 and 80 input coefficients are set to zero. The output of the IFFT is a set of 432 QPSK orthogonal sine coefficients which constitute - 4 - 432 narrow band, orthogonal carriers supporting a symbol rate of 3550 per second and having a symbol period of 280 microseconds. A frequency plan for a full-diversity, two-satellite broadcast system 10 is depicted in Fig. 3. In a preferred embodiment of the system 10, the satellites 12 and 16 of Fig. 1 each broadcast the same programs A and B. The "early" satellite 12 transmits the programs A and B prior to their transmission via the "late" satellite 16. The frequency plan assigns frequency bands for each of the four QPSK-modulated satellite signals as indicated at 42, 44, 46 and 48, respeαively, in Fig. 3. In addition, two frequency bands 50 and 52 are assigned to the multicarrier-modulated program A and B signals transmitted from the terrestrial repeaters, which retransmit the signal from the early satellite 12 with a delay sufficient to time-synchronize the signal with that transmitted by the late satellite 16. In the frequency plan, channel separation is relatively small, with each of the six frequency bands 42-52 occupying approximately 2.07 megahertz (MHz), in an overall frequency bandwidth of 12.5 MHz. With reference to Fig. 4, a dual-mode radio receiver is illustrated which comprises a first arm 54 for receiving a QPSK signal from the early satellite 12, a second arm 56 for receiving a QPSK signal from the late satellite 16 and an MCM signal from a terrestrial repeater 18, and a combining unit 58 for generating a receiver output from two received signals. The two arms 54 and 56 allow for full diversity reception. The QPSK/MCM arm 56 of the radio receiver is implemented as a dual-mode receiver arm for satellite and terrestrial signal reception. QPSK signals received from the satellites 12 and 16 are demodulated in block 60 and 62, respeαively. An MCM signal from a terrestrial repeater 18 (consisting of a delayed, multicarrier-modulated version of the signal transmitted by the early satellite 12) is also demodulated, as indicated in block 64. A depuncturing unit 66 reinserts bits into the demodulated signal from the terrestrial repeater to increase the bit rate to that of the original TDM bit stream. The QPSK/MCM receiver arm 56 is configured to detect when a terrestrial repeater signal is present, as indicated in block 67, and selects the terrestrial repeater signal in lieu of the signal from the late satellite 16 via a seleαion unit 68. In a broadcast system having at least one satellite and at least one terrestrial repeater, the terrestrial signals in bands 50 and 52 of Fig. 3 can be absent or negligible when a radio receiver is in a rural area - 5 - outside the range of the terrestrial repeater. If the radio receiver is mobile and in use while the user is approaching a city or urban area, both satellite and terrestrial signals can be received. If the radio receiver is mobile and in use while driving within a city, however, only the terrestrial signals can be received in many instances because no LOS signal reception from a satellite is possible. If the strength of the terrestrial signal exceeds a predetermined threshold, the dual-mode receiver arm 56 of the radio receiver switches from receiving signals from the satellite 16 to receiving signals from the terrestrial repeater 18. The QPSK demodulators 60 and 62, the MCM demodulator 64, the depuncturing unit 66, the terrestrial detection unit 67 and the selection unit 68 depiαed in Fig. 4 are described in further detail below. The signal at the output of the QPSK demodulator 60 in the receiver arm 54, and the signal at the output of the selection unit 68 in the receiver arm 56, are TDM demultiplexed and decoded, as indicated in blocks 70 and 72 in Fig. 4, to recover the baseband bit stream. As indicated in block 74, the bit stream recovered from the satellite 12 in the receiver arm 54 is delayed by the amount of delay between the broadcasts from the early satellite 12 and late satellite 16 to bring the bit stream into time synchronization with the bit stream produced by the receiver arm 56. The signals from the receiver arms 54 and 56 are then subjeαed to post-deteαion diversity combining, as indicated in block 58, prior to MPEG audio decoding in block 78. It is to be understood that a radio receiver need not support satellite diversity and therefore can be implemented with only the QPSK/MCM arm 56 and without the QPSK arm 54. In such a radio receiver, the post- deteαion combining unit 58 can also be omitted. As shown in Fig. 3, the level of a terrestrial signal is substantially higher than a satellite signal and can be, for example, on the order of 30 decibels (dB) higher than the satellite signal. As stated previously, channel separation in the frequency plan is relatively small. Accordingly, filtering with high stop band attenuation is required to decode the satellite signal if a terrestrial signal is present in the adjacent channel as shown in Fig. 3. Typically, such filtering is avoided by increasing the frequency separation between the satellite and terrestrial signal channels. Channel filters are used to suppress the adjacent channel when separation is sufficient. - 6 - In accordance with an embodiment of the present invention, channel seleαion is implemented using filters which do not entirely suppress the adjacent channel. As will be discussed below, the location of the terrestrial repeater frequency bands 50 and 52 in the lower part of the frequency plan (and adjacent to the frequency bands 46 and 48, respeαively, of the late satellite 16) facilitates seleαion of the satellite signal or the terrestrial signal for receiver output. Filtering will be described in conneαion with QPSK demodulation, and then in conneαion with MCM demodulation, before describing filtering in a combined QPSK/MCM dual-mode receiver arm (Fig. 12) constructed in accordance with the preferred embodiment of the present invention. QPSK Satellite Signal Demodulation A schematic block diagram of a QPSK satellite signal receiver arm 80 is depiαed in Fig. 5. An antenna 82 and low noise amplifier (LNA) 84 at the radio receiver receive a signal at a carrier frequency of approximately 2.3 gigahertz (GHz). The signal is downconverted to a first intermediate frequency ( F) of approximately 135 MHz by a mixer 86 and a local oscillator 88. Signals from the mixer 86 are provided to a low-loss surface acoustic wave (SAW) intermediate frequency (IF) filter 90 and to a second mixer 92 and local oscillator 94 for downconversion to a second IF of approximately 3.68 MHz. A weak or "leaky" SAW filter is preferred to a strong SAW filter having better adjacent channel suppression. As depiαed in Fig. 6, a terrestrial channel 50 located direαly adjacent to a satellite channel 46 is partially within the SAW filter passband and the attenuation of this interferer channel 50 is only approximately 6 dB. Although a strong SAW filter is better able to remove an adjacent channel (e.g., channel 50), a strong SAW filter can introduce phase distortion and is also more expensive to implement then a weak SAW filter. The QPSK satellite signal arm 80 of Fig. 5 includes a sampler 96 which samples the received signal at the output of the SAW filter 90 at a sampling rate of four times the second IF. An analog-to-digital (A/D) converter 100 performs A/D conversion of the sampled signal, and a digital filter 102 removes the adjacent channel (e.g., channel 50) from - 7 - the digitized satellite signal. The digital filter 102 is preferably matched to a transmitter filter at the broadcast station. The digital filter 102 may have a stopband attenuation of 30 dB or higher, depending on the SAW filter 90, and the signal-to-noise ratio (SNR) after the SAW filter 90. The output of the digital filter 102 is then processed via a sampling switch and latch device 104 to recover the TDM signal from the QPSK modulation performed at the broadcast station. The digital filter 102 is preferably a root-raised-cosine (RRC) filter which is conventional for QPSK modulation and demodulation. In the preferred embodiment, the RRC filter has a sampling rate of 4 times the IF or 8 times the rate of the symbols transmitted in the satellite signal from the originating broadcast station. Further, a roll-off factor of α = 0.15 is selected. As shown in Fig. 7, the frequency response of the RRC filter has a passband ripple of 0.1 dB and a stopband ripple of 40 dB. Fig. 7 provides three plots representing the ideal RRC frequency response, the result of the Remez algorithm, and the RRC frequency response after coefficient quantization. Such an RRC filter specification can be fulfilled by a 136-tap, linear phase, finite impulse response (FIR) filter with a 10-bit fixed-point coefficient and a word length of 16 bits. MCM Demodulation of Terrestrial Signal An MCM demodulator is depiαed in Fig. 8. For MCM demodulation, an FFT is used to implement the filter bank at the radio receiver and corresponds to the IFFT described above in conneαion with MCM modulation at the terrestrial repeater. The input of the FFT as depiαed in Fig. 8 is sampled in accordance with a number of parameters seleαed for MCM transmission. The sampling frequency depends on an MCM symbol frequency F s which corresponds to the number of MCM symbols transmitted per second from the terrestrial repeater. In addition, the sampling frequency depends on the length of the FFT and the length of the guard interval associated with each MCM symbol. The sampling frequency is preferably F 4 = F s * FFTLEN * (1 + GUARDLEN_REL), where FFTLEN corresponds to the length of the FFT (e.g., 512), GUARDLEN_REL corresponds to the length of the guard interval relative to the useful - 8 - length or symbol duration (e.g., 280 microseconds), and F s corresponds to the MCM symbol frequency. The MCM symbol frequency F s is the bit rate divided by the number of bits per MCM symbol. As an example, the MCM signal bit rate may be 3.067 megabits per second (Mbps) and the number of bits per symbol may be 864 or 432. With continued reference to Fig. 8, an MCM signal of approximately 2.3 GHz is received at a radio receiver via an antenna 106 and a low noise amplifier (LNA) 108, and is downconverted to an IF of approximately 135 MHz by a mixer 110 and a local oscillator 112 before being processed by a SAW filter 114. The signal is band-limited by the SAW filter 114 to avoid aliasing components. The sampling frequency in the bandwidth of the SAW filter fulfills the Nyquist criterion for sampling signals. The received MCM signal is then downconverted to a second IF of approximately 4.60 MHz using a second mixer 116 and a second local oscillator 118. The signal is sampled by a sampler 120 at a frequency higher than the bandwidth of the signal, that is, at a sampling frequency F 2 > = 2 * ¥ x . The required sampling frequency is high in comparison with the bandwidth of the desired terrestrial signal (e.g., 4 times higher than the bandwidth of the desired signal) as shown in Fig. 9. Following A/D conversion in block 122, a digital filter 124 is used to suppress the adjacent channels. The digital filter 124 can be a low pass filter, as opposed to a bandpass filter, since the level of the adjacent satellite channel 46 is significantly lower than the terrestrial signal 50 (i.e., on the order of 30 dB lower). The adjacent satellite channel 46 merely appears as noise after A/D conversion and downsampling. The resulting speαrum after digital low pass filtering is shown in Fig. 10. The bandwidth is now equivalent to F 3 . The signal is then subjeαed to downsampling so as to be represented by a lower sampling frequency F 4 > 2 * F 3 . The frequencies F 2 and F 4 are selected such that F 4 is equal to N * F 2 where N is an integer number such as 4. The output of the digital low pass filter 124 after downsampling in block 126 is provided to the FFT as part of the MCM demodulation process, as shown in Fig. 8. Samples are converted to a vector by serial-to-parallel conversation and are then transformed into the frequency domain by the FFT prior to being decoded via an inverse mapping process. The mapping process converts the output of the FFT in the form of a data vector with complex values into an output bit stream. Dual-Mode Receiver QPSK modulation is an efficient method for satellite broadcasting, while MCM modulation is useful for terrestrial broadcasting. For systems utilizing satellite broadcasting for rural and suburban areas and terrestrial broadcasting for urban centers in which satellite signals are blocked by high buildings, for example, combined receivers are needed to receive both satellite signals and terrestrial signals. One possible dual-mode receiver is depicted in Fig. 11. This dual-mode receiver can be used as the satellite/terrestrial arm 56 in the radio receiver 14 of Fig. 1. If the satellite signal and terrestrial signal use the same frequency, a common tuner 129 can be used. The QPSK arm 130 and the MCM arm 132 of the dual-mode receiver can be identical to the QPSK demodulator and the MCM demodulator described above in connection with Figs. 5 and 8, respectively. In accordance with a preferred embodiment of the present invention, a dual-mode receiver for implementing, for example, the satellite/terrestrial arm of a radio receiver, is implemented using a combined architecture for both QPSK and MCM demodulation. The common architecture is depicted in Fig. 12. The dual-mode receiver depiαed in Fig. 12 is advantageous because it employs only one SAW filter and only one digital filter, and therefore reduces the cost and complexity of the receiver compared to that depiαed in Fig. 11. With continued reference to Fig. 12, an antenna 134 and LNA 136 are provided to receive satellite and terrestrial signals which are preferably in the frequency range of 2.332 to 2.345 GHz. The received satellite and terrestrial signals are supplied to the same SAW filter 132, which is preferably a weak or "leaky" SAW filter. As stated previously, a weak SAW filter is preferred to a strong SAW filter having better adjacent channel suppression, since the strong SAW filter can introduce phase distortion and is also more expensive to implement. As shown in Fig. 6, the passband of the weak SAW filter attenuates the terrestrial signal in the adjacent channel by only about 6 dB. This partial suppression of the adjacent channel is advantageous in that it allows for the deteαion of the terrestrial signal. The dual-mode receiver is configured to seleα a received terrestrial signal for - 10 - receiver output over a received satellite signal whenever the terrestrial signal exceeds a predetermined threshold. Thus, the dual-mode receiver searches essentially constantly for a terrestrial signal and selects the satellite signal only when the terrestrial signal is not present. In the illustrated embodiment, a super-heterodyne phase locked loop (PLL) 139 for seleαively tuning two voltage controlled local oscillators 140 and 142 together with corresponding mixers 144 and 146 is provided for downconverting the QPSK and MCM signals to the two different second IFs (i.e., 3.68 and 4.60 MHz, respeαively) as described above in connection with Figs. 5 and 8. For example, both oscillators 140 and 142 can be locked to one reference oscillator of 14.72 MHz, and a phase comparator frequency of 230 kHz can be used. In mixer 144, the satellite and terrestrial signals, which have different frequency bands, are mixed with different local oscillator input frequencies to do nconvert the signals to the same IF of approximately 135 MHz. For example, the mixer input frequencies are F LO1 is F terr - Fj for terrestrial signals and F terr - 2.07 MHz - Fj for satellite signals. With regard to mixer 146, the sampling frequency is different depending on whether the receiver is using terrestrial signals or satellite signals for receiver output. The mixer 146 is preferably re-tuned to achieve a second IF which is one-fourth of the used sampling frequency. The used sampling frequency is preferably an integer multiple of 2.3 MHz for MCM terrestrial signals and an integer multiple of 1.84 MHz for QPSK/TDM satellite signals. Accordingly, re-tuning the mixer 146 facilitates simplification of I/Q generation. Feedback data from a terrestrial deteαion circuit described below is provided to the super-heterodyne PLL circuit 139 to control the operation of the local oscillators 140 and 142 depending upon whether sufficiently strong terrestrial signals have been deteαed and are being used for receiver output in lieu of satellite signals. The digital filter 148 of Fig. 12 is implemented such that the frequency response of the matched filter (e.g., the RRC filter described above in conneαion with Figs. 5 and 7) required for QPSK demodulation also fulfills the requirements of the digital filter used prior to downsampling of an MCM signal (e.g., at a sampling frequency F 4 = N * 2.3 MHz where N=8) for FFT processing. When the receiver is initially powered on, the receiver configures the local oscillators 140 and 142 for downconversion of satellite signals - 11 - to a second IF of 3.68 MHz. The sampling and A/D conversion of the satellite signals in blocks 150 and 152 is as described previously. For an RRC filter, a sampling rate of four times the IF (or eight times the symbol rate) is used. The passband of the RRC filter is such that the filter will not pass the energy of an adjacent terrestrial signal. If a terrestrial signal of sufficient energy is present in the channel 50 adjacent to the satellite signal passed by the SAW filter, a difference in signal energy can be detected between the input and the output of the RRC filter. This is implemented by means of a terrestrial signal detector 154 in Fig. 12. The terrestrial signal detector 154 compares the signal energy at the input of the filter with the signal energy at the output of the filter. If the energy at the input of the filter is significantly higher than at the filter output (e.g., on the order of three times higher, depending on the SAW filter frequency response), a terrestrial signal is assumed to have been received. If a terrestrial signal is present in the adjacent channel, a signal is produced by the terrestrial signal detector 154 which retunes the local oscillators 144 and 146 to downconvert terrestrial signals. Thus, the center frequency of the terrestrial signal is shifted by approximately 2.07 MHz and the second IF becomes 4.60 MHz. Following sampling and A/D conversion, the terrestrial signal is applied to the RRC-type digital filter 148. Since the roll-off frequency of the digital filter 148 is seleαed to fulfill the requirements of both QPSK and MCM demodulation and the terrestrial and satellite signals have similar bandwidth, the digital filter passes the MCM terrestrial signal to block 156 for downsampling prior to FFT processing in block 158. The output of the digital filter 148 is also supplied to a sampling switch and latch device 160 to recover a TDM signal from the QPSK modulation performed at the broadcast station. A switch 162 is then used to seleα an output signal from either the sampling switch and latch device 160 or the FFT 158 for further processing via a TDM demultiplexing and decoding circuit 164 and the post-detection diversity combining unit 58 (Fig. 4). The operation of the switch 162 is controlled by the terrestrial signal deteαor 154. Thus, the location of the terrestrial repeater frequency bands 50 and 52 in the lower part of the frequency plan (Fig. 3), adjacent to the frequency bands 46 and 48 of the satellite signals, facilitates seleαion of satellite signals or the terrestrial signals for receiver output. Since a portion of an adjacent terrestrial signal remains in the output of the SAW - 12 - filter during satellite signal reception, a comparison of signal power can be used to detect the terrestrial signal. While a certain advantageous embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.

Claims: - 13 - What Is Claimed Is: 1. A receiver configured to receive broadcast signals of both a first signal type and a second signal type and to select broadcast signals of one of the signal types for output, said receiver comprising: a first oscillator and mixer circuit (140,144) for downconverting said broadcast signals of both said first signal type and said second signal type to a first intermediate frequency; a first filter (138) having a center frequency corresponding to said first intermediate frequency and frequency response selected to pass said broadcast signals of said first signal type and at least a portion of said broadcast signals of said second signal type; a second oscillator and mixer circuit (142, 146) for downconverting said broadcast signals of said first signal type and said broadcast signals of said second signal type to a second intermediate frequency; a sampling and analog-to-digital conversion circuit (150, 152) for converting said broadcast signals of said first signal type and said broadcast signals of said second signal type into digital signals; and a second filter (148) connected to the output of said sampling and analog- to-digital conversion circuit (150, 152) for filtering said digital signals, said digital signals generated from said broadcast signals of said first signal type being phase shift keying or PSK-modulated, and said digital signals generated from said broadcast signals of said second signal type being modulated in accordance with a second modulation scheme different from PSK modulation. 2. A receiver as claimed in claim 1, wherein a third filter is used when generating said broadcast signals of said first signal type, said second filter (148) being configured to have a frequency response of a matched filter corresponding to said third filter. 3. A receiver as claimed in claim 1, said second modulation scheme is multicarrier modulation or MCM, and said second filter (148) is configured to have a frequency response to facilitate MCM and PSK demodulation. - 14 - 4. A receiver as claimed in claim 3, wherein said frequency response accommodates a sampling frequency for MCM demodulation corresponding to at least one of the length of a fast Fourier transform for MCM demodulation and the length of a guard interval used during MCM modulation. 5. A receiver as claimed in claim 1, further comprising a signal detection circuit (154) connected to said second filter (148) for determining whether a characteristic of said second signal type exceeds a predetermined threshold and for generating an output signal for controlling the operation of said first oscillator and mixer circuit (140, 144), said first oscillator and mixer circuit (140, 144) being configured to use one of first and second input frequencies, depending on said output signal, to generate said first intermediate frequency by mixing with a corresponding one of said first signal type and said second signal type. 6. A receiver as claimed in claim 1, further comprising a first signal demodulation device (160) connected to said second filter (148) to process said first signal type, a second signal demodulation device (156, 158) connected to said second filter to process said second signal type, and a switching device (162) to select the output of one of said first signal demodulation device (160) and said second signal demodulation device (156, 158) in accordance with said output signal from said signal detection device. 7. A receiver as claimed in claim 1, wherein said second signal type is a multicarrier modulated signal that is demodulated using downsampling and fast Fourier transform processing, said second filter (148) being configured perform in accordance with filter parameters selected to facilitate said downsampling and said fast Fourier transform processing of said second signal type and PSK demodulation of said first signal type. 8. A receiver as claimed in claim 7, wherein said second filter (148) is a root-raised cosine filter. - 15 - 9. A receiver as claimed in claim 8, wherein said second filter (148) is a matched filter configured to substantially correspond to a third filter located at a broadcast station which produced said first signal type. 10. A method of receiving and selecting from broadcast signals transmitted in first and second frequency channels comprising the steps of: receiving signals at a carrier frequency; downconverting said received signals to a first intermediate frequency; filtering said received signals to pass said broadcast signals in both said first and second frequency channels; downconverting said broadcast signals in said first frequency channel and said second frequency channel to a second intermediate frequency; sampling and converting said broadcast signals into digital signals; and filtering said digital signals, wherein said digital signals generated from said first frequency channel are phase shift keying or PSK-modulated and said filtering is implemented using a root-raised-cosine filter to facilitate PSK demodulation, and said digital signals generated from said second frequency channel are modulated in accordance with a second modulation scheme different from PSK modulation and said filtering thereof is implemented using said root-raised-cosine filter. 11. A method as claimed in claim 10, wherein said filtering step comprises the step of selecting said root-raised-cosine filter to have a frequency response of a matched filter corresponding to a third filter used to generate said broadcast signals on said first frequency channel. 12. A method as claimed in claim 10, wherein said second modulation scheme is multicarrier modulation or MCM, and said providing step comprises the step of selecting said frequency response to facilitate MCM demodulation. - 16 - 13. A method as claimed in claim 12, wherein said selecting step comprises the step of selecting said frequency response in accordance with a sampling frequency for MCM demodulation, said sampling frequency corresponding to at least one of the length of a fast Fourier transform used for MCM demodulation and the length of a guard interval used during MCM modulation. 14. A method as claimed in claim 10, wherein said broadcast signals in said second frequency channel having a higher signal level than said broadcast signals in said first frequency channel, and further comprising the steps of: comparing the signal level of said digital signals before and after said filtering step to determine if a characteristic of said broadcast signals in said second frequency channel exceeds a predetermined threshold; generating a detection signal indicating whether said broadcast signals in said second frequency channel have been detected; and adjusting an input frequency to a local oscillator in accordance with said detection signal to downconvert said received signals to said first intermediate frequency by mixing with said received signals in said first and second frequency channels. 15. A receiver configured to receive both a first signal type corresponding to phase shift keying or PSK modulated signals and a second signal type corresponding to multicarrier modulated or MCM signals and to select one of the signal types for output, said receiver comprising: a first oscillator and mixer circuit (140, 144) for downconverting both of said first signal type and said second signal type to a first intermediate frequency; a first filter (138) having a center frequency corresponding to said first intermediate frequency and frequency response selected to pass said first signal type and at least a portion of said second signal type; a second oscillator and mixer circuit (142, 146) for downconverting said first signal type and said second signal type to a second intermediate frequency and a third intermediate frequency, respectively; - 17 - a sampling and analog-to-digital conversion circuit (150, 152) for digitizing said first signal type and said second signal type; and a second filter (148) connected to the output of said sampling and analog- to-digital conversion circuit (150, 152), said second filter (148) being a root-raised-cosine filter having a frequency response selected to facilitate demodulation of both said first signal type and said second signal type. 16. A receiver as claimed in claim 15, wherein said first filter (138) is a leaky filter selected from the group consisting of a surface acoustic wave filter and a ceramic filter. 17. A receiver as claimed in claim 15, further comprising a signal detection circuit (154) connected to said second filter (148) for determining whether a characteristic of said second signal type exceeds a predetermined threshold and for generating an output signal for controlling the operation of said first oscillator and mixer circuit (140, 144) to use one of first and second input frequencies, depending on said output signal, to generate said first intermediate frequency by mixing with a corresponding one of said first signal type and said second signal type. 18. A receiver as claimed in claim 17, wherein said second signal type is characterized by a higher signal level than said first signal type, said signal detection circuit (154) being configured to determine the difference in the signal level of said second signal type at the input and the output of said second filter (148). 19. A receiver as claimed in claim 17, further comprising a signal detection circuit (154) connected to said second filter for determining whether a characteristic of said second signal type exceeds a predetermined threshold and for generating an output signal for controlling the operation of said second oscillator and mixer circuit (142, 146) to control said second oscillator and mixer circuit (142, 146) to perform downconversion using said third intermediate frequency if said predetermined threshold is exceeded.

Inventors: BADRI SABAH, BUCHHOLZ STEPHAN, EBERLEIN ERNST, GERHAEUSER HEINZ, HEUBERGER ALBERT, LIPP STEFAN, BADRI, SABAH, BUCHHOLZ, STEPHAN, EBERLEIN, ERNST, GERHAEUSER, HEINZ, HEUBERGER, ALBERT, LIPP, STEFAN
Applicants: BADRI SABAH, BUCHHOLZ STEPHAN, EBERLEIN ERNST, FRAUNHOFER GES FORSCHUNG, GERHAEUSER HEINZ, HEUBERGER ALBERT, LIPP STEFAN, BADRI, SABAH, BUCHHOLZ, STEPHAN, EBERLEIN, ERNST, FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG E. V., GERHAEUSER, HEINZ, HEUBERGER, ALBERT, LIPP, STEFAN
Application year: 1998
Main classification: H04L27
IPC codes: H04H1, H04H40, H04L27