Method and apparatus for performing differential modulation over frequency in an orthogonal frequency division multiplexing (OFDM) communication system
First Claim
1. A method of transmitting a signal in an orthogonal frequency division multiplexing (OFDM) system having a plurality of sub-carriers, comprising the steps of:
- differentially encoding said signal in the frequency domain using adjacent sub-carriers to produce differentially encoded symbols;
storing said differentially encoded symbols and one or more pilot tones in an IFFT buffer to produce an analog signal centered at a desired carrier frequency; and
transforming said analog signal to create said OFDM signal.
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Abstract
A terrestrial repeater is disclosed for use in a satellite transmission system that may also include a plurality of satellites. The terrestrial repeaters are implemented as OFDM transmitters to minimize the channel impairments caused by multi-path propagation. The OFDM terrestrial repeaters differentially encode the transmitted signal over frequency, as opposed to time, in order to avoid channel phase distortion. The OFDM receiver allocates two unmodulated sub-carriers as pilot bins and thereby provides two sub-carriers as a reference within each OFDM symbol. At the OFDM receiver, the two unmodulated pilot bins contain no phase information and provide a reference for the differential demodulation performed by the OFDM receiver. The OFDM transmitter operates in two modes, namely, a normal mode and a transmitter identifier information (TII) mode. The TII mode contains all of the functions of the normal mode, and also transmits a repeater identification signal with the customer data, using the inactive OFDM sub-carriers (bins). In a normal mode, the OFDM transmitter optionally fills unused (inactive) sub-carriers with zeroes and in a TII mode, the OFDM transmitter uses at least some of the unused (inactive) sub-carriers to transmit the transmitter identifier information (TII). The predefined TII value is mapped onto a set of complex symbols. The TII signal facilitates the receiver test equipment in associating the received signal with the terrestrial repeater(s) that generate the TII signal.
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Citations
22 Claims
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1. A method of transmitting a signal in an orthogonal frequency division multiplexing (OFDM) system having a plurality of sub-carriers, comprising the steps of:
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differentially encoding said signal in the frequency domain using adjacent sub-carriers to produce differentially encoded symbols; storing said differentially encoded symbols and one or more pilot tones in an IFFT buffer to produce an analog signal centered at a desired carrier frequency; and transforming said analog signal to create said OFDM signal. - View Dependent Claims (2, 3, 4, 5, 6)
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7. An orthogonal frequency division multiplexing (OFDM) transmitter for transmitting an OFDM signal having a plurality of sub-carriers, comprising:
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a differential encoder for modulating said OFDM signal in the frequency domain using adjacent sub-carriers to produce differentially encoded symbols; an IFFT buffer for storing said differentially encoded symbols and one or more pilot tones to produce an analog signal centered at a desired carrier frequency; and a transformer for creating said OFDM signal. - View Dependent Claims (8, 9, 10, 11, 12)
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13. A method of receiving a signal in an orthogonal frequency division multiplexing (OFDM) system having a plurality of sub-carriers, comprising the steps of:
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transforming said received signal to recover an OFDM signal in the frequency domain having a plurality of sub-carriers, wherein said transformed signal contains differentially encoded symbols and one or more pilot tones and wherein said transformed signal is centered at a desired carrier frequency; and differentially decoding said OFDM signal in the frequency domain wherein said differential decoding is performed using adjacent sub-carriers. - View Dependent Claims (14, 15, 16, 17)
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18. An orthogonal frequency division multiplexing (OFDM) receiver for receiving an OFDM signal having a plurality of sub-carriers, comprising:
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a transformer for recovering said OFDM signal having a plurality of sub-carriers, wherein said recovered signal contains differentially encoded symbols and one or more pilot tones and wherein said recovered signal is centered at a desired carrier frequency; and a differential decoder for demodulating said OFDM signal in the frequency domain wherein said differential decoding is performed using adjacent sub-carriers. - View Dependent Claims (19, 20, 21, 22)
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Specification