Transmission system for reduction of amateur radio interference

Abstract

The invention is a novel method and transmission system for mitigating the adverse effects of amateur (HAM) radio interference on digital signals transmitted across a twisted pair and reducing the interference caused by the digital signals to amateur radio communications. At the heart of the invention is strategical placement of the frequency spectrum of the transmitted signal relative to the center frequency of the HAM bands. In particular, the signal spectrum is permitted to straddle one or more HAM bands, which permits the transport of high data rates with a single carrier. At the transmitter, the signal is filtered with a real-valued symmetric baseband filter in order to remove signal content in the HAM bands. At the receiver, the signal undergoes adaptive interference cancellation, followed by processing through a decision feedback equalizer. In some cases, the signal is filtered at baseband with a real-valued filter to remove interference in the HAM bands. In the context of a telecommunications access system, the inventive technique can be applied to either or both directions of traffic flow, i.e., downstream and upstream, and multiple carriers can be added to increase capacity in any given direction.

We claim:  
     
         1 . A transmitter for transmitting a modulated signal across a transmission medium, the transmitter comprising: 
 an encoder for encoding a digital data stream into one or more encoded digital signals;    one or more substantially identical baseband notching filters connected to the encoder, for respectively receiving the one or more encoded digital signals, each baseband notching filter having a notch at zero frequency; and    a modulator connected to the one or more baseband notching filters, for producing the modulated signal centered about a carrier frequency, wherein the carrier frequency is approximately equal to the center frequency of an interference band.    
     
     
         2 . A transmitter according to  claim 1 , further comprising one or more substantially identical Nyquist filters respectively connected between the one or more notching filters and the modulator.  
     
     
         3 . A transmitter according to  claim 1 , wherein the baseband notching filters have real-valued coefficients.  
     
     
         4 . A transmitter according to  claim 1 , wherein the encoder is a quadrature amplitude modulation (QAM) encoder producing two encoded signals, of which one is an in-phase encoded signal and the other is an in-quadrature encoded signal, and wherein the modulator is a quadrature modulator.  
     
     
         5 . A transmitter according to  claim 1 , wherein the interference band is an amateur radio band having a center frequency approximately equal to one of 1.9, 3.75, 7.15, 10.125, 14.175, 18.118 and 21.225 Megahertz.  
     
     
         6 . A transmitter according to  claim 1 , wherein the interference band has a bandwidth of less than or approximately equal to 10% of the carrier frequency.  
     
     
         7 . A transmitter for transmitting a modulated signal across a transmission medium, the transmitter comprising: 
 an encoder for encoding a digital data stream into one or more encoded digital signals having a symbol rate f S ;    one or more substantially identical baseband notching filters connected to the encoder, for respectively receiving the one or more encoded digital signals, each baseband notching filter having a pair of notches symmetrically disposed about zero frequency at frequencies approximately equal to ±f S /4; and    a modulator connected to the one or more baseband notching filters, for producing the modulated signal centered about a carrier frequency f C ;    wherein f S  and f C  are related to the center frequencies f H1  and f H2  of two interference bands by f C  being approximately equal to ½(f H1 +f H2 ) and f S  being approximately equal to 2|f H2 −f H1 |.    
     
     
         8 . A transmitter according to  claim 7 , further comprising one or more substantially identical Nyquist filters respectively connected between the one or more notching filters and the modulator.  
     
     
         9 . A transmitter according to  claim 7 , wherein the baseband notching filters have real-valued coefficients.  
     
     
         10 . A transmitter according to  claim 7 , wherein the encoder is a quadrature amplitude modulation (QAM) encoder producing two encoded signals, of which one is an in-phase encoded signal and the other is an in-quadrature encoded signal, and wherein the modulator is a quadrature modulator.  
     
     
         11 . A transmitter according to  claim 7 , wherein the interference bands are adjacent amateur radio bands, having corresponding center frequencies f H1  and f H2  chosen from the set of 1.9, 3.75, 7.15, 10.125, 14.175, 18.118 and 21.225 Megahertz, approximately.  
     
     
         12 . A transmitter according to  claim 7 , wherein the interference bands have bandwidths of less than or approximately equal to 0.1×f C .  
     
     
         13 . A receiver for extracting a digital data stream from a modulated signal, said modulated signal being centered about a carrier frequency, the receiver comprising: 
 adaptive interference cancellation (AIC) means for controllably reducing narrowband interference present in the modulated signal around the carrier frequency, thereby to produce an interference-reduced signal;    a demodulator connected to the AIC means for receiving and demodulating the interference-reduced signal, thereby to produce one or more baseband demodulated signals;    a power estimator connected to the demodulator and to the AIC means, for receiving the one or more baseband demodulated signals, calculating the power of residual interference present in the one or more baseband demodulated signals around the carrier frequency and providing the AIC means with an interference estimate signal;    one or more substantially identical baseband notching filters connected to the demodulator, for receiving the one or more baseband demodulated signals and producing respective filtered demodulated signals, each baseband notching filter having a notch at zero frequency; and    a decision-feedback equalizer (DFE) connected to the one or more baseband notching filters, for receiving the filtered demodulated signals, decoding digital data embedded therein and producing the digital data stream.    
     
     
         14 . A receiver according to  claim 13 , wherein the baseband notching filters have real-valued coefficients.  
     
     
         15 . A receiver according to  claim 13 , further comprising one or more Nyquist filters respectively connected between the demodulator and the baseband notching filters.  
     
     
         16 . A receiver according to  claim 13 , further comprising a variable-gain amplifier connected between the AIC means and the demodulator, for maintaining a substantially constant signal level of the interference-reduced signal.  
     
     
         17 . A receiver according to  claim 13 , wherein the demodulator is a quadrature demodulator and wherein the number of baseband demodulated signals is two, one of which is an in-phase baseband demodulated signal and the other of which is an in-quadrature baseband demodulated signal.  
     
     
         18 . A receiver for extracting a digital data stream from a modulated signal, said modulated signal being centered about a carrier frequency f C  and having a bandwidth f S , the receiver comprising: 
 adaptive interference cancellation (AIC) means for controllably reducing narrowband interference present in the modulated signal around frequencies f C ±f S /4, thereby to produce an interference-reduced signal;    a demodulator connected to the AIC means for receiving and demodulating the interference-reduced signal, thereby to produce one or more baseband demodulated signals;    a power estimator connected to the demodulator and to the AIC means, for receiving the one or more baseband demodulated signals and providing the AIC means with an estimate of residual interference in the one or more baseband demodulated signals around frequencies f C ±f S /4;    one or more substantially identical baseband notching filters connected to the demodulator, for receiving the one or more baseband demodulated signals and producing respective filtered demodulated signals, each baseband notching filter having a pair of notches symmetrically disposed about zero frequency at frequencies approximately equal to ±f S /4; and    a decision-feedback equalizer (DFE) connected to the one or more baseband notching filters, for receiving the filtered demodulated signals, decoding digital data embedded therein and producing the digital data stream.    
     
     
         19 . A receiver according to  claim 18 , wherein the baseband notching filters have real-valued coefficients.  
     
     
         20 . A receiver according to  claim 18 , further comprising one or more Nyquist filters respectively connected between the demodulator and the baseband notching filters.  
     
     
         21 . A receiver according to  claim 18 , further comprising a variable-gain amplifier connected between the AIC means and the demodulator, for maintaining a substantially constant signal level of the interference-reduced signal.  
     
     
         22 . A receiver according to  claim 18 , wherein the demodulator is a quadrature demodulator.  
     
     
         23 . A receiver for extracting a digital data stream from a modulated signal, said modulated signal being centered about a carrier frequency, the receiver comprising: 
 adaptive interference cancellation (AIC) means for controllably reducing narrowband interference present in the modulated signal around the carrier frequency in accordance with first and second residual interference estimates, thereby to produce an interference-reduced signal;    a demodulator connected to the AIC means for receiving and demodulating the interference-reduced signal, thereby to produce one or more baseband demodulated signals, and for providing the AIC means with the first estimate of residual interference in the one or more baseband demodulated signals around the carrier frequency;    one or more substantially identical baseband notching filters connected to the demodulator, for receiving the one or more baseband demodulated signals and producing respective filtered demodulated signals, each baseband notching filter having a notch at zero frequency; and    a decision-feedback equalizer (DFE) connected to the one or more baseband notching filters, for receiving the filtered demodulated signals, decoding digital data embedded therein and producing the digital data stream, wherein the DFE is further connected to the AIC means for providing the AIC means with the second estimate of residual interference in the filtered demodulated signals around the carrier frequency;    
     
     
         24 . A receiver according to  claim 23 , wherein the baseband notching filters have real-valued coefficients.  
     
     
         25 . A receiver according to  claim 23 , further comprising one or more Nyquist filters respectively connected between the demodulator and the baseband notching filters.  
     
     
         26 . A receiver according to  claim 23 , further comprising a variable-gain amplifier connected between the AIC means and the demodulator, for maintaining a substantially constant signal level of the interference-reduced signal.  
     
     
         27 . A receiver according to  claim 23 , wherein the demodulator is a quadrature demodulator.  
     
     
         28 . A method of transmitting digital data, comprising the steps of: 
 encoding the data into one or more baseband digital signals;    filtering the one or more baseband digital signals with respective baseband filters having notches at zero frequency;    modulating the filtered baseband digital signals about a carrier frequency, thereby to produce a modulated signal; and    transmitting the modulated signal across a transmission medium;    wherein the carrier frequency is approximately equal to the center frequency of an interference band.    
     
     
         29 . A method according to  claim 28 , wherein the center frequency of the interference band is approximately equal to one of 1.9, 3.75, 7.15, 10.125, 14.175, 18.118 and 21.225 Megahertz.  
     
     
         30 . A method according to  claim 28 , wherein the encoding step further comprises further encoding the data in accordance with a partial response filter having a null at zero frequency.  
     
     
         31 . A method of transmitting digital data, comprising the steps of: 
 encoding the data into one or more baseband digital signals;    filtering the one or more baseband digital signals with respective baseband filters having notches at frequencies approximately equal to ±Δf;    modulating the filtered baseband digital signals about a carrier frequency f C , thereby to produce a modulated signal; and    transmitting the modulated signal across a transmission medium;    wherein f C +Δf is approximately equal to the center frequency of a first interference band and f C −Δf is approximately equal to the center frequency of a second interference band.    
     
     
         32 . A method according to  claim 31 , the baseband digital signals having a common symbol rate, wherein Δf is approximately equal to one-quarter the symbol rate.  
     
     
         33 . A method according to  claim 31 , wherein the first and second interference bands are adjacent, and wherein the center frequencies of the first and second interference bands are chosen from the set of 1.9, 3.75, 7.15, 10.125, 14.175, 18.118 and 21.225 Megahertz, approximately.  
     
     
         34 . A method of recovering a digital data stream from a received signal modulated about a carrier frequency, comprising: 
 demodulating the modulated signal, thereby to produce one or more baseband demodulated signals;    filtering the one or more baseband demodulated signals with respective baseband filters having notches at zero frequency; and    decoding the filtered baseband demodulated signals, thereby to recover the digital data;    wherein the carrier frequency is approximately equal to the center frequency of an interference band.    
     
     
         35 . A method according to  claim 34 , wherein the center frequency of the interference band is approximately equal to one of 1.9, 3.75, 7.15, 10.125, 14.175, 18.118 and 21.225 Megahertz.  
     
     
         36 . A method of recovering a digital data stream from a received signal modulated about a carrier frequency f C , comprising: 
 demodulating the modulated signal, thereby to produce one or more baseband demodulated signals;    filtering the one or more baseband demodulated signals with respective baseband filters having notches at frequencies ±Δf; and    decoding the filtered baseband demodulated signals, thereby to recover the digital data;    wherein f C ±Δf is approximately equal to the center frequency of a first interference band and f C −Δf is approximately equal to the center frequency of a second interference band.    
     
     
         37 . A method according to  claim 36 , wherein the first and second interference bands are adjacent, and wherein the center frequencies of the first and second interference bands are chosen from the set of 1.9, 3.75, 7.15, 10.125, 14.175, 18.118 and 21.225 Megahertz, approximately.  
     
     
         38 . A modem for transmitting a first modulated signal onto a twisted pair and for receiving a second modulated signal from the twisted pair, comprising: 
 a hybrid for interfacing with the twisted pair;    a transmitter connected to the hybrid, for producing the first modulated signal from a first digital data stream, the transmitter comprising an encoder for encoding the first digital data stream into one or more encoded digital signals; one or more substantially identical baseband notching filters connected to the encoder, for respectively receiving the one or more encoded digital signals, each baseband notching filter having a notch at zero frequency; and a modulator connected to the one or more baseband notching filters, for producing the first modulated signal centered about a carrier frequency, wherein the carrier frequency is approximately equal to the center frequency of an interference band; and    a receiver connected to the hybrid, for extracting a digital data stream from the second modulated signal, the receiver comprising adaptive interference cancellation (AIC) means for controllably reducing narrow band interference present in the second modulated signal around the carrier frequency, thereby to produce an interference-reduced signal; a demodulator connected to the AIC means for receiving and demodulating the interference-reduced signal, thereby to produce one or more baseband demodulated signals; a power estimator connected to the demodulator and to the AIC means, for receiving the one or more baseband demodulated signals and providing the AIC means with an estimate of residual interference in the one or more baseband demodulated signals around the carrier frequency; one or more substantially identical baseband notching filters connected to the demodulator, for receiving the one or more baseband demodulated signals and producing respective filtered demodulated signals, each baseband notching filter having a notch at zero frequency; and a decision-feedback equalizer (DFE) connected to the one or more baseband notching filters, for receiving the filtered demodulated signals, decoding digital data embedded therein and producing the second digital data stream.    
     
     
         39 . A modem according to  claim 38  wherein the twisted pair carries a differential signal.  
     
     
         40 . A modem according to  claim 38 , wherein the center frequency of the interference band is approximately equal to one of 1.9, 3.75, 7.15, 10.125, 14.175, 18.118 and 21.225 Megahertz.  
     
     
         41 . A method of allocating a frequency spectrum, comprising: 
 selecting a first portion of the frequency spectrum for transmission of a first signal, said first portion having a first center frequency f C1 ;    wherein f C1  is related to the center frequency f H1  of an interference band by f C1  being approximately equal to f H1 .    
     
     
         42 . A method according to  claim 41 , further comprising: 
 selecting a second portion of the frequency spectrum for transmission of a second signal, said second portion having a second center frequency f C2 ;    wherein f C2  is related to the center frequency f H2  of an interference band by f C  being approximately equal to f H2 .    
     
     
         43 . A method according to  claim 41 , further comprising: 
 selecting a second portion of the frequency spectrum for transmission of a second signal, said second portion having a second center frequency f C2  and a bandwidth f S ;    wherein f S  and f C2  are related to the center frequencies f H2  and f H3  of two interference bands by f C2  being approximately equal to ½(f H2 +f H3 ) and f S  being approximately equal to 2|f H3 −f H2 |.    
     
     
         44 . A method of allocating a frequency spectrum, comprising: 
 selecting a first portion of the frequency spectrum for transmission of a first signal, said first portion having a center frequency f C1  and a bandwidth f S1 ;    wherein f S1  and f C1  are related to the center frequencies f H1  and f H2  of two interference bands by f C1  being approximately equal to ½(f H1 +f H2 ) and f S1  being approximately equal to 2|f H2 −f H1 |.    
     
     
         45 . A method according to  claim 44 , further comprising: 
 selecting a second portion of the frequency spectrum for transmission of a second signal, said second portion having a second center frequency f C2  and a bandwidth f S2 ;    wherein f S2  and f C2  are related to the center frequencies f H3  and f H4  of two interference bands by f C2  being approximately equal to ½(f H3 +f H4 ) and f S2  being approximately equal to 2 |f H4 −f H3 |.