Transceiver front end for software radio system
A digital front end transceiver system ( 300 ) for a software radio system comprising: a transmitter module ( 310 ) adapted to digitally interpolate, up-convert and combine baseband digital in-phase (I) and quadrature (Q) channels to form a digital intermediate frequency (IF) IQ signal for conversion to an analogue IF IQ signal for transmission; and a receiver module ( 320 ) adapted to separate, down-convert and decimate a received digital IF IQ signal into baseband digital I and Q channels. The transmitter and receiver modules use a common fixed sampling and IF conversion rate f s , where f s is a quadruple of the bandwidth of the baseband signal, such that the system can be applied in a software radio system for a wireless communication application having any baseband bandwidth by selecting the fixed sampling rate based on the baseband bandwidth.
1. A digital front end transceiver system for a software radio system comprising:
a transmitter module adapted to digitally interpolate, up-convert and combine baseband digital in-phase (I) and quadrature (Q) channels to form a digital intermediate frequency (IF) IQ signal for conversion to an analogue IF IQ signal for transmission; and
a receiver module adapted to separate, down-convert and decimate a received digital IF IQ signal into baseband digital I and Q channels,
wherein the transmitter and receiver modules use a common fixed sampling and IF conversion rate f s , where f s is a quadruple of the bandwidth of the baseband signal, such that the system can be applied in a software radio system for a wireless communication application having any baseband bandwidth by selecting the fixed sampling rate based on the baseband bandwidth,
wherein the sampling rate f s =4 BW where BW is the bandwidth of the baseband signal, such that the center frequency f IF of the intermediate frequency signal IF is f IF =BW, and
wherein the receiver module includes a down-converting digital IF sampler/mixer having a numerically controlled oscillator (NCO) and two sign converters for separating and down-converting from an intermediate frequency to a baseband the a digital IQ signal received from a digital to analogue converter and separating the signal into the respective I and Q signal components, and a decimator having, for each of the I and Q signals, a low pass filter for pulse shaping and decimating and a sampler for down-sampling the I and Q channel signals to reduce the signal bandwidth to that of the a desired digital baseband, wherein the NCO and two sign converters are implemented using two four input multiplexers and a 2 bit counter wherein the sign change mechanism uses 2's complement sign inversion performed by an inverter and one bit adder.
2. The digital front end transceiver system as claimed in claim 1 wherein the down-converting digital mixer/sampler performs down-conversion from baseband to the intermediate frequency by applying a periodic sign changing sequence of 0, 1, 0, −1 representing phases of 0°, 90°, 180° and 270° of a sinusoidal function at a sampling rate of f s =4 BW and sampling with a 90° phase separation to separate the I and Q signal components.
3. The digital front end transceiver system as claimed in claim 1 further comprising an analogue to digital converter (ADC) for converting a received analogue IF IQ signal to a digital IF IQ signal.
4. The digital front end transceiver system as claimed in claim 3 wherein the ADC is adapted to naturally down-convert a received analogue signal having a first intermediate frequency IF1 having a center frequency of f IF1 =f s +BW, to a second intermediate frequency IF2 having a center frequency of f 1F2 =BW by using a sampling rate of f s for the analogue to digital conversion.
5. The digital front end transceiver system as claimed in claim 1 wherein the LPF is a root-raised cosine low-pass filter (LPF) adapted to perform both decimator and pulse-shaping operations.
6. The digital front end transceiver system as claimed in claim 5 wherein the impulse response of the LPF is defined by:
h
(
t
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=
4
β
cos
(
(
1
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β
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π
t
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T
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+
sin
(
(
1
-
β
)
π
t
/
T
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4
β
t
T
π
T
(
1
-
4
β
t
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T
)
2
)
where β determines the roll off factor of the filter response.
7. A digital front end transceiver system for a software radio system comprising:
a transmitter module adapted to digitally interpolate, up-convert and combine baseband digital in-phase (I) and quadrature (Q) channels to form a digital intermediate frequency (IF) IQ signal for conversion to an analogue IF IQ signal for transmission; and
a receiver module adapted to separate, down-convert and decimate a received digital IF IQ signal into baseband digital I and Q channels,
wherein the transmitter and receiver modules use a common fixed sampling and IF conversion rate f s , where f s is a quadruple of the bandwidth of the baseband signal, such that the system can be applied in a software radio system for a wireless communication application having any baseband bandwidth by selecting the fixed sampling rate based on the baseband bandwidth,
wherein the sampling rate f s =4 BW where BW is the bandwidth of the baseband signal, such that the center frequency f 1F of the intermediate frequency signal IF is f 1F =BW, and
wherein the transmitter module includes a digital transmitter baseband interpolator having for each of the I and Q channels a digital sampler adapted to sample the digital signals for transmission, and a low pass filter (LPF) for interpolating and pulse shaping the sampled digital signals; and a digital up-converting intermediate frequency sampler/mixer having a numerically controlled oscillator (NCO), two sign converters and a mixer for combining and up-converting to the IF the digital signals for the I and Q channels, and
wherein the NCO and two sign converters are implemented using two four input multiplexers and a 2 bit counter wherein the sign change mechanism uses 2's complement sign inversion performed by an inverter and one bit adder.