Method and apparatus for channel response calculation in an OFDM receiver
View Patent ↗An efficient algorithm is described for use with OFDM receivers that characterizes the impulse response of a communication channel using frequency domain techniques that reduce computational and memory requirements, relative to time-domain cross-correlation methods, without sacrificing algorithm performance. An FFT engine is used to transform a time domain input sequence, the transformed sequence is multiplied by the conjugate of the expected sequence, the product of several sequences is averaged, then the FFT engine transforms the average back to a time domain sequence, the magnitude of which is the impulse response of the channel.
1. A device for computing the impulse response of a communication channel comprising:
an FFT engine for computing a Fast Fourier Transform (FFT) of an input sequence to produce an HT output sequence;
a data selector feeding the FFT engine to select between an input signal and a feedback path;
a multiplier that multiplies the FFT output sequence by a predetermined Error Vector Magnitude (EVM) sequence;
wherein when the data selector selects the input signal, the averaging the product of the EVM sequence and the FFT of the input signal is calculated, and when the data selector selects the feedback path, the FFT engine computes the FFT of the impulse response of the communication channel.
2. The device of claim 1 wherein the EVM is computed on a per-carrier basis as a difference between a known transmitted sequence and a received sequence.
3. The device of claim 2 wherein the known transmitted sequence is generated by a linear feedback shift register with a generator polynomial X 10 +X 7 +1.
4. The device of claim 3 wherein the linear feedback shift register can be initialized at the start of each transmitted sequence.
5. The device of claim 4 wherein the known transmitted sequence comprises the first set of values generated by the generator polynomial, the first set comprising a predetermined number of the first values generated.
6. The device of claim 5 wherein the sequence is transmitted with pi/4-offset BPSK modulation using single carrier modulation.
7. The device of claim 6 , wherein the sample rate of the sequence is 50 million samples per second.
8. The device of claim 7 , wherein the sequence is used for both bitloading and for channel characterization.
9. The device of claim 1 , wherein FFT engine computers the impulse response as (I 2 +Q 2 ) from the complex result of a final FFT pass.
10. A system for processing received signals to determine the impulse response of a channel, the system comprising:
a) a selector switch having a first and second input and an output, the first input for receiving a time domain signal;
b) a fast Fourier Transform calculation engine having an input and an output, the input coupled to the output of the selector switch;
c) a data symbol switch having a signal input, a control input and an output, the signal input coupled to the output of the fast Fourier Transform calculation engine and controlled by a control signal coupled to the control input, the switch governing which data symbols of a received packet are to be added into an average of channel response under the control of the control signal;
d) a multiplier having a first input, a second input and an output, the first input coupled to the output of the data symbol switch, the second input coupled to receive an EVM sequence, the multiplier multiplying the signal applied to the first input with the signal applied to the second input;
e) an EVM average memory, the EVM average memory storing the output of the multiplier and providing the stored values to the second input of the data selector switch; and
f) a sum and square computer coupled to the output of the fast Fourier Transform, the sum and square computer outputting the impulse response of the time domain channel.
11. A method of computing the impulse response of a communication channel comprising:
computing a Fast Fourier Transform (FFT) of an input sequence to produce an output sequence with an FFT engine;
feeding the FFT engine with a data selector to select between an input signal and a feedback path;
multiplying the FFT output sequence by a predetermined Error Vector Magnitude (EVM) sequence with a multiplier;
wherein when the data selector selects the input signal, the averaging the product of the EVM sequence and the FFT of the input signal is calculated, and when the data selector selects the feedback path, the HT engine computes the FFT of the impulse response of the communication channel.
12. The method of claim 11 wherein the EVM is computed on a per-carrier basis as a difference between a known transmitted sequence and a received sequence.
13. The method of claim 12 wherein the known transmitted sequence is generated by a linear feedback shift register with a generator polynomial X 10 +X 7 +1.
14. The method of claim 13 wherein the linear feedback shift register can be initialized at the start of each transmitted sequence.
15. The method of claim 14 wherein the known transmitted sequence comprises the first set of values generated by the generator polynomial, the first set comprising a predetermined number of the first values generated.
16. The method of claim 15 wherein the sequence is transmitted with pi/4-offset BPSK modulation using single carrier modulation.
17. The method of claim 16 , wherein the sample rate of the sequence is 50 million samples per second.