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**Language:** Matlab

**Processor:** Not Relevant

**Submitted by kadhiem Ayob on Oct 22 2011**

Licensed under a Creative Commons Attribution 3.0 Unported License

Complex FIR filtering of complex input requires 4 multipliers per tap. It is possible to reduce that to 3 at the expense of extra adder/two subtractors. This can be verified by simple algebra, as follows:

(A+jB)(C+jD) = AC-BD + j(AD+BC)

Add BC-BC to the real term: AC-BD = AC-BD + BC-BC

= B(C-D) -C(B-A)

Add AC-AC to the imaginary term: AD+BC = AD+BC + AC-AC

= A(C+D) +C(B-A)

Thus, the term C*(B-A) is shared between real and imaginary outputs. The new structure is made up of three subfilters. The code below describes and verifies the structure functionality and can be modified into bit true model.

Additionally, if your design can run at three times the speed of input then you can time fold the three subfilters neatly.

%random input

I_in = randn(1,1024);

Q_in = randn(1,1024);

%random test filter

h = randn(1,20)+j*randn(1,20);

%split up into 3 subfilters

h1 = real(h)+imag(h); %extra adder

h2 = real(h);

h3 = real(h)-imag(h); %extra subtractor

%apply subfilter

ch1 = filter(h1,1,I_in);

ch2 = filter(h2,1,Q_in-I_in); %extra subtractor

ch3 = filter(h3,1,Q_in);

%combine outputs

yr = round((ch3-ch2)*2^15);

yi = round((ch1+ch2)*2^15);

IQ_out = complex(yr,yi);

%direct computation

ref = round(2^15*filter(h,1,complex(I_in,Q_in)));

plot(IQ_out - ref);

I_in = randn(1,1024);

Q_in = randn(1,1024);

%random test filter

h = randn(1,20)+j*randn(1,20);

%split up into 3 subfilters

h1 = real(h)+imag(h); %extra adder

h2 = real(h);

h3 = real(h)-imag(h); %extra subtractor

%apply subfilter

ch1 = filter(h1,1,I_in);

ch2 = filter(h2,1,Q_in-I_in); %extra subtractor

ch3 = filter(h3,1,Q_in);

%combine outputs

yr = round((ch3-ch2)*2^15);

yi = round((ch1+ch2)*2^15);

IQ_out = complex(yr,yi);

%direct computation

ref = round(2^15*filter(h,1,complex(I_in,Q_in)));

plot(IQ_out - ref);

Experienced FPGA Engineer, focussed on DSP functionality within FPGAs

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