down_converter.m

完整的空时码链路仿真多发射天线多接收天线空时分集系统的具体代码实现和误码性能评估

function [quad,inphase]=down_converter(received,fc,fs,T,cut_off,order)

%   [quad,inphase]=down_converter(received,fc,fs,T,cut_off,order)
%
%	Output:
%	quad		- Quadrature component of the received signal, in baseband
%	inphase		- Inphase component of the received signal, in baseband
%	
%   Input:
%   received    - Received signal, in passband 
%   fc          - Carrier frequency
%   fs          - Sampling frequency
%   T           - Symbol period
%   cut_off     - Cut off frequency for the lowpass filter
%   order       - Order of the lowpass filter
%
%   Short Theoretical Background for the Function:
%   
%   Downconverts the received passband signal to a baseband signal which
%   is lowpass filtered and split into a quadrature and an inphase component 
%   The function low_pass is used for lowpass filtering
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%     
%%% Function part of simulation for Space-Time
%%% coding project, group Grey-2001.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%   Author:  Fredrik Hansson
%   Date:    2001-03-20
%   Version: 1.0
%   Revision (Name & Date):
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

oversamp = fs*T;

t = 0:T/oversamp:(length(received)*T-T)/oversamp; % timescale


ri = received.*(2*cos(-2*pi*fc*t));      % downconversion, still containing
                                         % high frequency components
rq = received.*(2*sin(-2*pi*fc*t));      % downconversion, still containing
                                         % high frequency components

inphase = low_pass(cut_off,order,ri);    % The inphase component only
quad    = low_pass(cut_off,order,rq);    % The quadrature component only