Fully functional modulation/demodulation functions

scripts/fDSQPSKDemodulator.m

@@ -14,4 +14,61 @@
 % bitsOut (Px1 Integers) = P demodulated bits of 1's and 0's
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
-function [bitsOut]=fDSQPSKDemodulator(symbolsIn,GoldSeq,phi)
+function [ bitsOut ] = fDSQPSKDemodulator( symbolsIn, goldSeq, phi )
+
+% chipPeriod and carrierPeriod are given nominal values
+T_carrier = 5; % carrier period
+T_chip = 1; % chip period
+chipsPerBit = T_carrier/T_chip; % chipsPerBit must be an integer value
+
+chipsLength = length(symbolsIn);
+symbolLength = chipsLength/chipsPerBit;
+seqLength = length(goldSeq);
+
+%% De-Spread using Gold Sequence
+
+% Change gold sequence form
+goldSeq = 1 - 2*goldSeq; % 1 -> -1, 0 -> 1
+
+symbolsModulated = complex(zeros(symbolLength, 1), zeros(symbolLength, 1));
+
+for chipIndex = 1:chipsLength
+    seqIndex = mod(chipIndex-1, seqLength) + 1; % index for gold sequence
+    symbolIndex = ceil(chipIndex/chipsPerBit); % index for QPSK symbols
+
+    % Accumulate chips to be averaged later
+    symbolsModulated(symbolIndex) = symbolsModulated(symbolIndex) + goldSeq(seqIndex) * symbolsIn(chipIndex);    
+
+    if mod(chipIndex, chipsPerBit) == 0
+        symbolsModulated(symbolIndex) = symbolsModulated(symbolIndex) / chipsPerBit;
+    end
+end
+
+%% Perform QPSK de-modulation
+
+phi_rad = deg2rad(phi);
+bitsOut = zeros(symbolLength*2, 1, 'uint8');
+
+for symbolIndex = 1:symbolLength
+
+    % Find phase angle
+    theta = angle(symbolsModulated(symbolIndex));
+    if theta < 0
+        theta = theta + 2*pi;
+    end
+
+    % Convert phase angle to binary values    
+    bitPair = uint8((theta - phi_rad)*2/pi);
+
+
+    % check for invalid values
+    if bitPair < 0 || bitPair > 3
+        fprintf('ERROR: bitsInPaired contained value outside range 0-3: %i @ index %i\n', symbolsPaired(symbolIndex), symbolIndex)
+        return
+    end
+
+    % Split bit pairs into bits
+    bitsOut(2*symbolIndex - 1 : 2*symbolIndex) = de2bi(bitPair, 2, 'left-msb');
+end
+
+end

scripts/fDSQPSKModulator.m

@@ -15,4 +15,56 @@
 % symbolsOut (Rx1 Complex) = R channel symbol chips after DS-QPSK Modulation
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
-function [symbolsOut]=fDSQPSKModulator(bitsIn,goldseq,phi)
+function [ symbolsOut ] = fDSQPSKModulator( bitsIn, goldSeq, phi )
+
+% chipPeriod and carrierPeriod are given nominal values
+T_carrier = 5; % carrier period
+T_chip = 1; % chip period
+chipsPerBit = T_carrier/T_chip; % chipsPerBit must be an integer value
+
+symbolsLength = length(bitsIn)/2;
+chipsLength = symbolsLength * chipsPerBit;
+seqLength = length(goldSeq);
+
+symbolsPaired = zeros(symbolsLength, 1, 'double');
+symbolsModulated = complex( symbolsPaired, symbolsPaired ); % initialise as a symbolsBinary sized complex vector
+
+%% Perform QPSK modulation
+
+phi_rad = deg2rad(phi);
+
+for symbolIndex = 1:symbolsLength
+
+    symbolsPaired(symbolIndex) = bitsIn(symbolIndex*2 - 1) * 2 + bitsIn(symbolIndex*2); % group bits into pairs
+
+    %check for invalid values
+    if symbolsPaired(symbolIndex) < 0 || symbolsPaired(symbolIndex) > 3
+        fprintf('ERROR: bitsInPaired contained value outside range 0-3: %i @ index %i\n', bitsInPaired(symbolIndex), symbolIndex)
+        return
+    end
+
+    % Find symbols in polar form
+    theta = phi_rad + symbolsPaired(symbolIndex)*pi/2; % NOTE: diagram was vague on what phi should be for n ~= 00
+    r = sqrt(2);
+
+    % Convert to a + bi form
+    symbolsModulated(symbolIndex) = complex(r*cos(theta), r*sin(theta));
+end
+
+%% Spread using Gold Sequence
+% As specified in SomeNotes.1, "The messages are first modulated using QPSK,
+% then spread by the gold sequences"
+
+% Change gold sequence form
+goldSeq = 1 - 2*goldSeq; % 1 -> -1, 0 -> 1
+
+symbolsOut = complex(zeros(chipsLength, 1), zeros(chipsLength, 1));
+
+for chipIndex = 1:chipsLength
+    seqIndex = mod(chipIndex-1, seqLength) + 1; % index for gold sequence
+    symbolIndex = ceil(chipIndex/chipsPerBit); % index for QPSK symbols
+
+    symbolsOut(chipIndex) = goldSeq(seqIndex) * symbolsModulated(symbolIndex);
+end
+
+end