Since I plan on using the zero crossing detector for the timing error detector (TED), the matched filter output was downsampled from 8 samples/symbol to 2 samples/symbol. As such, the loop operates at 2 samples/symbol but the input data is sampled at 8 samples/symbol. Note that the matched filter operates at 8 samples/symbol is used to remove the double frequency term resulting from the heterodyne operation. The output of the matched filter is downsampled by 4 to produce a data stream at 2 samples/symbol.

Use a Farrow filter for your interpolator. I used a piecewise-parabolic interpolation filter. You could use that as well or use the cubic interpolator. These structures are described in detail in Section 5.4.2 of the text

For the timing error detector (TED) I suggest you use the zero crossing detector described in Section 5.4.1 of the text. The TED operates at 2 samples/symbol. When the strobe is high, the TED output is the error value give by the equation. When the strobe is low, TED output is zero. In this way, the timing error, which is computed at one sample/symbol, is upsampled by zero insertion.

Interpolation control is performed by a decrementing modulo-1 counter as described in Section 5.4.3 of the text.

The enabled hold block keeps the input value when the enable signal is 0 and accepts a new input when the enable signal is > 0. This block is needed since the fractional interval is updated by the mod-1 counter only once per symbol but the interpolator needs it 2 times per symbol. The hold operation ensures that the interpolator is using the proper value for the fractional interval.

Simulink provides skeletal enabled subsystems:

Simulink -> Ports & Subsystems -> Enabled Subsystem

The default enabled subsystem is shown below:
The subsystem consists of a simple wire with the enable icon above it. This means the wire is only active when the enable signal exceeds a predefined threshold (double click on the enable block to set the threshold). When the enable signal is below the predefined threshold, nothing happens and the output holds its value.

The decision subsystem is also enabled. This is because decisions should be made on every other interpolator output on average. The enable signal indicates which interpolator outputs represent the desired signal space projections. An example of the enabled decision subsystem for BPSK is shown below

Incorporate the symbol timing synchronization subsystem into a BPSK detector.

For an input to the detector, use the From File block with the Filename field set to bpsktrdata.mat and the Sample Time Field set to 1.

Run the simulation.

The detector produces a whole bunch of decisions.

To find the data bits, look for the 8 bit SYNC pattern in the detector output. Once you find it, the following 448 bits correspond to 64 7-bit ASCII characters.

Plot the timing error (loop filter input) for the timing PLL and use this plot to estimate how long (measured in symbols) it took for the PLL to phase lock. Either attach the plot to your email message (preferred method) or print it out and turn it in to the simulink TA.

Plot the interpolation interval (mu) as a function of time. Either attach the plot to your email message (preferred method) or print it out and turn it in to the simulink TA.