Timing synchronizer using polyphase filterbanks. More...
#include <gr_pfb_clock_sync_fff.h>
Public Member Functions | |
| ~gr_pfb_clock_sync_fff () | |
| void | set_taps (const std::vector< float > &taps, std::vector< std::vector< float > > &ourtaps, std::vector< gr_fir_fff * > &ourfilter) |
| std::vector< float > | channel_taps (int channel) |
| std::vector< float > | diff_channel_taps (int channel) |
| void | print_taps () |
| void | print_diff_taps () |
| void | set_alpha (float alpha) |
| void | set_beta (float beta) |
| void | set_max_rate_deviation (float m) |
| bool | check_topology (int ninputs, int noutputs) |
| Confirm that ninputs and noutputs is an acceptable combination. | |
| int | general_work (int noutput_items, gr_vector_int &ninput_items, gr_vector_const_void_star &input_items, gr_vector_void_star &output_items) |
| compute output items from input items | |
Friends | |
| gr_pfb_clock_sync_fff_sptr | gr_make_pfb_clock_sync_fff (double sps, float gain, const std::vector< float > &taps, unsigned int filter_size, float init_phase, float max_rate_deviation) |
Detailed Description
Timing synchronizer using polyphase filterbanks.
This block performs timing synchronization for PAM signals by minimizing the derivative of the filtered signal, which in turn maximizes the SNR and minimizes ISI.
This approach works by setting up two filterbanks; one filterbanke contains the signal's pulse shaping matched filter (such as a root raised cosine filter), where each branch of the filterbank contains a different phase of the filter. The second filterbank contains the derivatives of the filters in the first filterbank. Thinking of this in the time domain, the first filterbank contains filters that have a sinc shape to them. We want to align the output signal to be sampled at exactly the peak of the sinc shape. The derivative of the sinc contains a zero at the maximum point of the sinc (sinc(0) = 1, sinc(0)' = 0). Furthermore, the region around the zero point is relatively linear. We make use of this fact to generate the error signal.
If the signal out of the derivative filters is d_i[n] for the ith filter, and the output of the matched filter is x_i[n], we calculate the error as: e[n] = (Re{x_i[n]} * Re{d_i[n]} + Im{x_i[n]} * Im{d_i[n]}) / 2.0 This equation averages the error in the real and imaginary parts. There are two reasons we multiply by the signal itself. First, if the symbol could be positive or negative going, but we want the error term to always tell us to go in the same direction depending on which side of the zero point we are on. The sign of x_i[n] adjusts the error term to do this. Second, the magnitude of x_i[n] scales the error term depending on the symbol's amplitude, so larger signals give us a stronger error term because we have more confidence in that symbol's value. Using the magnitude of x_i[n] instead of just the sign is especially good for signals with low SNR.
The error signal, e[n], gives us a value proportional to how far away from the zero point we are in the derivative signal. We want to drive this value to zero, so we set up a second order loop. We have two variables for this loop; d_k is the filter number in the filterbank we are on and d_rate is the rate which we travel through the filters in the steady state. That is, due to the natural clock differences between the transmitter and receiver, d_rate represents that difference and would traverse the filter phase paths to keep the receiver locked. Thinking of this as a second-order PLL, the d_rate is the frequency and d_k is the phase. So we update d_rate and d_k using the standard loop equations based on two error signals, d_alpha and d_beta. We have these two values set based on each other for a critically damped system, so in the block constructor, we just ask for "gain," which is d_alpha while d_beta is equal to (gain^2)/4.
The clock sync block needs to know the number of samples per second (sps), because it only returns a single point representing the sample. The sps can be any positive real number and does not need to be an integer. The filter taps must also be specified. The taps are generated by first conceiving of the prototype filter that would be the signal's matched filter. Then interpolate this by the number of filters in the filterbank. These are then distributed among all of the filters. So if the prototype filter was to have 45 taps in it, then each path of the filterbank will also have 45 taps. This is easily done by building the filter with the sample rate multiplied by the number of filters to use.
The number of filters can also be set and defaults to 32. With 32 filters, you get a good enough resolution in the phase to produce very small, almost unnoticeable, ISI. Going to 64 filters can reduce this more, but after that there is very little gained for the extra complexity.
The initial phase is another settable parameter and refers to the filter path the algorithm initially looks at (i.e., d_k starts at init_phase). This value defaults to zero, but it might be useful to start at a different phase offset, such as the mid- point of the filters.
The final parameter is the max_rate_devitation, which defaults to 1.5. This is how far we allow d_rate to swing, positive or negative, from 0. Constraining the rate can help keep the algorithm from walking too far away to lock during times when there is no signal.
Constructor & Destructor Documentation
| gr_pfb_clock_sync_fff::~gr_pfb_clock_sync_fff | ( | ) |
Member Function Documentation
| std::vector<float> gr_pfb_clock_sync_fff::channel_taps | ( | int | channel | ) |
Returns the taps of the matched filter
| bool gr_pfb_clock_sync_fff::check_topology | ( | int | ninputs, |
| int | noutputs | ||
| ) | [virtual] |
Confirm that ninputs and noutputs is an acceptable combination.
- Parameters:
-
ninputs number of input streams connected noutputs number of output streams connected
- Returns:
- true if this is a valid configuration for this block.
This function is called by the runtime system whenever the topology changes. Most classes do not need to override this. This check is in addition to the constraints specified by the input and output gr_io_signatures.
Reimplemented from gr_basic_block.
| std::vector<float> gr_pfb_clock_sync_fff::diff_channel_taps | ( | int | channel | ) |
Returns the taps in the derivative filter
| int gr_pfb_clock_sync_fff::general_work | ( | int | noutput_items, |
| gr_vector_int & | ninput_items, | ||
| gr_vector_const_void_star & | input_items, | ||
| gr_vector_void_star & | output_items | ||
| ) | [virtual] |
compute output items from input items
- Parameters:
-
noutput_items number of output items to write on each output stream ninput_items number of input items available on each input stream input_items vector of pointers to the input items, one entry per input stream output_items vector of pointers to the output items, one entry per output stream
- Returns:
- number of items actually written to each output stream, or -1 on EOF. It is OK to return a value less than noutput_items. -1 <= return value <= noutput_items
general_work must call consume or consume_each to indicate how many items were consumed on each input stream.
Implements gr_block.
| void gr_pfb_clock_sync_fff::print_diff_taps | ( | ) |
Print all of the filterbank taps of the derivative filter to screen.
| void gr_pfb_clock_sync_fff::print_taps | ( | ) |
Print all of the filterbank taps to screen.
| void gr_pfb_clock_sync_fff::set_alpha | ( | float | alpha | ) | [inline] |
Set the gain value alpha for the control loop
| void gr_pfb_clock_sync_fff::set_beta | ( | float | beta | ) | [inline] |
Set the gain value beta for the control loop
| void gr_pfb_clock_sync_fff::set_max_rate_deviation | ( | float | m | ) | [inline] |
Set the maximum deviation from 0 d_rate can have
| void gr_pfb_clock_sync_fff::set_taps | ( | const std::vector< float > & | taps, |
| std::vector< std::vector< float > > & | ourtaps, | ||
| std::vector< gr_fir_fff * > & | ourfilter | ||
| ) |
Resets the filterbank's filter taps with the new prototype filter
Friends And Related Function Documentation
| gr_pfb_clock_sync_fff_sptr gr_make_pfb_clock_sync_fff | ( | double | sps, |
| float | gain, | ||
| const std::vector< float > & | taps, | ||
| unsigned int | filter_size, | ||
| float | init_phase, | ||
| float | max_rate_deviation | ||
| ) | [friend] |
Build the polyphase filterbank timing synchronizer.
- Parameters:
-
sps (double) The number of samples per second in the incoming signal gain (float) The alpha gain of the control loop; beta = (gain^2)/4 by default. taps (vector<int>) The filter taps. filter_size (uint) The number of filters in the filterbank (default = 32). init_phase (float) The initial phase to look at, or which filter to start with (default = 0). max_rate_deviation (float) Distance from 0 d_rate can get (default = 1.5).
The documentation for this class was generated from the following file:
