GNU Radio 3.6.3.1 C++ API
digital_pfb_clock_sync_fff Class Reference

Timing synchronizer using polyphase filterbanks. More...

#include <digital_pfb_clock_sync_fff.h>

Inheritance diagram for digital_pfb_clock_sync_fff:
gr_block gr_basic_block gr_msg_accepter gruel::msg_accepter

List of all members.

Public Member Functions

 ~digital_pfb_clock_sync_fff ()
void update_gains ()
 update the system gains from omega and eta
void set_taps (const std::vector< float > &taps, std::vector< std::vector< float > > &ourtaps, std::vector< gr_fir_fff * > &ourfilter)
std::vector< std::vector< float > > get_taps ()
std::vector< std::vector< float > > get_diff_taps ()
std::vector< float > get_channel_taps (int channel)
std::vector< float > get_diff_channel_taps (int channel)
std::string get_taps_as_string ()
std::string get_diff_taps_as_string ()
void set_loop_bandwidth (float bw)
 Set the loop bandwidth.
void set_damping_factor (float df)
 Set the loop damping factor.
void set_alpha (float alpha)
 Set the loop gain alpha.
void set_beta (float beta)
 Set the loop gain beta.
void set_max_rate_deviation (float m)
float get_loop_bandwidth () const
 Returns the loop bandwidth.
float get_damping_factor () const
 Returns the loop damping factor.
float get_alpha () const
 Returns the loop gain alpha.
float get_beta () const
 Returns the loop gain beta.
float get_clock_rate () const
 Returns the current clock rate.
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
- Public Member Functions inherited from gr_block
virtual ~gr_block ()
unsigned history () const
void set_history (unsigned history)
bool fixed_rate () const
 Return true if this block has a fixed input to output rate.
virtual void forecast (int noutput_items, gr_vector_int &ninput_items_required)
 Estimate input requirements given output request.
virtual bool start ()
 Called to enable drivers, etc for i/o devices.
virtual bool stop ()
 Called to disable drivers, etc for i/o devices.
void set_output_multiple (int multiple)
 Constrain the noutput_items argument passed to forecast and general_work.
int output_multiple () const
bool output_multiple_set () const
void set_alignment (int multiple)
 Constrains buffers to work on a set item alignment (for SIMD)
int alignment () const
void set_unaligned (int na)
int unaligned () const
void set_is_unaligned (bool u)
bool is_unaligned () const
void consume (int which_input, int how_many_items)
 Tell the scheduler how_many_items of input stream which_input were consumed.
void consume_each (int how_many_items)
 Tell the scheduler how_many_items were consumed on each input stream.
void produce (int which_output, int how_many_items)
 Tell the scheduler how_many_items were produced on output stream which_output.
void set_relative_rate (double relative_rate)
 Set the approximate output rate / input rate.
double relative_rate () const
 return the approximate output rate / input rate
virtual int fixed_rate_ninput_to_noutput (int ninput)
 Given ninput samples, return number of output samples that will be produced. N.B. this is only defined if fixed_rate returns true. Generally speaking, you don't need to override this.
virtual int fixed_rate_noutput_to_ninput (int noutput)
 Given noutput samples, return number of input samples required to produce noutput. N.B. this is only defined if fixed_rate returns true. Generally speaking, you don't need to override this.
uint64_t nitems_read (unsigned int which_input)
 Return the number of items read on input stream which_input.
uint64_t nitems_written (unsigned int which_output)
 Return the number of items written on output stream which_output.
tag_propagation_policy_t tag_propagation_policy ()
 Asks for the policy used by the scheduler to moved tags downstream.
void set_tag_propagation_policy (tag_propagation_policy_t p)
 Set the policy by the scheduler to determine how tags are moved downstream.
int max_noutput_items ()
 Return the maximum number of output items this block will handle during a call to work.
void set_max_noutput_items (int m)
 Set the maximum number of ouput items htis block will handle during a call to work.
void unset_max_noutput_items ()
 Clear the switch for using the max_noutput_items value of this block.
bool is_set_max_noutput_items ()
 Ask the block if the flag is or is not set to use the internal value of max_noutput_items during a call to work.
void expand_minmax_buffer (int port)
long max_output_buffer (size_t i)
 Returns max buffer size on output port i.
void set_max_output_buffer (long max_output_buffer)
 Sets max buffer size on all output ports.
void set_max_output_buffer (int port, long max_output_buffer)
 Sets max buffer size on output port port.
long min_output_buffer (size_t i)
 Returns min buffer size on output port i.
void set_min_output_buffer (long min_output_buffer)
 Sets min buffer size on all output ports.
void set_min_output_buffer (int port, long min_output_buffer)
 Sets min buffer size on output port port.
gr_block_detail_sptr detail () const
void set_detail (gr_block_detail_sptr detail)
- Public Member Functions inherited from gr_basic_block
virtual ~gr_basic_block ()
long unique_id () const
long symbolic_id () const
std::string name () const
std::string symbol_name () const
gr_io_signature_sptr input_signature () const
gr_io_signature_sptr output_signature () const
gr_basic_block_sptr to_basic_block ()
bool alias_set ()
std::string alias ()
pmt::pmt_t alias_pmt ()
void set_block_alias (std::string name)
void message_port_register_in (pmt::pmt_t port_id)
void message_port_register_out (pmt::pmt_t port_id)
void message_port_pub (pmt::pmt_t port_id, pmt::pmt_t msg)
void message_port_sub (pmt::pmt_t port_id, pmt::pmt_t target)
void message_port_unsub (pmt::pmt_t port_id, pmt::pmt_t target)
virtual bool message_port_is_hier (pmt::pmt_t port_id)
virtual bool message_port_is_hier_in (pmt::pmt_t port_id)
virtual bool message_port_is_hier_out (pmt::pmt_t port_id)
pmt::pmt_t message_ports_in ()
 Get input message port names.
pmt::pmt_t message_ports_out ()
 Get output message port names.
void _post (pmt::pmt_t which_port, pmt::pmt_t msg)
bool empty_p (pmt::pmt_t which_port)
 is the queue empty?
bool empty_p ()
void insert_tail (pmt::pmt_t which_port, pmt::pmt_t msg)
pmt::pmt_t delete_head_nowait (pmt::pmt_t which_port)
pmt::pmt_t delete_head_blocking (pmt::pmt_t which_port)
msg_queue_t::iterator get_iterator (pmt::pmt_t which_port)
void erase_msg (pmt::pmt_t which_port, msg_queue_t::iterator it)
virtual bool has_msg_port (pmt::pmt_t which_port)
template<typename T >
void set_msg_handler (pmt::pmt_t which_port, T msg_handler)
 Set the callback that is fired when messages are available.
- Public Member Functions inherited from gr_msg_accepter
 gr_msg_accepter ()
 ~gr_msg_accepter ()
void post (pmt::pmt_t which_port, pmt::pmt_t msg)
 send msg to msg_accepter on port which_port
- Public Member Functions inherited from gruel::msg_accepter
 msg_accepter ()
virtual ~msg_accepter ()

Friends

DIGITAL_API
digital_pfb_clock_sync_fff_sptr 
digital_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, int osps)

Additional Inherited Members

- Public Types inherited from gr_block
enum  { WORK_CALLED_PRODUCE = -2, WORK_DONE = -1 }
 Magic return values from general_work. More...
enum  tag_propagation_policy_t { TPP_DONT = 0, TPP_ALL_TO_ALL = 1, TPP_ONE_TO_ONE = 2 }
- Protected Member Functions inherited from gr_block
 gr_block (void)
 gr_block (const std::string &name, gr_io_signature_sptr input_signature, gr_io_signature_sptr output_signature)
void set_fixed_rate (bool fixed_rate)
void add_item_tag (unsigned int which_output, uint64_t abs_offset, const pmt::pmt_t &key, const pmt::pmt_t &value, const pmt::pmt_t &srcid=pmt::PMT_F)
 Adds a new tag onto the given output buffer.
void add_item_tag (unsigned int which_output, const gr_tag_t &tag)
 Adds a new tag onto the given output buffer.
void get_tags_in_range (std::vector< gr_tag_t > &v, unsigned int which_input, uint64_t abs_start, uint64_t abs_end)
 Given a [start,end), returns a vector of all tags in the range.
void get_tags_in_range (std::vector< gr_tag_t > &v, unsigned int which_input, uint64_t abs_start, uint64_t abs_end, const pmt::pmt_t &key)
 Given a [start,end), returns a vector of all tags in the range with a given key.
- Protected Member Functions inherited from gr_basic_block
 gr_basic_block (void)
 gr_basic_block (const std::string &name, gr_io_signature_sptr input_signature, gr_io_signature_sptr output_signature)
 Protected constructor prevents instantiation by non-derived classes.
void set_input_signature (gr_io_signature_sptr iosig)
 may only be called during constructor
void set_output_signature (gr_io_signature_sptr iosig)
 may only be called during constructor
void set_color (vcolor color)
 Allow the flowgraph to set for sorting and partitioning.
vcolor color () const
- Protected Attributes inherited from gr_block
std::vector< long > d_max_output_buffer
std::vector< long > d_min_output_buffer
- Protected Attributes inherited from gr_basic_block
std::string d_name
gr_io_signature_sptr d_input_signature
gr_io_signature_sptr d_output_signature
long d_unique_id
long d_symbolic_id
std::string d_symbol_name
std::string d_symbol_alias
vcolor d_color
msg_queue_map_t msg_queue
pmt::pmt_t message_subscribers

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 filterbank 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 block's parameters are:

  • sps: The clock sync block needs to know the number of samples per symbol, because it defaults to return a single point representing the symbol. The sps can be any positive real number and does not need to be an integer.
  • taps: One of the most important parameters for this block is the taps of the filter. One of the benefits of this algorithm is that you can put the matched filter in here as the taps, so you get both the matched filter and sample timing correction in one go. So create your normal matched filter. For a typical digital modulation, this is a root raised cosine filter. The number of taps of this filter is based on how long you expect the channel to be; that is, how many symbols do you want to combine to get the current symbols energy back (there's probably a better way of stating that). It's usually 5 to 10 or so. That gives you your filter, but now we need to think about it as a filter with different phase profiles in each filter. So take this number of taps and multiply it by the number of filters. This is the number you would use to create your prototype filter. When you use this in the PFB filerbank, it segments these taps into the filterbanks in such a way that each bank now represents the filter at different phases, equally spaced at 2pi/N, where N is the number of filters.
  • filter_size (default=32): 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.
  • init_phase (default=0): 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.
  • max_rate_deviation (default=1.5): The next 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.
  • osps (default=1): The osps is the number of output samples per symbol. By default, the algorithm produces 1 sample per symbol, sampled at the exact sample value. This osps value was added to better work with equalizers, which do a better job of modeling the channel if they have 2 samps/sym.

Constructor & Destructor Documentation

digital_pfb_clock_sync_fff::~digital_pfb_clock_sync_fff ( )

Member Function Documentation

bool digital_pfb_clock_sync_fff::check_topology ( int  ninputs,
int  noutputs 
)
virtual

Confirm that ninputs and noutputs is an acceptable combination.

Parameters:
ninputsnumber of input streams connected
noutputsnumber 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.

int digital_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_itemsnumber of output items to write on each output stream
ninput_itemsnumber of input items available on each input stream
input_itemsvector of pointers to the input items, one entry per input stream
output_itemsvector 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.

Reimplemented from gr_block.

float digital_pfb_clock_sync_fff::get_alpha ( ) const

Returns the loop gain alpha.

float digital_pfb_clock_sync_fff::get_beta ( ) const

Returns the loop gain beta.

std::vector<float> digital_pfb_clock_sync_fff::get_channel_taps ( int  channel)

Returns the taps of the matched filter for a particular channel

float digital_pfb_clock_sync_fff::get_clock_rate ( ) const

Returns the current clock rate.

float digital_pfb_clock_sync_fff::get_damping_factor ( ) const

Returns the loop damping factor.

std::vector<float> digital_pfb_clock_sync_fff::get_diff_channel_taps ( int  channel)

Returns the taps in the derivative filter for a particular channel

std::vector< std::vector<float> > digital_pfb_clock_sync_fff::get_diff_taps ( )

Returns all of the taps of the derivative filter

std::string digital_pfb_clock_sync_fff::get_diff_taps_as_string ( )

Return the derivative filter taps as a formatted string for printing

float digital_pfb_clock_sync_fff::get_loop_bandwidth ( ) const

Returns the loop bandwidth.

std::vector< std::vector<float> > digital_pfb_clock_sync_fff::get_taps ( )

Returns all of the taps of the matched filter

std::string digital_pfb_clock_sync_fff::get_taps_as_string ( )

Return the taps as a formatted string for printing

void digital_pfb_clock_sync_fff::set_alpha ( float  alpha)

Set the loop gain alpha.

Set's the loop filter's alpha gain parameter.

This value should really only be set by adjusting the loop bandwidth and damping factor.

Parameters:
alpha(float) new alpha gain
void digital_pfb_clock_sync_fff::set_beta ( float  beta)

Set the loop gain beta.

Set's the loop filter's beta gain parameter.

This value should really only be set by adjusting the loop bandwidth and damping factor.

Parameters:
beta(float) new beta gain
void digital_pfb_clock_sync_fff::set_damping_factor ( float  df)

Set the loop damping factor.

Set the loop filter's damping factor to df. The damping factor should be sqrt(2)/2.0 for critically damped systems. Set it to anything else only if you know what you are doing. It must be a number between 0 and 1.

When a new damping factor is set, the gains, alpha and beta, of the loop are recalculated by a call to update_gains().

Parameters:
df(float) new damping factor
void digital_pfb_clock_sync_fff::set_loop_bandwidth ( float  bw)

Set the loop bandwidth.

Set the loop filter's bandwidth to bw. This should be between 2*pi/200 and 2*pi/100 (in rads/samp). It must also be a positive number.

When a new damping factor is set, the gains, alpha and beta, of the loop are recalculated by a call to update_gains().

Parameters:
bw(float) new bandwidth
void digital_pfb_clock_sync_fff::set_max_rate_deviation ( float  m)
inline

Set the maximum deviation from 0 d_rate can have

void digital_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

void digital_pfb_clock_sync_fff::update_gains ( )

update the system gains from omega and eta

This function updates the system gains based on the loop bandwidth and damping factor of the system. These two factors can be set separately through their own set functions.


Friends And Related Function Documentation

DIGITAL_API digital_pfb_clock_sync_fff_sptr digital_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,
int  osps 
)
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).
osps(int) The number of output samples per symbol (default=1).

The documentation for this class was generated from the following file: