GNU Radio Manual and C++ API Reference  3.7.3
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gr::digital::pfb_clock_sync_ccf Class Referenceabstract

Timing synchronizer using polyphase filterbanks. More...

#include <gnuradio/digital/pfb_clock_sync_ccf.h>

Public Types

typedef boost::shared_ptr
< pfb_clock_sync_ccf
sptr
 
- 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 }
 

Public Member Functions

virtual void update_gains ()=0
 update the system gains from omega and eta More...
 
virtual void set_taps (const std::vector< float > &taps, std::vector< std::vector< float > > &ourtaps, std::vector< gr::filter::kernel::fir_filter_ccf * > &ourfilter)=0
 
virtual std::vector
< std::vector< float > > 
taps () const =0
 
virtual std::vector
< std::vector< float > > 
diff_taps () const =0
 
virtual std::vector< float > channel_taps (int channel) const =0
 
virtual std::vector< float > diff_channel_taps (int channel) const =0
 
virtual std::string taps_as_string () const =0
 
virtual std::string diff_taps_as_string () const =0
 
virtual void set_loop_bandwidth (float bw)=0
 Set the loop bandwidth. More...
 
virtual void set_damping_factor (float df)=0
 Set the loop damping factor. More...
 
virtual void set_alpha (float alpha)=0
 Set the loop gain alpha. More...
 
virtual void set_beta (float beta)=0
 Set the loop gain beta. More...
 
virtual void set_max_rate_deviation (float m)=0
 
virtual float loop_bandwidth () const =0
 Returns the loop bandwidth. More...
 
virtual float damping_factor () const =0
 Returns the loop damping factor. More...
 
virtual float alpha () const =0
 Returns the loop gain alpha. More...
 
virtual float beta () const =0
 Returns the loop gain beta. More...
 
virtual float clock_rate () const =0
 Returns the current clock rate. More...
 
virtual float error () const =0
 Returns the current error of the control loop. More...
 
virtual float rate () const =0
 Returns the current rate of the control loop. More...
 
virtual float phase () const =0
 Returns the current phase arm of the control loop. More...
 
- Public Member Functions inherited from gr::block
virtual ~block ()
 
unsigned history () const
 
void set_history (unsigned history)
 
void declare_sample_delay (int which, unsigned delay)
 
void declare_sample_delay (unsigned delay)
 
unsigned sample_delay (int which) const
 
bool fixed_rate () const
 Return true if this block has a fixed input to output rate. More...
 
virtual void forecast (int noutput_items, gr_vector_int &ninput_items_required)
 Estimate input requirements given output request. More...
 
virtual 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 More...
 
virtual bool start ()
 Called to enable drivers, etc for i/o devices. More...
 
virtual bool stop ()
 Called to disable drivers, etc for i/o devices. More...
 
void set_output_multiple (int multiple)
 Constrain the noutput_items argument passed to forecast and general_work. More...
 
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) More...
 
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. More...
 
void consume_each (int how_many_items)
 Tell the scheduler how_many_items were consumed on each input stream. More...
 
void produce (int which_output, int how_many_items)
 Tell the scheduler how_many_items were produced on output stream which_output. More...
 
void set_relative_rate (double relative_rate)
 Set the approximate output rate / input rate. More...
 
double relative_rate () const
 return the approximate output rate / input rate More...
 
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. More...
 
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. More...
 
uint64_t nitems_read (unsigned int which_input)
 Return the number of items read on input stream which_input. More...
 
uint64_t nitems_written (unsigned int which_output)
 Return the number of items written on output stream which_output. More...
 
tag_propagation_policy_t tag_propagation_policy ()
 Asks for the policy used by the scheduler to moved tags downstream. More...
 
void set_tag_propagation_policy (tag_propagation_policy_t p)
 Set the policy by the scheduler to determine how tags are moved downstream. More...
 
int min_noutput_items () const
 Return the minimum number of output items this block can produce during a call to work. More...
 
void set_min_noutput_items (int m)
 Set the minimum number of output items this block can produce during a call to work. More...
 
int max_noutput_items ()
 Return the maximum number of output items this block will handle during a call to work. More...
 
void set_max_noutput_items (int m)
 Set the maximum number of output items this block will handle during a call to work. More...
 
void unset_max_noutput_items ()
 Clear the switch for using the max_noutput_items value of this block. More...
 
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. More...
 
void expand_minmax_buffer (int port)
 
long max_output_buffer (size_t i)
 Returns max buffer size on output port i. More...
 
void set_max_output_buffer (long max_output_buffer)
 Sets max buffer size on all output ports. More...
 
void set_max_output_buffer (int port, long max_output_buffer)
 Sets max buffer size on output port port. More...
 
long min_output_buffer (size_t i)
 Returns min buffer size on output port i. More...
 
void set_min_output_buffer (long min_output_buffer)
 Sets min buffer size on all output ports. More...
 
void set_min_output_buffer (int port, long min_output_buffer)
 Sets min buffer size on output port port. More...
 
float pc_noutput_items ()
 Gets instantaneous noutput_items performance counter. More...
 
float pc_noutput_items_avg ()
 Gets average noutput_items performance counter. More...
 
float pc_noutput_items_var ()
 Gets variance of noutput_items performance counter. More...
 
float pc_nproduced ()
 Gets instantaneous num items produced performance counter. More...
 
float pc_nproduced_avg ()
 Gets average num items produced performance counter. More...
 
float pc_nproduced_var ()
 Gets variance of num items produced performance counter. More...
 
float pc_input_buffers_full (int which)
 Gets instantaneous fullness of which input buffer. More...
 
float pc_input_buffers_full_avg (int which)
 Gets average fullness of which input buffer. More...
 
float pc_input_buffers_full_var (int which)
 Gets variance of fullness of which input buffer. More...
 
std::vector< float > pc_input_buffers_full ()
 Gets instantaneous fullness of all input buffers. More...
 
std::vector< float > pc_input_buffers_full_avg ()
 Gets average fullness of all input buffers. More...
 
std::vector< float > pc_input_buffers_full_var ()
 Gets variance of fullness of all input buffers. More...
 
float pc_output_buffers_full (int which)
 Gets instantaneous fullness of which input buffer. More...
 
float pc_output_buffers_full_avg (int which)
 Gets average fullness of which input buffer. More...
 
float pc_output_buffers_full_var (int which)
 Gets variance of fullness of which input buffer. More...
 
std::vector< float > pc_output_buffers_full ()
 Gets instantaneous fullness of all output buffers. More...
 
std::vector< float > pc_output_buffers_full_avg ()
 Gets average fullness of all output buffers. More...
 
std::vector< float > pc_output_buffers_full_var ()
 Gets variance of fullness of all output buffers. More...
 
float pc_work_time ()
 Gets instantaneous clock cycles spent in work. More...
 
float pc_work_time_avg ()
 Gets average clock cycles spent in work. More...
 
float pc_work_time_var ()
 Gets average clock cycles spent in work. More...
 
float pc_work_time_total ()
 Gets total clock cycles spent in work. More...
 
void reset_perf_counters ()
 Resets the performance counters. More...
 
void setup_pc_rpc ()
 Sets up export of perf. counters to ControlPort. Only called by the scheduler. More...
 
bool is_pc_rpc_set ()
 Checks if this block is already exporting perf. counters to ControlPort. More...
 
void no_pc_rpc ()
 If the block calls this in its constructor, it's perf. counters will not be exported. More...
 
void set_processor_affinity (const std::vector< int > &mask)
 Set the thread's affinity to processor core n. More...
 
void unset_processor_affinity ()
 Remove processor affinity to a specific core. More...
 
std::vector< int > processor_affinity ()
 Get the current processor affinity. More...
 
int active_thread_priority ()
 Get the current thread priority in use. More...
 
int thread_priority ()
 Get the current thread priority stored. More...
 
int set_thread_priority (int priority)
 Set the current thread priority. More...
 
bool update_rate () const
 
block_detail_sptr detail () const
 
void set_detail (block_detail_sptr detail)
 
- Public Member Functions inherited from gr::basic_block
pmt::pmt_t message_subscribers (pmt::pmt_t port)
 
virtual ~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
 
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. More...
 
pmt::pmt_t message_ports_out ()
 Get output message port names. More...
 
void _post (pmt::pmt_t which_port, pmt::pmt_t msg)
 
bool empty_p (pmt::pmt_t which_port)
 is the queue empty? More...
 
bool empty_p ()
 
bool empty_handled_p (pmt::pmt_t which_port)
 are all msg ports with handlers empty? More...
 
bool empty_handled_p ()
 
size_t nmsgs (pmt::pmt_t which_port)
 How many messages in the queue? More...
 
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)
 
virtual void setup_rpc ()
 Set up the RPC registered variables. More...
 
bool is_rpc_set ()
 Ask if this block has been registered to the RPC. More...
 
void rpc_set ()
 When the block is registered with the RPC, set this. More...
 
virtual bool check_topology (int ninputs, int noutputs)
 Confirm that ninputs and noutputs is an acceptable combination. More...
 
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. More...
 
- Public Member Functions inherited from gr::msg_accepter
 msg_accepter ()
 
 ~msg_accepter ()
 
void post (pmt::pmt_t which_port, pmt::pmt_t msg)
 send msg to msg_accepter on port which_port More...
 
- Public Member Functions inherited from gr::messages::msg_accepter
 msg_accepter ()
 

Static Public Member Functions

static sptr make (double sps, float loop_bw, const std::vector< float > &taps, unsigned int filter_size=32, float init_phase=0, float max_rate_deviation=1.5, int osps=1)
 

Additional Inherited Members

- Protected Types inherited from gr::basic_block
enum  vcolor { WHITE, GREY, BLACK }
 
- Protected Member Functions inherited from gr::block
 block (void)
 
 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. More...
 
void add_item_tag (unsigned int which_output, const tag_t &tag)
 Adds a new tag onto the given output buffer. More...
 
void remove_item_tag (unsigned int which_input, uint64_t abs_offset, const pmt::pmt_t &key, const pmt::pmt_t &value, const pmt::pmt_t &srcid=pmt::PMT_F)
 Removes a tag from the given input buffer. More...
 
void remove_item_tag (unsigned int which_input, const tag_t &tag)
 Removes a tag from the given input buffer. More...
 
void get_tags_in_range (std::vector< 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. More...
 
void get_tags_in_range (std::vector< 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. More...
 
void get_tags_in_window (std::vector< tag_t > &v, unsigned int which_input, uint64_t rel_start, uint64_t rel_end)
 Gets all tags within the relative window of the current call to work. More...
 
void get_tags_in_window (std::vector< tag_t > &v, unsigned int which_input, uint64_t rel_start, uint64_t rel_end, const pmt::pmt_t &key)
 Operates like gr::block::get_tags_in_window with the ability to only return tags with the specified key. More...
 
void enable_update_rate (bool en)
 
- Protected Attributes inherited from gr::block
std::vector< long > d_max_output_buffer
 
std::vector< long > d_min_output_buffer
 
gr::thread::mutex d_setlock
 
gr::logger_ptr d_logger
 
gr::logger_ptr d_debug_logger
 

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.

Reference: f. j. harris and M. Rice, "Multirate Digital Filters for Symbol Timing Synchronization in Software Defined Radios", IEEE Selected Areas in Communications, Vol. 19, No. 12, Dec., 2001.

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.127.1757

Member Typedef Documentation

Member Function Documentation

virtual float gr::digital::pfb_clock_sync_ccf::alpha ( ) const
pure virtual

Returns the loop gain alpha.

virtual float gr::digital::pfb_clock_sync_ccf::beta ( ) const
pure virtual

Returns the loop gain beta.

virtual std::vector<float> gr::digital::pfb_clock_sync_ccf::channel_taps ( int  channel) const
pure virtual

Returns the taps of the matched filter for a particular channel

virtual float gr::digital::pfb_clock_sync_ccf::clock_rate ( ) const
pure virtual

Returns the current clock rate.

virtual float gr::digital::pfb_clock_sync_ccf::damping_factor ( ) const
pure virtual

Returns the loop damping factor.

virtual std::vector<float> gr::digital::pfb_clock_sync_ccf::diff_channel_taps ( int  channel) const
pure virtual

Returns the taps in the derivative filter for a particular channel

virtual std::vector< std::vector<float> > gr::digital::pfb_clock_sync_ccf::diff_taps ( ) const
pure virtual

Returns all of the taps of the derivative filter

virtual std::string gr::digital::pfb_clock_sync_ccf::diff_taps_as_string ( ) const
pure virtual

Return the derivative filter taps as a formatted string for printing

virtual float gr::digital::pfb_clock_sync_ccf::error ( ) const
pure virtual

Returns the current error of the control loop.

virtual float gr::digital::pfb_clock_sync_ccf::loop_bandwidth ( ) const
pure virtual

Returns the loop bandwidth.

static sptr gr::digital::pfb_clock_sync_ccf::make ( double  sps,
float  loop_bw,
const std::vector< float > &  taps,
unsigned int  filter_size = 32,
float  init_phase = 0,
float  max_rate_deviation = 1.5,
int  osps = 1 
)
static

Build the polyphase filterbank timing synchronizer.

Parameters
sps(double) The number of samples per symbol in the incoming signal
loop_bw(float) The bandwidth of the control loop; set's alpha and beta.
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).
virtual float gr::digital::pfb_clock_sync_ccf::phase ( ) const
pure virtual

Returns the current phase arm of the control loop.

virtual float gr::digital::pfb_clock_sync_ccf::rate ( ) const
pure virtual

Returns the current rate of the control loop.

virtual void gr::digital::pfb_clock_sync_ccf::set_alpha ( float  alpha)
pure virtual

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
virtual void gr::digital::pfb_clock_sync_ccf::set_beta ( float  beta)
pure virtual

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
virtual void gr::digital::pfb_clock_sync_ccf::set_damping_factor ( float  df)
pure virtual

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
virtual void gr::digital::pfb_clock_sync_ccf::set_loop_bandwidth ( float  bw)
pure virtual

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
virtual void gr::digital::pfb_clock_sync_ccf::set_max_rate_deviation ( float  m)
pure virtual

Set the maximum deviation from 0 d_rate can have

virtual void gr::digital::pfb_clock_sync_ccf::set_taps ( const std::vector< float > &  taps,
std::vector< std::vector< float > > &  ourtaps,
std::vector< gr::filter::kernel::fir_filter_ccf * > &  ourfilter 
)
pure virtual

Resets the filterbank's filter taps with the new prototype filter

virtual std::vector< std::vector<float> > gr::digital::pfb_clock_sync_ccf::taps ( ) const
pure virtual

Returns all of the taps of the matched filter

virtual std::string gr::digital::pfb_clock_sync_ccf::taps_as_string ( ) const
pure virtual

Return the taps as a formatted string for printing

virtual void gr::digital::pfb_clock_sync_ccf::update_gains ( )
pure virtual

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.


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