GNU Radio 3.6.4.2 C++ API
gr_pfb_clock_sync_ccf Class Reference

Timing synchronizer using polyphase filterbanks. More...

#include <gr_pfb_clock_sync_ccf.h>

Inheritance diagram for gr_pfb_clock_sync_ccf:
gr_block

Public Member Functions

 ~gr_pfb_clock_sync_ccf ()
 
void update_gains ()
 update the system gains from omega and eta More...
 
void set_taps (const std::vector< float > &taps, std::vector< std::vector< float > > &ourtaps, std::vector< gr_fir_ccf * > &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. More...
 
void set_damping_factor (float df)
 Set the loop damping factor. More...
 
void set_alpha (float alpha)
 Set the loop gain alpha. More...
 
void set_beta (float beta)
 Set the loop gain beta. More...
 
void set_max_rate_deviation (float m)
 
float get_loop_bandwidth () const
 Returns the loop bandwidth. More...
 
float get_damping_factor () const
 Returns the loop damping factor. More...
 
float get_alpha () const
 Returns the loop gain alpha. More...
 
float get_beta () const
 Returns the loop gain beta. More...
 
float get_clock_rate () const
 Returns the current clock rate. More...
 
bool check_topology (int ninputs, int noutputs)
 
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...
 
- Public 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)
 
long unique_id (void) const
 
std::string name (void) const
 
virtual ~gr_block (void)
 
gr_io_signature_sptr input_signature (void) const
 
gr_io_signature_sptr output_signature (void) const
 
void set_input_signature (gr_io_signature_sptr sig)
 
void set_output_signature (gr_io_signature_sptr sig)
 
virtual void forecast (int, std::vector< int > &)
 Overload me! I am the forecast. More...
 
virtual bool start (void)
 
virtual bool stop (void)
 
void consume_each (const int how_many_items)
 Call during work to consume items. More...
 
void consume (const size_t i, const int how_many_items)
 
void produce (const size_t o, const int how_many_items)
 
uint64_t nitems_read (const size_t which_input=0)
 Get absolute count of all items consumed on the given input port. More...
 
uint64_t nitems_written (const size_t which_output=0)
 Get absolute count of all items produced on the given output port. More...
 
void add_item_tag (const size_t which_output, const gr_tag_t &tag)
 
void add_item_tag (const size_t which_output, uint64_t abs_offset, const pmt::pmt_t &key, const pmt::pmt_t &value, const pmt::pmt_t &srcid=pmt::PMT_F)
 
void get_tags_in_range (std::vector< gr_tag_t > &tags, const size_t which_input, uint64_t abs_start, uint64_t abs_end, const pmt::pmt_t &key=pmt::pmt_t())
 
void set_alignment (const size_t alignment)
 
bool is_unaligned (void)
 
size_t fixed_rate_noutput_to_ninput (const size_t noutput_items)
 
size_t interpolation (void) const
 
void set_interpolation (const size_t)
 
size_t decimation (void) const
 
void set_decimation (const size_t)
 
int max_noutput_items (void) const
 
void set_max_noutput_items (int)
 
void unset_max_noutput_items (void)
 
bool is_set_max_noutput_items (void) const
 
unsigned history (void) const
 
void set_history (unsigned history)
 
void set_fixed_rate (const bool fixed_rate)
 
bool fixed_rate (void) const
 Get the fixed rate setting. More...
 
void set_relative_rate (const double relative_rate)
 
double relative_rate (void) const
 Get the relative rate setting. More...
 
void set_output_multiple (const size_t multiple)
 
size_t output_multiple (void) const
 Get the output multiple setting. More...
 
tag_propagation_policy_t tag_propagation_policy (void)
 
void set_tag_propagation_policy (tag_propagation_policy_t p)
 
void set_max_output_buffer (long)
 
void set_max_output_buffer (int, long)
 
long max_output_buffer (size_t)
 
void set_min_output_buffer (long)
 
void set_min_output_buffer (int, long)
 
long min_output_buffer (size_t)
 
std::string symbol_name () const
 
bool alias_set ()
 
std::string alias ()
 
pmt::pmt_t alias_pmt ()
 
void set_block_alias (std::string name)
 
template<typename T >
void set_msg_handler (pmt::pmt_t which_port, T msg_handler)
 
void message_port_register_in (pmt::pmt_t)
 
void message_port_register_out (pmt::pmt_t)
 
void message_port_pub (pmt::pmt_t, pmt::pmt_t)
 
void message_port_sub (pmt::pmt_t, pmt::pmt_t)
 
void message_port_unsub (pmt::pmt_t, pmt::pmt_t)
 
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...
 
bool empty_p (pmt::pmt_t which_port)
 is the queue empty? More...
 
bool empty_p ()
 
void insert_tail (pmt::pmt_t, pmt::pmt_t)
 
pmt::pmt_t delete_head_nowait (pmt::pmt_t)
 
pmt::pmt_t delete_head_blocking (pmt::pmt_t)
 
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)
 
bool has_msg_handler (pmt::pmt_t which_port)
 Tests if there is a handler attached to port which_port. More...
 
virtual void dispatch_msg (pmt::pmt_t which_port, pmt::pmt_t msg)
 
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...
 
void work (const InputItems &, const OutputItems &)
 implements work -> calls general work More...
 
void notify_topology (const size_t, const size_t)
 notifications of new topological commits More...
 
void notify_active (void)
 start notification More...
 
void notify_inactive (void)
 stop notification More...
 
virtual void propagate_tags (const size_t, const gras::TagIter &)
 implements tag_propagation_policy() More...
 
void _update_input_reserve (void)
 
gras::BufferQueueSptr input_buffer_allocator (const size_t, const gras::SBufferConfig &)
 
gras::BufferQueueSptr output_buffer_allocator (const size_t, const gras::SBufferConfig &)
 

Friends

GR_CORE_API
gr_pfb_clock_sync_ccf_sptr 
gr_make_pfb_clock_sync_ccf (double sps, float loop_bw, 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 }
 Return options for the work call. More...
 
enum  tag_propagation_policy_t { TPP_DONT = 0, TPP_ALL_TO_ALL = 1, TPP_ONE_TO_ONE = 2 }
 
typedef std::deque< pmt::pmt_tmsg_queue_t
 
typedef std::map< pmt::pmt_t,
msg_queue_t,
pmt::pmt_comperator
msg_queue_map_t
 
typedef std::map< pmt::pmt_t,
msg_queue_t,
pmt::pmt_comperator >
::iterator 
msg_queue_map_itr
 
typedef boost::function< void(pmt::pmt_t)> msg_handler_t
 
typedef std::map< pmt::pmt_t,
msg_handler_t,
pmt::pmt_comperator
d_msg_handlers_t
 
- Public Attributes inherited from gr_block
long _unique_id
 
std::string _name
 
std::string d_symbol_alias
 
std::string d_symbol_name
 
msg_queue_map_t msg_queue
 
pmt::pmt_t message_subscribers
 
d_msg_handlers_t d_msg_handlers
 
gruel::mutex d_setlock
 
std::vector< int > d_affinity
 
gr_vector_int _work_ninput_items
 
gr_vector_int _fcast_ninput_items
 
size_t _num_outputs
 
ptrdiff_t _work_io_ptr_mask
 
size_t _output_multiple_items
 
double _relative_rate
 
bool _enable_fixed_rate
 
size_t _input_history_items
 
tag_propagation_policy_t _tag_prop_policy
 
size_t _interp
 
size_t _decim
 
gr_io_signature_sptr _in_sig
 
gr_io_signature_sptr _out_sig
 

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

gr_pfb_clock_sync_ccf::~gr_pfb_clock_sync_ccf ( )

Member Function Documentation

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

Reimplemented from gr_block.

int gr_pfb_clock_sync_ccf::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 gr_pfb_clock_sync_ccf::get_alpha ( ) const

Returns the loop gain alpha.

float gr_pfb_clock_sync_ccf::get_beta ( ) const

Returns the loop gain beta.

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

Returns the taps of the matched filter for a particular channel

float gr_pfb_clock_sync_ccf::get_clock_rate ( ) const

Returns the current clock rate.

float gr_pfb_clock_sync_ccf::get_damping_factor ( ) const

Returns the loop damping factor.

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

Returns the taps in the derivative filter for a particular channel

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

Returns all of the taps of the derivative filter

std::string gr_pfb_clock_sync_ccf::get_diff_taps_as_string ( )

Return the derivative filter taps as a formatted string for printing

float gr_pfb_clock_sync_ccf::get_loop_bandwidth ( ) const

Returns the loop bandwidth.

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

Returns all of the taps of the matched filter

std::string gr_pfb_clock_sync_ccf::get_taps_as_string ( )

Return the taps as a formatted string for printing

void gr_pfb_clock_sync_ccf::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 gr_pfb_clock_sync_ccf::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 gr_pfb_clock_sync_ccf::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 gr_pfb_clock_sync_ccf::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 gr_pfb_clock_sync_ccf::set_max_rate_deviation ( float  m)
inline

Set the maximum deviation from 0 d_rate can have

void gr_pfb_clock_sync_ccf::set_taps ( const std::vector< float > &  taps,
std::vector< std::vector< float > > &  ourtaps,
std::vector< gr_fir_ccf * > &  ourfilter 
)

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

void gr_pfb_clock_sync_ccf::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

GR_CORE_API gr_pfb_clock_sync_ccf_sptr gr_make_pfb_clock_sync_ccf ( double  sps,
float  loop_bw,
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 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).

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