Abstract:
A Viterbi decoder module includes a branch metric module configured to calculate branch metrics corresponding to a continuous phase modulated signal. Each of the branch metrics corresponds to a respective path between stages of the Viterbi decoder module. A path metric module is configured to calculate a first cost metric associated with the first state of the next stage based on the first branch metric and the second branch metric, and calculate a second cost metric associated with the second state of the next stage based on the third branch metric and the fourth branch metric. A traceback module is configured to determine a maximum likelihood path between stages of the Viterbi decoder based on the first cost metric and the second cost metric. The Viterbi decoder module is configured to output decoded data based on the maximum likelihood path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/578,180, filed on Dec. 20, 2011. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to Viterbi decoding for Bluetooth communication. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Signals transmitted and received in Gaussian frequency shift keying (GFSK) communication systems (e.g., a Bluetooth network) include a sequence of symbols corresponding to data packets. A device operating in the Bluetooth network includes a receiver (or a transceiver) that receives, for example only, basic data rate (BDR) signals, enhanced data rate (EDR) signals, and/or Bluetooth low energy (BLE) signals including symbols of a corresponding type of data packets. The receiver generally includes a maximum-likelihood decoder such as a Viterbi decoder. 
     The Viterbi decoder implements a Viterbi algorithm to decode a bitstream (e.g. of data packets) represented by the symbols. For example, a twelve state Viterbi decoder implementing a corresponding twelve state trellis may be used to decode BDR packets. Conversely, an eight state Viterbi decoder implementing an eight state trellis may be used to decode BLE packets. Each state of a corresponding one of the trellises is associated with two branch metrics and respective correlators. The branch metrics correspond to two possible next states (i.e., for respective next bits/symbols). For example, in a twelve state Viterbi decoder, the two branch metrics for each possible current state (i.e., a current bit/symbol) point to two different possible next states out of the remaining eleven states. Conversely, in an eight state Viterbi decoder, the two branch metrics of each current state point to two different possible next state out of the remaining seven states. 
     The Viterbi decoder correlates the received signal, over the duration of a symbol, to all possible signals using the corresponding trellis to generate correlation values for each branch metric. Accordingly, for a current state, a twelve state Viterbi decoder calculates correlation values for 24 (i.e., two per state) branch metrics and an eight state Viterbi decoder calculates correlation values for 16 branch metrics. Each correlation value corresponds to one of the branch metrics between the current state (a state of a current bit/symbol) and a next state (a state of a next bit/symbol). 
     Two of the branch metrics point to each subsequent state. Accordingly, each subsequent state has two branch metrics terminating at the subsequent state. Further, each subsequent state has two associated cost metrics. A first cost metric corresponds to a sum of one of the branch metrics (i.e., a first branch metric) terminating at the state and a cost metric of the state the first branch metric originated from. A second cost metric corresponds to a sum of the other of the branch metrics (i.e., a second branch metric) terminating at the state and a cost metric of the state the second branch metric originated from. The greater of the first cost metric and the second cost metric is the cost metric for the subsequent state. In other words, the cost metric having the greater magnitude corresponds to the most likely path (i.e., the path having the maximum likelihood) to the subsequent state. 
     The cost metric of each subsequent state is calculated after receiving a corresponding symbol. In other words, branch metrics pointing from a first state to a second state are associated with decoding a first received symbol, and branch metrics pointing from the second state to a third state are associated with decoding a second received symbol. After a predetermined number of symbols are received and the corresponding cost metrics for the latest state are calculated (i.e., a traceback depth is reached), the Viterbi decoder selects the latest state having the greatest cost metric, and each previous state having the greatest cost metric in a path leading to the selected state (i.e., a survivor path). The survivor path represents the decoded bits corresponding to the predetermined number of symbols. Each transition from one state to another in a twelve state Viterbi decoder involves calculating and storing correlation values for 24 branch metrics. Conversely, each transition from one state to another in an eight state Viterbi decoder involves calculating and storing correlation values for 16 branch metrics. 
     Each of BDR signaling and BLE signaling include continuous phase modulated (CPM) signaling. A transmitted CPM signal for a time t, where nT&lt;t&lt;(n+1)T, and T is a symbol duration (e.g., 1 μs for BDR and BLE signaling) can be represented as x(t)=e −j(θ     n     +I     n     Q(t−nT)+I     n−1     Q(t−(n−1)T)) , where I n  is the n th  bit transmitted, I n−1  is the (n−1) th  bit transmitted, θ n  is an accumulated phase angle prior to transmission of the n th  bit, where θ n =θ n−1 +I n−2 hπn, and h corresponds to a modulation index (e.g., π/3 for BDR signaling and π/2 for BLE signaling). An ideal signal corresponding to the transmitted signal can be represented as e j(θ     n     +I     n     Q(t)+I     n−1     Q(t+T)) . For Q(t), Q(t)=2πh∫ 0   t g(t)dt, and g(t), a Gaussian pulse shaping filter, can be represented as 
                   g   ⁡     (   t   )       =         π     α     ⁢     ⅇ     -       (       π   ⁢           ⁢   t     α     )     2             ,     
     ⁢   where     ⁢                         α   =           ln   ⁢           ⁢   2         2       ⁢     T   BT         ,         
and B corresponds to 3 dB bandwidth. For example only, for BDR and BLE signaling, 3 dB bandwidth is 500 KHz.
 
     Accordingly, for BDR and BLE signaling, each current state of the Viterbi decoder and corresponding trellis is represented by both an accumulated angle θ n  (e.g., a multiple of the modulation index π/3 or π/2) and an (n−1) th  bit I n−1 , or (θ n , I n−1 ). Each next state of the Viterbi decoder and trellis is represented by a next accumulated angle value θ n+1  and the n th  bit, or (θ n+1 , I n ). The bits in BDR and BLE signaling correspond to 1 or −1. An example twelve state trellis  100  corresponding to BDR signaling is shown in  FIG. 1A . An example eight state trellis  104  corresponding to BLE signaling is shown in  FIG. 1B . 
     SUMMARY 
     A Viterbi decoder module includes a branch metric module configured to calculate branch metrics including a first branch metric, a second branch metric, a third branch metric, and a fourth branch metric corresponding to a continuous phase modulated signal. The continuous phase modulated signal corresponds to a basic data rate (BDR) or a Bluetooth low energy (BLE) signal, and each of the branch metrics correspond to a respective path between one of a first state and a second state of a current stage of the Viterbi decoder module, and one of the first state and the second state of a next stage of the Viterbi decoder module. A path metric module is configured to calculate a first cost metric associated with the first state of the next stage based on the first branch metric and the second branch metric, and calculate a second cost metric associated with the second state of the next stage based on the third branch metric and the fourth branch metric. A traceback module is configured to determine a maximum likelihood path between stages of the Viterbi decoder based on the first cost metric and the second cost metric. The Viterbi decoder module is configured to output decoded data based on the maximum likelihood path. 
     A method of operating a Viterbi decoder includes calculating branch metrics including a first branch metric, a second branch metric, a third branch metric, and a fourth branch metric corresponding to a continuous phase modulated signal. The continuous phase modulated signal corresponds to a basic data rate (BDR) or a Bluetooth low energy (BLE) signal, and each of the branch metrics corresponds to a respective path between one of a first state and a second state of a current stage of the Viterbi decoder and one of the first state and the second state of a next stage of the Viterbi decoder. The method further includes calculating a first cost metric associated with the first state of the next stage based on the first branch metric and the second branch metric, calculating a second cost metric associated with the second state of the next stage based on the third branch metric and the fourth branch metric, determining a maximum likelihood path between stages of the Viterbi decoder based on the first cost metric and the second cost metric, and outputting decoded data based on the maximum likelihood path. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1A  illustrates a twelve state BDR trellis according to the prior art; 
         FIG. 1B  illustrates an eight state BLE trellis according to the prior art; 
         FIG. 2  is a functional block diagram of a device including a two state Viterbi decoder module according to the principles of the present disclosure. 
         FIG. 3  illustrates a two state trellis according to the principles of the present disclosure; 
         FIG. 4  is a functional block diagram of a two state Viterbi decoder module according to the principles of the present disclosure; 
         FIG. 5  illustrates a branch metric module according to the principles of the present disclosure; and 
         FIG. 6  illustrates a path metric module according to the principles of the present disclosure. 
     
    
    
     DESCRIPTION 
     A device configured to operate in a Bluetooth network includes a transmitter and a receiver (or, a transceiver) for transmitting and receiving Bluetooth signals. For Bluetooth communication including, for example only, basic data rate (BDR) signaling and/or Bluetooth low energy (BLE) signaling, the receiver may include a Viterbi decoder. Further, BDR and BLE signaling correspond to a relatively high number of Viterbi decoder (and trellis) states. For example, BRD and BLE signaling require twelve and eight states, respectively. In a Viterbi decoder, a greater number of states and associated branch metrics corresponds to increased computational complexity and cost, which further corresponds to, for example, increased hardware and increased power consumption. 
     In the Viterbi architecture according to the principles of the present disclosure, a Viterbi decoder reduces the number of states required for decoding, for example, BDR signals and BLE signals. For example only, the Viterbi decoder according to the present disclosure is configurable to decode both BDR and BLE signals using two states instead of twelve and eight, respectively. 
     In  FIG. 2 , a device  200  is configured to transmit and receive Bluetooth signals including, but not limited to BDR signals and BLE signals. The device  200  includes a receiver  204 . The receiver  204  receives a signal  208 , which may include a BDR signal and/or a BLE signal. The signal  208  is downconverted by a downconverter  212  and provided to a filter  216  (e.g., a low pass filter). The filter  216  provides a filtered signal r(t) to a differential detector  220 , which communicates with a slicer  224 . The filter  216  also provides the signal r(t) to a Viterbi decoder module  228 . 
     For example only, the signal r(t) is complex and is provided to the Viterbi decoder module  228  at 8 Msps. The Viterbi decoder module  228  has two states, each having two associated branch metrics, for a total of four branch metrics. The Viterbi decoder module  228  calculates a corresponding ideal signal for each of the branch metrics and stores the ideal signals for a predetermined period (e.g., 1 μs). The ideal signal for correlation of each of the branch metrics may be represented as e j(I     n     Q(t)+I     n−1     Q(t+T)) , where Q(t)=2πh∫ 0   t g(t)dt, and g(t) corresponds to the Gaussian pulse shaping filter for BDR signaling and BLE signaling. In other words, instead of an ideal signal represented as e j(θ     n     +I     n     Q(t)+I   n−1   Q(t+T))  as described for the twelve state BDR Viterbi decoder/trellis and the eight state BLE Viterbi decoder/trellis, the ideal signal used for correlation in the two state Viterbi decoder module according to the principles of the present disclosure is represented as e j(I     n     Q(t)+I     n+1     Q(t+T)) . 
     In  FIG. 3 , a two state trellis  300  corresponding to the Viterbi decoder module  228  includes branches (i.e., branch metrics) b 0 , b 1 , b 2 , and b 3 . A current stage of the trellis  300  include a first state I n−1 (prev_bit)=1 (i.e., a previous bit was decoded as a 1) and a second state I n−1 (prev_bit)=−1 (i.e., a previous bit was decoded as a −1). A next stage of the trellis  300  include a first state I n (curr_bit)=1 (i.e., a current bit is a 1) and a second state I n (curr_bit)=−1 (i.e., a current bit is a −1). Assuming eight samples per symbol (i.e., bit) and a sampling instant idx, an array of the ideal signals can be represented as corr_sig[branch_no][idx], where 0≦idx&lt;8, and “branch_no” can be 1, 2, 3, or 4, corresponding to branch b 0 , b 1 , b 2 , or b 3 , respectively. 
     In  FIG. 4 , an example two state Viterbi decoder module  400  includes a branch metric module  404 , a path metric module  408 , a traceback module  412 . The branch metric module  404  calculates the branch metrics (i.e., four branch metrics, two per state, for each stage of the Viterbi decoder module  400 ) for the signal r(t). The path metric module  408  calculates the best, or optimal, survivor paths for each state, which corresponds to the paths having the highest cost metric into each respective state. The traceback module  412  restores a maximum-likelihood path based on the survivor paths calculated by the path metric module  408 . 
     In  FIG. 5 , an example branch metric module  500  includes branch metric paths  504 - 1 ,  504 - 2 ,  504 - 3 , and  504 - 4 , referred to collectively as branch metric paths  504 , for each of the respective four branch metrics. Each of the paths  504  receives a sample of the signal r(t) at idx. For example only, the received signal is sampled eight times over the duration of 1 μs, or at 8 MHz, for eight samples of r(t). The samples are correlated to the corresponding ideal signal, which also has a 1 μs duration, by multiplying each sample by the ideal signal at multiplier  508  and summing the result with an accumulated sum of previous multiplications. For example, the result of the multiplication is summed with the output of delay element  512  at summer  516  and provided to a branch metric accumulator  520 . The branch metric accumulator  520  accumulates each of the samples (e.g., eight samples) over the sample period and outputs a final branch metric  524 . For example only, the branch metric for a branch b corresponds to 
               ∑     idx   =   0     7     ⁢           ⁢       r   ⁡     [   idx   ]       *           
corr_sig[b][idx].
 
     Each of the states of the trellis  300  shown in  FIG. 3  can be reached via two paths. For example, the first state I n (curr_bit)=1 can be reached via the branch b 0  or the branch b 1 . Accordingly, the cost metrics of the first state depend on the branch metrics calculated for each of the branches b 0  and b 1 . Conversely, the second state I n (curr_bit)=−1 can be reach via the branch b 2  or the branch b 3 . Accordingly, the cost metrics of the second state depend on the branch metrics calculated for each of the branches b 2  and b 3 . 
     In  FIG. 6 , an example path metric module  600  calculates and stores two cost metrics for each state of the trellis  300 . The two cost metrics include an old cost metric old_c[state_no] for the state and a new cost metric new_c[state_no] for the state, where “state_no” is a 1 or a 2 to identify the state. For example only, the cost metric values of old_c[1], new_c[1], old_c[2], and new_c[2] are stored in respective registers  604 ,  608 ,  612 , and  616 . The registers  604 ,  608 ,  612 , and  616  may be updated at 1 MHz. 
     For each new cost metric corresponding to a state, the two possible corresponding branch metrics terminating at the state are summed with respective old cost metric values, multiplied by a programmable multiplier M, and compared to one another to determine the new cost metric for the state. For example, for state 1, the branch metric b 0  is summed with the old cost metric for state 2 at summer  620 , multiplied by the programmable multiplier M at multiplier  624 , and provided to multiplexer  628 . The output of the summer  620  may be stored in a temporary register  632 . Conversely, the branch metric b 1  is summed with the old cost metric for state 1 at summer  636 , multiplied by M* (the conjugate of the programmable multiplier M) at multiplier  640 , and provided to the multiplexer  628 . The output of the summer  620  may be stored in a temporary register  644 . 
     The values stored in the registers  632  and  644  are also provided to a comparator  648 , which selects an output of the multiplexer  628 . For example, if the value stored in the register  632  is greater, then the multiplexer  628  outputs the value stored in the register  632 . Conversely, if the value stored in the register  644  is greater, then the multiplexer  628  outputs the value stored in the register  644 . Accordingly, the multiplexer  628  outputs a new cost metric corresponding to the greater of the cost metrics associated with the branch metric b 0  and the branch metric b 1 , which is stored as the new cost metric new_c[1] in the register  616 . 
     Conversely, for state 2, the branch metric b 2  is summed with the old cost metric for state 2 at summer  652 , multiplied by the programmable multiplier M at multiplier  656 , and provided to multiplexer  660 . The output of the summer  652  may be stored in a temporary register  664 . Conversely, the branch metric b 3  is summed with the old cost metric for state 1 at summer  668 , multiplied by M* at multiplier  672 , and provided to the multiplexer  660 . The output of the summer  668  may be stored in a temporary register  676 . 
     The values stored in the registers  664  and  676  are also provided to a comparator  680 , which selects an output of the multiplexer  660 . For example, if the value stored in the register  664  is greater, then the multiplexer  660  outputs the value stored in the register  664 . Conversely, if the value stored in the register  676  is greater, then the multiplexer  660  outputs the value stored in the register  676 . Accordingly, the multiplexer  660  outputs a new cost metric corresponding to the greater of the cost metrics associated with the branch metric b 2  and the branch metric b 3 , which is stored as the new cost metric new_c[2] in the register  608 . For subsequent states, the values stored in the registers  608  and  616  are copied to the registers  604  and  612 . In other words, the current new cost metrics become the old cost metrics for the subsequent states. 
     The programmable multiplier M (M*, the conjugate of the programmable multiplier M) is based on the modulation index of the received signal  208 . In particular, the programmable multiplier M is adjusted based on whether the signal corresponds to BDR signaling or BLE signaling. Accordingly, the same Viterbi decoder module  400  and corresponding branch metric module  500  and path metric module  600  can be used to decode both BDR signals and BLE signals by adjusting the programmable multiplier. For example, for BDR signaling, 
             M   =       ⅇ     (     j   *     π   3       )       .           
Conversely, for BLE signaling,
 
             ,     M   =       ⅇ     (     j   *     π   2       )       .             
For example only, the Viterbi decoder module  400  or another component of the device  200  may determine whether BDR or BLE signaling is being used and adjust the programmable multiplier M (and M*) accordingly.
 
     In one implementation, Viterbi decoding is performed on a data portion of BDR and BLE packets, and a preamble, access code, tail, and header portion of the BDR packets are decoded using, for example only, the differential detector  220  as shown in  FIG. 2 . Conversely, for EDR packets, the entire packet may be decoded using the differential detector  220 . For example only, the header portion of a packet indicates whether the packet is a BDR packet, a BLE packet, or an EDR packet. After decoding the header portion, the receiver  204  determines whether to switch to Viterbi decoding based on the packet type. If the header portion indicates that the packet is a BDR or BLE packet, then the receiver  204  switches to Viterbi decoding, and adjusts the programmable multiplier M accordingly. 
     The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. 
     As used herein, the term module may refer to, be part of, or include an Application Specific Integrated Circuit (ASIC); a discrete circuit; an integrated circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor (shared, dedicated, or group) that executes code; other suitable hardware components that provide the described functionality; or a combination of some or all of the above, such as in a system-on-chip. The term module may include memory (shared, dedicated, or group) that stores code executed by the processor. 
     The term code, as used above, may include software, firmware, and/or microcode, and may refer to programs, routines, functions, classes, and/or objects. The term shared, as used above, means that some or all code from multiple modules may be executed using a single (shared) processor. In addition, some or all code from multiple modules may be stored by a single (shared) memory. The term group, as used above, means that some or all code from a single module may be executed using a group of processors. In addition, some or all code from a single module may be stored using a group of memories. 
     The apparatuses and methods described herein may be partially or fully implemented by one or more computer programs executed by one or more processors. The computer programs include processor-executable instructions that are stored on at least one non-transitory tangible computer readable medium. The computer programs may also include and/or rely on stored data. Non-limiting examples of the non-transitory tangible computer readable medium include nonvolatile memory, volatile memory, magnetic storage, and optical storage.