# Patent

## Title
Device and frame structure for powerline communications

## Abstract

Embodiments of the invention provide a device and a frame structure for powerline communications. The header may comprise two parts that are separately encoded. A common header segment is encoded alone, and an embedded header segment is encoded with payload data.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of and claims benefit to of U.S. patent application Ser. No. 13/101,938, filed May 5, 2011, which in turn claims the benefit of the filing dates of U.S. Provisional Application No. 61/333,601, which is titled “Header and Method for ITU G-HNEM” and was filed May 11, 2010; U.S. Provisional Application No. 61/363,003, which is titled “Header and Method for ITU G-HNEM” and was filed Jul. 9, 2010; U.S. Provisional Application No. 61/443,326, which is titled “Header and Method for ITU G-HNEM” and was filed Feb. 16, 2011; U.S. Provisional Application No. 61/333,614, which is titled “Interleaver Design and Method for ITU G.HNEM” and was filed May 11, 2010; U.S. Provisional Application No. 61/360,559, which is titled “Interleaver Design and Method for ITU G.HNEM” and was filed Jul. 1, 2010; U.S. Provisional Application No. 61/372,646, which is titled “Interleaver Design and Method for ITU G.HNEM” and was filed Aug. 11, 2010; U.S. Provisional Application No. 61/383,960, which is titled “Interleaver Design and Method for ITU G.HNEM” and was filed Sep. 17, 2010; U.S. Provisional Application No. 61/388,863, which is titled “Interleaver Design and Method for ITU G.HNEM” and was filed Oct. 1, 2010; U.S. Provisional Application No. 61/391,407, which is titled “Interleaver Design and Method for ITU G.HNEM” and was filed Oct. 8, 2010; U.S. Provisional Application No. 61/392,677, which is titled “Interleaver Design and Method for ITU G.HNEM” and was filed Oct. 13, 2010; U.S. Provisional Application No. 61/416,809, which is titled “Interleaver Design and Method for ITU G.HNEM” and was filed Nov. 24, 2010; and U.S. Provisional Application No. 61/418,955, which is titled “Interleaver Design and Method for ITU G.HNEM” and was filed Dec. 2, 2010, the disclosures of which are hereby incorporated by reference herein in their entirety.


TECHNICAL FIELD
Embodiments are directed, in general, to power line communication systems and, more specifically, to an interleaver and a header structure for power line communications.
BACKGROUND
The International Telecommunication Union (ITU) Telecommunication Standardization Bureau is developing new standards—identified as G.hnem—to enable cost-effective smart grid applications such as distribution automation, smart meters, smart appliances and advanced recharging systems for electric vehicles. The G.hnem standards link electrical grids and communications networks, enabling utilities to exercise a higher level of monitoring and to support power lines as a communications medium. The G.hnem standard supports Ethernet, IPv4 and IPv6 protocols, and G.hnem-based networks can be integrated with IP-based networks. The G.hnem standards define the physical layer and the data link layer for narrowband OFDM power line communications over alternating current and direct current electric power lines at frequencies below 500 kHz.
The format of the PHY frame and the interleaver that will be used in the G.hnem standards are being considered by the ITU.
SUMMARY OF THE INVENTION
In one embodiment, a transmitter comprises a scrambler circuit for receiving PHY frames for transmission. The PHY frames comprise a common header segment and a data segment. The data segment further comprises an embedded header segment and a payload segment. A header encoder circuit is coupled to the scrambling circuit and provides forward error correction encoding to the common header segment of the PHY frames. A payload encoder circuit is coupled to the scrambling circuit and provides forward error correction encoding to the data segment including the embedded header segment.
The header encoder circuit may encode the common header segment using a more robust modulation and coding scheme than used by the payload encoder circuit to encode the data segment including the embedded header. The header encoder circuit may encode the common header segment using a maximum available Reed-Solomon codeword size. The common header segment may include data identifying the modulation and coding scheme used to encode the data segment and/or identifying the length of the payload segment. The embedded header segment and the payload portion may be encoded using the same modulation and coding scheme. The embedded header segment may include a separate cyclic redundancy check.
In another embodiment, a transmitter for a powerline communications network comprises a forward error correction (FEC) encoder receiving payload data and appending parity-check data to the payload data to create a FEC bit stream output. An aggregation and fragmentation module receives the FEC bit stream and segments the FEC bit stream into a plurality of blocks. An interleaving module interleaves the data in the plurality of blocks.
A fragment repetition encoder is coupled to the aggregation and fragmentation module and receives the plurality of blocks. The fragment repetition encoder copies the plurality of blocks a selected number of times and provides the plurality of blocks and copies of the plurality of blocks to the interleaving module. The interleaving module may interleave each block and each copy of the blocks separately, or may interleave each block and all copies of the block together.
The fragment repetition encoder may combine each block and copies of each block in a buffer and pad the buffer with additional bits so that the buffer is equivalent to an integer number of symbols for transmission. The additional bits may be cyclically repeated bits from the buffer.
The length of each block may be selected based upon the number of modulated bits transmitted in one half-cycle of the power carrier on the powerline network, or the total number of input bits in a FEC codeword block, or the total number of bits loaded on symbols that span at least 10 ms for a 50 Hz AC powerline communications network, and at least 8.33 ms for a 60 Hz AC powerline communications network or for a network with no AC signal, or a maximum fragment size of 3072 bits.
A transmitter in one embodiment comprises a header generation circuit that provides a set of common header bits and a set of embedded header bits. A header encoder circuit includes a forward error correction encoder. The header encoder circuit provides a common header segment using the common header bits. A payload encoder circuit includes a forward error correction encoder. The payload encoder circuit provides a data segment that includes an embedded header segment generated from the set of embedded header bits. The data segment may further include a set of embedded payload bits.



BRIEF DESCRIPTION OF THE DRAWINGS
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, wherein:
FIG. 1 illustrates a G.hnem PHY frame;
FIG. 2 illustrates where padding is used in the signal chain;
FIG. 3 illustrates a functional model of a physical medium attachment sublayer according to one embodiment;
FIG. 4 is a block diagram of a system in accordance with one embodiment;
FIG. 5 illustrates synchronizing a cyclic prefix with short impulsive noise bursts;
FIG. 6 illustrates time-domain link adaptation;
FIG. 7 illustrates an interleaver structure according to one embodiment;
FIG. 8 illustrates a functional model of a payload encoder according to one embodiment;
FIG. 9 illustrates the steps performed in the generation of an encoded payload block;
FIG. 10 illustrates the interleaving process according to one embodiment;
FIG. 11 illustrates the order of writing and reading into a permutation matrix according to one embodiment; and
FIG. 12 is an example of row-column permutation that displays the spreading behavior of the interleaver.



DETAILED DESCRIPTION
The invention now will be described more fully hereinafter with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. One skilled in the art may be able to use the various embodiments of the invention.
FIG. 1 illustrates a G.hnem PHY frame 100, which contains a preamble segment 101, a header segment 102 and a segment payload 103. The preamble 101 enables packet detection. The header 102 enables the receiver to determine information needed for packet decoding. The header 102 can be into two parts: a common header 104 that is transmitted with a fixed modulation and coding scheme, and an embedded header 105 that is transmitted with the same modulation and coding scheme as the payload data 103. The embedded header 105 may have a variable length depending upon the frame type. The embedded header 105 and the payload 103 form a data segment 106 in the PHY frame.
Proposals for the format and contents of the PHY frame header (PFH) to be used in the ITU-T G.hnem standard are under consideration. For example, Tables 1 and 2 list the header fields in two proposals. The proposals include some initial values for the contents of the G.hnem header and the number of symbols. Of particular concern is the fact that even with optimistic assumptions about the header transmission rate (spectral efficiency of 0.5), a large number of header symbols are needed for the Cenelec bands, resulting in high overhead. The large overhead is primarily because of the header size. The common portion of the header field totals 42 bits—with additional bits possible for different frame types. Since the header needs to be transmitted with the most robust modulation and coding scheme (MCS), it is desirable to minimize the number of bits transmitted in the header to limit the overhead.










TABLE 1






NUMBER 




FIELD
OF BITS
DESCRIPTION


















FT
4
Frame type
Common part


DOD
4
Domain ID (identifier)



SID
8
The DEVICE_ID of the





source node



DID
8
The DEVICE_ID,





MULTICAST_ID or





BROADCAST_ID of the





destination node(s)



MI
1
Multicast indication identifies





whether the DID is a unicast





or multicast destination



DRI
1
Duration indication -





indicates whether the frame





duration field is present



FTSF
TBD
Frame-type specific field
Variable part


HCS
16
Header check sequence (16
Common part




bits)


















TABLE 2






NUMBER 



FIELD
OF BITS
DESCRIPTION

















MPDU Length
7
Indicates the length of the frame payload




expressed in bytes at different scales depending on




the length of the MAC protocol data unit (MPDU).


Transmission
14 + (variable)
Indicates the set of parameters used to transmit the


Parameters

payload.


(CM + TM + RS




code word size)




Ack Req
1
Indicates whether the receiver should respond with




an acknowledgement to indicate MPDU reception




status


Op Code
4
Indicates the encoding method of the remaining




bits reserved by ITU in the PFH.


Reserved bits for
variable
Reserved bits for future use by ITU.


upper layer use


















TABLE 3






NUMBER 



FIELD
OF BITS
DESCRIPTION


















FT
2
Frame Type
Common fields


HCS
12
Header Check





Sequence











Sub-total:




 14











TM
9 - Cenelec
Tone Map
Variable fields



40 - FCC




FL
6
Frame Length



TX profile
6




ACK
3
Acknowledgement



FEC size
8
Forward Error





Correction size











Sub-total:




 32 - Cenelec




 63 - FCC



Reserved
0 - Cenelec
Variable field



3 - FCC




Total :




 46 - Cenelec




 80 - FCC









Embodiments disclosed herein provide methods for more efficient header transmission, which includes both optimizing the bits contained in the header, and the modulation scheme for the header.
Header Organization
To minimize the header overhead, the header is split in two parts: a common header portion (104) and a payload-embedded portion (105). The common part 104 is transmitted with the most robust modulation and coding scheme (MCS) and the embedded portion (105) is transmitted with the same MCS as the payload data. The common part 104 signals the MCS used for the payload. The common part 104 also signals the length of the payload portion. The embedded portion 105 may be embedded with separate cyclic redundancy check (CRC) in the payload. The embedded portion also signals the source ID and domain ID.
The contents of the common header 104 portion of header 102 are shown in Table 4for one embodiment. Note that it only contains information necessary for the common header to receive the data portion: namely the modulation & coding scheme, the bit loading map, and the length of the data.









TABLE 4






NUMBER 



FIELD
OF BITS
DESCRIPTION







MCS
4
Modulation & coding scheme for data


Packet Type
0-3
Indicates packet type


LEN
6-8
Length of the payload in symbols


LEN_EMBHDR
0-3
Length of the embedded 




header (if necessary)


BITLOAD
0-6
Bit loading indicator


HCS_1
K
First K bits of 16-bit header checksum, 




XOR-ed with K-bit destination 




ID or broadcast group ID




(Only an intended device 




passes this portion of the CRC)


HCS_2
16-K
Last K bits of 16-bit header checksum




(All devices can check 




this portion of the CRC)









The following points may be considered in certain embodiments.
The common header (104) may contain an optional length of the embedded header, if there is any variability in the embedded header length. Note that this optional length could be replaced by a packet type, if there is a unique mapping between the packet type and the embedded header length.
The destination ID (or equivalent broadcast group ID) is sent implicitly by masking the first K bits of the CRC. Only the intended receivers pass this portion of the CRC. Other receivers may check the remaining portion of the CRC to ensure that they have received the length parameter correctly, and could use this for Carrier Sense Multiple Access (CSMA) or to save power by going to sleep.
The embedded portion (105) of the header 102 contains other fields, such as those described in Tables 1-4 above, along with a MAC header, if any. A separate CRC may be used for the embedded portion (105) to enable receivers to know if they have received the “control” information correctly, even if the actual data is not received correctly.
In another embodiment, the embedded header comprises: a 16-bit source identifier (Source ID); a 16-bit destination identifier (Destination ID); data identifying the number of bits in the payload, which may exclude padding bits; other segmentation information; a 16-bit CRC; and ACK/NAK control. If the embedded header is coded separately from the payload data, then it may be possible to also add optional power control information in the embedded header to further improve the performance of data decoding.
Header Fields
In addition to the organization of the header as discussed above, the choice of the header fields for various embodiments is discussed below.
MCS: In one embodiment, the total possible combinations of modulation and coding scheme (including repetitions) are restricted to twelve or less for all subbands. This leaves room for four reserved values, with sixteen combinations of modulation and coding scheme.
LEN: The length field signals the number of OFDM symbols used. It will be understood that the actual signaled value may be a multiple of a basic quantum. In addition, the embedded portion of the header may signal the number of padding bits for the payload.
LEN_EMBHDR: The length of the embedded header may be restricted to a few combinations. In one embodiment, the value is limited to four combinations.
BITLOAD: The bit-loading parameter signals the modulation scheme used on all tones. In one embodiment, the available bandwidth may be divided into subbands. The numbers of tones in the Cenelec A, B, C and D bands are 36, 20, 6 and 6, respectively, in one embodiment. On the other hand, for the FCC bands, the numbers of tones may be, for example, 143, 34, or 106. Considering the wide disparity in the number of tones, a different number of BITLOAD bits may be used for different bands. Table 5 summarizes the suggested subband size and the number of bits used for signaling the bit-loading for each band option.












TABLE 5









SUBBAND SIZE
NO. OF 




CARRIER 
(NO. TONES/BW 
BITS IN



BAND
SPACING
IN KHZ)
BITLOAD





















Cenelec-A
24.4140625/16
4/6.1035 kHz
3



Cenelec-B


2



Cenelec-C


2



Cenelec-D


2



FCC/ARIB
24.4140625/8 
4/24.414 kHz
4-5










Length Indication

The main uncertainty in the length comes from padding in the signal chain. This is done in several places, as shown in FIG. 2, which is illustrative of padding used in the signal chain. Padding 0 (201) represents padding of the MPDU that may occur in the data link layer (DLL) before encoding. After passing through a Reed-Solomon (RS) encoder and convolutional encoder, which provide FEC, padding 1 (202) may occur to ensure an integer number of fragments prior to interleaving. Padding 2 (203) may occur during interleaving, repetition and concatenation. Finally, padding 3 (204) ensures that a valid frame length is processed.
From the frame duration, the receiver only knows the total number of bits at the end of the processing chain. It is necessary to define some method to ensure the receiver can obtain the other parameters. Two methods have been proposed to handle this.
In G3, padding 2 and 3 are not performed. The specification does not explicitly state how padding 0 and 1 are performed, but in one embodiment padding 0 (201) is done to ensure that the number of bits in padding 3 (204) is minimized, given the total number of bits to be transmitted. With this assumption, the receiver can uniquely process the received bits up till the Reed-Solomon decoder. After that point, DLL signaling gives the number of bytes added in the padding 0 (201) stage.
This method would work adequately for G.hnem, except for the Interleave-over-AC-cycle (IoAC) mode in which case the padding 2 (203) could add an unpredictable number of bits. To combat this, the following method has been suggested.
Restrict the payload size (i.e. the number of bytes at the input to the RS encoder) to a pre-determined set, and signal the index within this set. In this method, padding 0 (201) is performed to achieve a target size at the RS encoder input, which is then signaled in the PFH. This allows the receiver to do the RS decoding, after which DLL signaling gives the number of bytes added in the padding 0 block, just as in G3. This method has the disadvantage of restricting the RS input size, and increasing the signaling required in PFH.
An alternative method described below builds on both the above methods.
High Level Description of Suggested Signaling Method
In this method, the main idea is to do most of the padding in the DLL (padding 0, 201) as is done in G3. Padding 0 (201) adds an integer number of bytes before segmentation into LCDUs. The number of bytes padded is indicated in DLL signaling. LCDU segmentation ensures roughly equal Reed Solomon codeword lengths, subject to a constraint on the maximum LCDU length. By controlling the maximum LCDU length, the transmitter can adapt to channel conditions. Given the maximum LCDU length Kmax, the actual codeword length is some number between Kmax/2 and Kmax depending on the input MPDU length.
Padding 1 (202) adds very few bits—fewer than can be generated by 1 byte at the convolutional encoder input. Thus, the maximum length of padding 1 (202) is 15 bits (corresponding to rate-1/2 code). Since this rule is known a priori, the number of padding 1 (202) bits is not signaled, but is inferred by the receiver. The values in padding 0 (201) and padding 1 (202) ensure that an integer number of interleaver fragments are produced, while adding the fewest number of pad bits.
Padding 2 (203) is only done for IoAC mode. Note that for each interleaver fragment, padding 2 (203) always adds the same number of OFDM symbols, in order to ensure that each interleaver fragment spans roughly an integer number of OFDM symbols. Note further the number of OFDM symbols added for each fragment is at most equal 14 for Cenelec bands and 28 for FCC band. Consequently, padding 2 (203) can be signaled with 4 bits for Cenelec band and 5 bits for FCC band.
Padding 3 (204) is only done to ensure a valid number of OFDM symbols. Assuming this is a multiple of four, then two bits are needed to signal it. Note that padding 3 (204) should not be used in IoAC mode because the aim is to spread each frame over an integer number of zero crossings.
Table 6 lists fields used in one embodiment.









TABLE 6





FIELD
NUMBER OF BITS
NOTE







FT
1
Common 


HCS
12
fields


BAT type
1, indicates run-time Bit Allocation Table (BAT)
Variable 



or not
fields


TM
9 - Cenelec




40 - FCC




If run-time BAT is not used, each bit indicates




whether or not a group is used. If run-time BAT




is used, these fields indicate BAT id



FL
9 - Cenelec




10 - FCC




In IoF mode, gives the number of symbols in the




payload. In IoAC mode, gives the number of




zero crossings, and the number of pad symbols




used for each fragment during interleaving



Coding &
5 (also indicates interleaver type)



modulation




ACK
1



Maximum RS
2



codeword




(LCDU) length




Reserved
2 - Cenelec




8 - FCC



Total
42 - Cenelec




80 - FCC









Operation at the Transmitter

DLL Padding

The transmitter receives an MPDU size of NMPDU-IN bytes and decides on a maximum LCDU codeword size Kmax, which may be one 4 values in one embodiment (such as 239, 136, 88, and 42, for example). The transmitter goes through the following calculations.
First-Cut MPDU Division into LPDUs, and Corresponding Number of Coded Bits: The number of codewords (LCDUs) is NCWnom=ceil(NMPDU-IN/Kmax). If each RS codeword adds RCW bytes of parity (it is the same for all codewords, since their sizes are approximate the same), then the total number of bytes at the Reed Solomon output is NRSOUTnom=NMPDU-IN +NCWnom*RCW bytes. This produces NFECB=(NRSOUT*8+6)/rI bits at the convolutional encoder output.
Adjust Number of Reed Solomon Output Bytes: The number of pad bits (BP) is calculated, namely as BP=B0×Nfrg−NFECB. These are the number of pad bits that would have been needed, if there were no padding in the DLL. Instead, DLL padding is done so that 0≦BP<8/rI. More precisely, the desired RS output size is NRSOUTadj=floor((B0×Nfrg*rI−6)/8). Note that with this number of RS bytes, BP is always less than 8/rI as required.
Adjust Number of Codewords, MPDU Size: The actual number of codewords is adjusted to NCW=ceil(NRSOUTadj/(Kmax+RCW)). The adjusted MPDU size is NMPDUadj=NRSOUTadj−NCW*RCW. This is the number of input bytes at the Reed Solomon codeword in order to get an output of NRSOUTadj bytes.
DLL Padding: The number of DLL padding bytes is NPAD-DLL=NMPDUadj−NMPDU-IN bytes. These padding bytes are added at the DLL. They could contain all zeros, or maybe cyclic repeats.
Division into Codewords: The padding input is divided into NCW LCDUs of size KCW1=ceil(NMPDUadj/NCW) each, except the last (NCW*KCW1−NMPDUadj) codewords which have size KCW1−1 bytes.
At the end of the above process, the input has been padded to ensure the number of padding 1 (202) bits is always less than 8/rI, and the padded MPDU is divided into roughly equal sized LCDUs under the constraint of the maximum codeword length Kmax.
PHY Padding
The transmitter performs the padding as mentioned above, divides the padded MPDU into LCDUs, and performs forward error correction (FEC). The transmitter than calculates BP padding bits, and does interleaver fragmentation, etc. In Interleave-over-fragment (IoF) mode, no further padding is needed.
In IoAC mode, one further stage of padding is needed. With R repetitions, only B0*R bits are generated by the interleaver. The output, on the other hand, contains ceil(B0*R/NZC)*NZC bits. Thus, a difference of Pad2=ceil(B0*R/NZC)*NZC−B0*R bits, or equivalently Pad3/KP OFDM symbols is added. This number of padded OFDM symbols may be signaled, given by
Npad2-IoAc=(ceil(B0*R/NZC)*NZC−B0*R)/kP in the FL field for the IoAC mode.
In addition, to the above padding 3 (204) may be done to get to a desired frame length. If this is done, the value should be signaled.
Summary of Signaling
DLL: Number of pad bytes given by NPAD-DLL=NMPDUadj NMPDU-IN bytes.
PHY:

    
    
        1. Maximum codeword size: Kmax. Suggested length=2 bits
        2. FL
        
            a. IoF mode: Number of symbols in frame: 9 bits for Cenelec, 10 bits for FCC band.
            b. IoAC mode:
            
                i. Number of zero crossing cycles: 5 bits for Cenelec, 5 bits for FCC band.
                ii. Number of symbols in Pad2: NPad2-IoAC=(ceil(B0*R/\NZC)*NZC−B0*R)/kP. 4 bits for Cenelec, 5 bits for FCC band.

Receiver Operation

            
            
        
        
    
    


IoF Mode: Btot=kP*FL/R is the total number of bits before repetitions. B0 is the maximum divisor of Btot that is less than or equal to min(Btot, NZC, Bmax).
IoAC Mode: In IoAC mode, the effective frame length is the total frame length (some multiple of the number of symbols corresponding to NZC bits) minus the number of symbols added in pad 2. (Note that the latter is signaled). With this number, the receiver again calculates:

Btot=kP*FLeff/R as the total number of bits before repetition and padding by NPad2 bits.

B0 is the maximum divisor of Btot that is less than or equal to min(Btot, NZC, Bmax).
Once B0 is known, deinterleaving is done. The number of pad bits in pad 1 stage is estimated as BP=mod(Btot, 8/rI). These pad bits are removed before decoding. The DLL contains the number of pad bytes added in stage 0.
Run-Time BAT Signaling
While run-time BATs (bit allocation tables) are supported in the ITU specification, there is currently no way for the receiver to determine which run-time BAT was used. In order to know this, the receiver has to know at least (i) the transmitter ID, and (ii) the BAT index.
In one embodiment, a flag may be used to indicate whether the BAT is a pre-determined BAT or a runtime BAT. If the pre-determined BAT is used, the tone map field indicates which tone groups are active, as proposed currently. If the runtime BAT is used, the tone map field carries the transmitter (16-bit) ID, the domain (16-bit short) ID and the BAT index. Since these require at least 36 bits, this is done only for FCC band.
For Cenelec band, run-time BAT can only be used with constraints (if at all) since the TM field is not large enough to hold the transmitter and domain ids. One possible constraint is to restrict run-time BATs in Cenelec band so that for a given receiver, the same BAT table is used for all transmitters. Thus, just from the BAT index, the receiver can determine the run-time BAT.
Physical Medium Attachment Sub-Layer (PMA)
FIG. 3 illustrates a functional model of the PMA according to one embodiment. In the transmit direction, an incoming PHY frame (301) comprises a header (PFH) and payload, such as the PHY frame illustrated in FIG. 1. The PFH may comprise a common part of the header, while the payload may include a variable or embedded portion of the header. Both the PFH bits and the payload bits of the incoming frame are scrambled in scrambler 302. The PFH bits of the incoming frame are encoded in PFH encoder 303, which includes a FEC encoder, repetition encoder and channel interleaver. The payload bits are separately encoded in payload encoder 304, which also includes a FEC encoder, repetition encoder and channel interleaver. The PFH and payload encoders 303, 304 may be the signal chain illustrated in FIG. 2, in one embodiment. The parameters of payload encoder 304 are controlled by the PHY management entity 307, which provides PMA MGMT primitives. The parameters of the PFH encoder are predefined for each particular bandplan to facilitate interoperability. After encoding, the PFH and payload are each mapped 305 into an integer number of symbol frames. The obtained symbol frames (306) of the PFH and the payload are submitted to a physical medium dependent sub-layer for modulation and transmission over the medium.
In the receive direction, all necessary inverse operations of decoding, and de-scrambling (309) are performed on the received symbol frames (308). The recovered PFH and payload are combined in received PHY frames (310) are further processing in a physical coding sub-layer.
Based on simulations in the Cenelec-A band, it has been agreed to use a bit interleaver spanning a half mains cycle. The bit interleaver is positioned between the FEC and the complex modulation. In extending to the FCC band, concerns were raised regarding the memory required by an interleaver with the same time span. On the other hand, for the Cenelec B band, it was pointed out that the interleaver may span too few bits for the lowest spectral efficiency.
Different interference scenarios may be used for evaluation of the different schemes considered for ITU G.hnem. The noise models to be considered for the simulations include white noise only, white noise+time domain periodic noise, and white noise+time domain periodic noise+frequency domain narrowband interference. Two key factors of an interleaver design are (1) the size of the interleaver, implying the time period that the interleaver spans, and (2) the complexity of implementing the de-interleaver. An interleaver with a longer time span is expected to spread out impulsive noises. However, an interleaver spanning a longer period in time, implies that the receiver has to do the de-interleaving operation fast enough to meet any latency requirements for acknowledgement of the reception of the packet. Thus, the interleaver length needs to be carefully chosen to achieve a tradeoff between performance and complexity. In one embodiment, the interleaver length is chosen to span the period between the zero crossings of the AC mains.
Two scenarios are considered, one with a longer interleaver and another with a shorter interleaver whose interleaver length is related to the periodicity of the time domain periodic noise. The simulation parameters are given in Table 7.








TABLE 7





SIMULATION PARAMETER
VALUE







Modulation
DBPSK


Bandwidth occupied
40-90 kHz


Impulse noise period
Various combinations:



11.12 ms,



 8.33 ms,



  10 ms


Impulse noise length
   2 ms


Interleaver size
Short & full-packet


Coding
Rate 1⁄2, K = 7 Convolutional code



+ Reed Solomon









FIG. 4 is a block diagram of a system in accordance with one embodiment. Forward error correction is provided by Reed Solomon coding 401 and convolutional coding 402. Channel interleaver 403 is 11.12 ms long in one scenario, and 77.84 ms long in another scenario. The output of channel interleaver is provided to ODFM modulation 404. The periodicity of the periodic impulse noise is 11.12 ms. Based on observed results from a set of simulation results for the case when the impulse noise period is 11.12 ms (an integer number of OFDM symbols), the longer interleaver offers minimal gains at the cost of increased memory and processing requirements.
Benefits of Tying Interleaver Length to AC Mains
The results noted above establish that choosing an interleaver having a length that is close to the AC mains suffices to achieve most of the interleaving gains for the case of periodic impulsive noise. Based on this, the interleaver length for the Cenelec-A band has been chosen to be an integer number of OFDM symbols, whose span does not exceed 10 ms.
In one embodiment, the symbol length is also chosen to be equal (or very close) to a divisor of the zero crossing duration. In other words, given the symbol period Tsym, the zero crossing period Tzc can be expressed as Tsym*Nsym+Trem, where 0<Trem<Tsym. In one embodiment, Tsym is chosen so that Trem is close to zero. The following potential benefits result from selecting Tsym in this manner:
A first minor benefit is that such a choice gives the maximum possible interleaver length, under the assumption that the interleaver length is Tsym*Nsym, which is clearly maximized by choosing Trem close to zero. Other choices, with a larger Trem, have poorer performance due to shorter interleaver length, with the degradation being visible above Trem ̃0.5 ms.
A second benefit of choosing Trem ̃0 is that it enables the system to combat short bursts of periodic impulsive noise. To be precise, consider the case where the periodic impulsive noise occurs during every zero crossing, and is less than one cyclic prefix in length (i.e., a few 10 s of microseconds). This case is observed quite frequently, in practice. If there are an integer number of OFDM symbols between successive impulsive noise bursts (i.e., between zero crossings of the mains), it is possible to synchronize the cyclic prefix of one symbol with the impulsive noise burst. Thus, the OFDM system becomes completely immune to the effect of impulsive noise.
FIG. 5 illustrates synchronizing a cyclic prefix with short impulsive noise bursts. Note that this is not possible unless there are an (nearly) integer number of OFDM symbols between impulse noise bursts. Even if one impulse noise burst is synchronized with the cyclic prefix, subsequent bursts will drift into the FFT symbol.
Note that achieving the above requires the transmitter to align the start of some OFDM symbol with the zero crossing. This imposes a granularity of one OFDM symbol length (<1 ms) in the medium access procedure. Note that the preamble typically lasts a few OFDM symbols, and so the additional granularity is small compared to the delay requirements from accurate channel sensing.
Advantages: The above method works without any additional signaling. Further, since the preamble can only start at discrete intervals, the receiver can use this information to reduce the probability of false preamble detections.
Disadvantages: The above method, by itself, is effective only when the impulsive noise burst length is of the order of the cyclic prefix duration. Further, unless the number of symbols per zero crossing mains is a multiple of 3, this does not protect against impulsive noise from all three phases.
For the case of differential PSK, this method provides performance benefits such that synchronized symbols offer about 0.2-0.3 dB gain over unsynchronized symbols. Further gains are expected with coherent modulation.
Time-Domain Link Adaptation
To handle the case where impulsive noise is much longer than the cyclic prefix, the system may use time-domain link adaptation as illustrated in FIG. 6. In every block of N symbols 601a-n, a few symbols are affected by impulsive noise 602. With simple receiver feedback, the transmitter can adapt the data rate in different symbols 601 to the signal-to-noise ratio (SNR) in the symbol. For example, no data may be transmitted in the first two symbols 601a, 602b, with maximum data rate being transmitted in the remaining (N−2) symbols.
Note that the link adaptation mentioned here is the time-domain equivalent of adapting the modulation parameters on different tones in response to channel variations or narrowband interference. In powerline communication, since periodic impulsive noise is a dominant noise source, time-domain link adaptation would be equally effective and should be combined with frequency domain link adaptation. In the extreme case, the system use a two-dimensional link adaptation for cases where the periodic impulsive noise is also frequency selective and concentrated on a few tones.
The use of time-domain link adaptation only requires receiver feedback of the phase on which it is present, and the SNR profile across different symbols. It does not necessarily require the transmitter to synchronize symbol start with the zero crossing.
A few different variations are possible, based on the general principle of time domain link adaptation. If some symbols have such high noise that they cannot be used, then, (i) dummy data can be transmitted, (ii) only pilots can be transmitted, (iii) neither pilots nor data may be transmitted, (iv) neither pilots nor data may be transmitted and the pilot phase may not count up.
In the transmitter, after optional repetition, the coded bit stream is interleaved 403 before modulation 404 (FIG. 4). To limit decoding latency, interleaving is done over a block of Nint or fewer OFDM symbols. The values of Nint for different bands is given in Table 8.











TABLE 8








FFT SYMBOL





DURATION




BAND
(EXCLUDING CP)
NINT




















Cenelec-A/B/C/D
655.36 us
14



FCC
327.68 us
28










The zero crossing duration (AC mains frequency) may be 50 or 60 Hz. The values in Table 8 were chosen to account for the worst case value zero crossing period of 10 ms. In other embodiments, different profiles may be defined for different regions, with correspondingly different interleaver lengths.
The total number of bits modulated in one OFDM symbol is denoted Bsym=ΣkBk. With Nsym symbols per frame, the coded bit stream contains Nsym*Bsym bits, and is divided into Nblock=|Nsym/Nint| blocks, where ┌f┌ denotes the smallest integer greater than or equal to f. The first Nblock−1 blocks contain Nint*Bsym bits, corresponding to Nint OFDM symbols. The remaining bits can only fill Nremaining=Nsym−(Nblock−1)*Nint symbols. To ensure simplicity of implementation and to ensure sufficient interleaving depth for all bits, repetition bits are padded to the last block to extend the length to Nint-last symbols. This is done by repeating the Nremaining*Bsym bits circularly till the desired number of bits is obtained. For a given value of Nremaining, the last block size Nint-last is drawn from the set {Nint, ┌Nint/2┐, ┌Nint/4┐}.
Given a number of symbols Nremaining, the choice of Nint-last from the allowed set needs to be determined. One possibility is to adaptively set the size depending on noise conditions. Another is to use a fixed mapping from Nremaining to Nint-last. For example, Nint-last is the minimum value in the allowed set which is greater than 2*Nremaining. If no such value exists, Nint-last is equal to Nint.
Thus, note that the ith block contains coded bits modulating a block of Nint(i) successive OFDM symbols, where




  
    
      
        
          
            N
            int
          
          ⁡
          
            (
            i
            )
          
        
        =
        
        ⁢
        
          N
          int
        
      
    
    
      
        
        ⁢
        
          
            
              for
              ⁢
              
                
              
              ⁢
              i
            
            =
            1
          
          ,
          2
          ,
          ...
          ⁢
          
            
          
          ,
          
            
              N
              block
            
            -
            1
          
          ,
          and
        
      
    
  
  
    
      
        =
        
        ⁢
        
          N
          
            int
            -
            last
          
        
      
    
    
      
        
        ⁢
        
          
            for
            ⁢
            
              
            
            ⁢
            i
          
          =
          
            
              N
              block
            
            .
          
        
      
    
  




In each block, interleaving of bits is done as follows. The input bits are S/P converted to a Bsym×Nint(i) matrix by serial-to-parallel conversion, as shown below.




  
    
      
        
      
    
    
      
        Bits
        ⁡
        
          (
          1
          )
        
      
    
    
      
        Bits
        ⁡
        
          (
          
            
              B
              sym
            
            +
            1
          
          )
        
      
    
    
      
        
      
    
    
      
        Bits
        ⁡
        
          (
          
            
              
                (
                
                  
                    
                      N
                      int
                    
                    ⁡
                    
                      (
                      i
                      )
                    
                  
                  -
                  1
                
                )
              
              *
              
                B
                sym
              
            
            +
            1
          
          )
        
      
    
  
  
    
      
        
      
    
    
      
        Bits
        ⁡
        
          (
          2
          )
        
      
    
    
      
        Bits
        ⁡
        
          (
          
            
              B
              sym
            
            +
            2
          
          )
        
      
    
    
      
        
      
    
    
      
        Bits
        ⁡
        
          (
          
            
              
                (
                
                  
                    
                      N
                      int
                    
                    ⁡
                    
                      (
                      i
                      )
                    
                  
                  -
                  1
                
                )
              
              *
              
                B
                sym
              
            
            +
            2
          
          )
        
      
    
  
  
    
      
        
      
    
    
      
        Bits
        ⁡
        
          (
          3
          )
        
      
    
    
      
        Bits
        ⁡
        
          (
          
            
              B
              sym
            
            +
            3
          
          )
        
      
    
    
      ...
    
    
      
        Bits
        ⁡
        
          (
          
            
              
                (
                
                  
                    
                      N
                      int
                    
                    ⁡
                    
                      (
                      i
                      )
                    
                  
                  -
                  1
                
                )
              
              *
              
                B
                sym
              
            
            +
            3
          
          )
        
      
    
  
  
    
      
        
          Bits
          ⁡
          
            (
            
              1
              ,
              2
              ,
              ...
              ⁢
              
                
              
              ,
              
                
                  B
                  sym
                
                *
                
                  
                    N
                    int
                  
                  ⁡
                  
                    (
                    i
                    )
                  
                
              
            
            )
          
        
        →
      
    
    
      ...
    
    
      ...
    
    
      
        
      
    
    
      ...
    
  
  
    
      
        
      
    
    
      
        Bits
        ⁡
        
          (
          
            B
            sym
          
          )
        
      
    
    
      
        Bits
        ⁡
        
          (
          
            2
            ⁢
            
              
            
            ⁢
            
              B
              sym
            
          
          )
        
      
    
    
      
        
      
    
    
      
        Bits
        ⁡
        
          (
          
            
              
                N
                int
              
              ⁡
              
                (
                i
                )
              
            
            *
            
              B
              sym
            
          
          )
        
      
    
  




Then, the matrix is interleaved as follows. The (K, L)th element of the output matrix is determined as the (k, l)th input element, wherein

k=(α*(K−βL)) % Bsym 

l=(γ*(L−δk)) % Nint(i), and the integers α, β, γ, δ are chosen to ensure

gcd(α,Bsym)=gcd(δ,Bsym)=gcd(β,Nint(i))=gcd(γ,Nint(i))=1.

After interleaving, the blocks are concatenated and the resultant Bsym×Nsym matrix is used for modulation.
As long as the interleaver size is matched to the periodicity of the impulse noise, the performance benefit of longer interleavers is minimal. Hence, in one embodiment, the length of the channel interleaver (the interleaver between the channel and the inner code) is limited to the periodicity of the impulse noise.
Interleaver Structure
FIG. 7 illustrates an interleaver structure according to one embodiment.
The convolutional code 701 receives and processes one or more Reed Solomon code words and produces NFEC bits. These bits need to be repeated appropriately, interleaved and fit into an integer number of OFDM symbol frames. Each OFDM symbol frame carries kH or kP bits depending on whether the symbol frame goes to the header or to the payload. This is done in the following steps.
The (NFEC bits) input stream is first segmented (702) into B blocks of roughly equal lengths, drawn from a set of valid block lengths. The length of each block is determined as the minimum of: (i) the input number of bits NFEC, (ii) 2048 (or some other number) bits, and (iii) the number of bits transmitted in one semi-zero crossing interval (either 8.33 ms or 10 ms depending on the region).
Some example combinations of the above-identified parameters are given in Table 9.













TABLE 9









No. of
No. of





Average

OFDM
modulated bits
Desired block



No. of
no. of bits

symbols in
in half-zero-
length



tones per
per
kP
half-zero-
crossing period
Minimum of



OFDM
modulated
(Column 2 *
crossing
(Column 4 *
(2048, Column


NFEC
symbols
carrier
Column 3)
period
Column 5)
1, Column 6)





















<1008
36
2 (QPSK)
72
14 (10 ms)  
1008
NFEC


≧1008
(Cenelec-




1008



A)







<2016
36
4 (16-
72
14 (10 ms)  
2016
NFEC


≧2016
(Cenelec-
QAM)



2016



A)







<2048
144 (FCC
2 (QPSK)
288
24 (8.33 ms)
6912
NFEC


≧2048
band)




2048









Each block is repeated R times (703) and applied to R interleavers (704). R is the number of repetitions, and R may be 1, 2, 4 or 8, for example.
Each interleaver (704) produces B bits. The output of the interleavers (704) is concatenated (705) to yield a buffer of length RB interleaved bits. The output of the buffer is read to yield the desired kP modulated bits for each OFDM symbol. When a buffer is exhausted, the buffer for the next block is created and used. If the last buffer is exhausted before the frame has been modulated, the last buffer is read again from the beginning.
Interleaving Depth
For the case of one repetition, the interleaving depth is limited to the half-zero crossing period. This is ensured by the condition that the maximum block length is the number of modulated bits in half mains cycle period. There are two exceptions to this pattern:

    
    
        1) If the number of repeats is larger than 1, the same coded bits can be spread across multiple 10 ms intervals.
        2) For the FCC band, the maximum interleaver depth of 2048 bits may span less than the half mains cycle period. Simulation results show that this does not excessively affect performance even when 30% of the band is erased in 20% of the symbols.

Memory Requirement

    
    


The interleaver (704) requires memory allotted for only one block at a time. Even if multiple repeats are interleaved, the log-likelihood ratios (LLRs) can be combined in place. In other words, the system only needs to store LLRs at the deinterleaver output. For multiple received blocks, the system only has to find the position where a given bit is deinterleaved, and accumulate the corresponding LLR. Given that the maximum block size is 2048 bits, only 2K bytes of memory is required assuming 8-bit LLRs.
Payload Encoder
FIG. 8 illustrates a functional model of a payload encoder 800 according to one embodiment. The payload encoder comprises FEC encoder 801, an aggregation and fragmentation block (AF) 802, a fragment repetition encoder (FRE) 803, and interleaver 804. FRE 803 supports a robust communication mode (RCM) and is turned off in case of normal mode of operation.
The incoming PHY-frame payload is divided into sequential information blocks of K bytes per block. Each information block is encoded by FEC encoder 801. The bytes in each information block shall be in the same order as they are in the corresponding LPDU. The payload bit to be transmitted first is the first in the corresponding information block.
AF block 802 first collects one or more FEC codewords and forms an FEC codeword block. One or more FEC codewords may make up the FEC codeword block. The FEC codewords may be concatenated into an FEC codeword block in the order they are output by the FEC encoder and with the same order of bits. Further, the FEC codeword block is partitioned into fragments of roughly the same size—B0 bits each.
FRE 803 provides repetitions of fragments with the repetition rate of R. If R>1 is set, each fragment shall be copied R−1 times and all copies concatenated into the fragment buffer, FB, so that the first bit of each copy follows the last bit of previous copy. The total size of the FB is B0×R bits. FRE 803 supports the values R=1, 2, 4, where the value of R=1 corresponds to normal mode of operation. If R=1, an FB contains a single fragment of B0 bits.
All fragments of each FB are interleaved. The first fragment of each FB is interleaved by the interleaver I1 (804). If repetitions are applied, the following copies of fragments in each FB shall be interleaved by interleavers I2, I3, . . . IR (804), respectively. The interleaver Ir is obtained by cyclically shifting I1 by round(B0(r−1)/R) bits.
If the number of bits in the fragment B0 doesn't fit integer number of symbols, the fragment shall be padded accordingly prior to the interleaving. The pad shall be generated by repeating the bits of the fragment starting from the first bit of the fragment, in ascending order, until the symbol is filled up.
If the value of B0 is selected to be less than NZC, the payload encoder may be set into Interleaving-over-AC-cycle (IoAC) mode or in Interleaving-over-Fragment (IoF) mode. If set to IoAC mode, each FB (containing interleaved fragments and their interleaved copies) shall be padded to the closest integer number of NZC. The pad shall be generated by cyclical repeating of the bits of this same FB, starting from its first bit. The first bit of the pad shall follow the last bit of the FB and shall be the repetition of the first bit of the same FB. After FB is padded, it's passed for concatenation 805.
If set to IoF mode, the FB shall be passed for concatenation with no additional bits padded (while the length of the FB is less than NZC).
The interleaving mode is set based on the number of symbol frames, k, to be generated (kP for payload symbol frames and kH for header symbol frames). If k is an integer multiple of the minimum number of symbols spanning an AC zero crossing period, the IoAC mode is always used. Otherwise, the IoF mode is used.
The FBs shall be concatenated into an encoded payload block, in the order of the sourcing fragments. In Normal mode of operation and IoF mode, the encoded payload block is a concatenation of Nfrg fragments generated by the AF 802.
The encoded payload block is passed for segmentation into symbol frames.
FIG. 9 illustrates the steps performed in the generation of the encoded payload block for the case Nfrg=4. The AF (802) forms FEC codeword block 901 comprising one or more FEC codewords 902. The FEC codewords 902 are concatenated into an FEC codeword block 901 in the order they are output by the FEC encoder (801) and with the same order of bits.
The value of B0 shall be calculated as a divisor of the total number of bits in the FEC codeword block 901. This shall be the maximum divisor which value is less than or equal to the minimum of:

    
    
        1) The total number of input bits (the FEC codeword block);
        2) The total number, NZC, of bits loaded in the symbols that span at least 10 ms for the case of 50 Hz AC lines, and at least 8.33 ms for the case of 60 Hz AC lines or lines with no AC; or
        3) The maximum fragment size Bmax.
    
    


In one embodiment, the value of Bmax=3072 bits is supported by all transmitter and receiver nodes. Support for Bmax=6144 bits may be optional at both the transmitter and receiver.
In one embodiment, B0 is rounded up to fit an integer number of OFDM symbol frames.
The number of fragments is Nfrg=ceil(s×NFEC/B0), where s is the number of FEC codewords in the FEC codeword block. To obtain integer number of fragments, the FEC codeword block 901 is padded (903) with Nfrg*B0−NFEC zeros.
The FRE (803) provides repetitions of fragments with the repetition rate of R. If R>1 is set, each fragment shall be copied R−1 times and all copies concatenated into the fragment buffer, FB (904), so that the first bit of each copy follows the last bit of previous copy. The total size of the FB is B0×R bits. FRE 803 supports the values R=1, 2, 4, where the value of R=1 corresponds to normal mode of operation. If R=1, an FB contains a single fragment of B0 bits.
At the output of the payload encoder 800, a bit stream comprising individual buffers {FBi} is available containing bits to be mapped to symbol frames. For any symbol frame, the desired number of bits, as determined by the bit allocation table, are read from the bit stream. The first symbol frame starts reading at the first (leftmost) bit of the symbol stream, and continues reading in the order FB1, FB2, etc.
Channel Interleaver
The fragment interleaver interleaves a block of B0 bits, based on:

    
    
        (i) the number of active subcarriers (ASC) per symbol frame; and
        (ii) the bit allocation table {bi} giving the number of bits allocated to the ith supported subcarrier.

Both the above quantities could depend on whether the interleaver fragment corresponds to header and data bits.

    
    


Given the above parameters, the fragment interleaver writes bits into a two-dimensional matrix, permutes them, and reads out the permuted bits. Specific rules are followed for reading and writing.
FIG. 10 illustrates the interleaving process according to one embodiment.
Step 1 (1001)—Calculation of parameters: The first step is to determine the number of rows and columns, and the row quantum parameter that determines the rules for reading and writing. The row quantum b is calculated as the largest integer less than or equal to the numerical average of {bi}, i.e., b=floor(mean({bi})).
The number of columns, denoted m, is calculated as the integer closest to (ASC* mean({bi})/b).
The number of rows, denoted n, is B0/m. The interleaver is defined only for sizes B0 for which the number of rows is an integer multiple of the row quantum b.
If the bit loading is flat, b equals the number of bits loaded in each active carrier, and m equals the number of active subcarriers. In this case, n equals the (total number of bits in the interleaver block/m), as required by G3. In this case, the number of bits in b rows exactly equals the total number of bits per symbol frame. If the bit loading is not flat, note that m does not necessarily represent the number of active subcarriers per OFDM symbol frame. Instead, it is chosen to ensure that the number of bits in b rows is within b bits of the total number of bits per OFDM symbol frame.
Step 2 (1002)—Writing into permutation matrix: With the above setup, input bits are now written into an n×m matrix as follows. Writing is done for one quantum of rows at a time. FIG. 11 illustrates the order of writing and reading into a permutation matrix according to one embodiment. Each box in FIG. 11 represents a bit. The number in the box indicates the position of the bit in the input bit stream. Thus, the first b rows are filled before writing into the next b rows, and so on. Within each quantum of rows, bits are written in column-first order. The first bit is written into the first row of the first column, the next is written in the second row of the first column, and so on until all b rows of the quantum are complete. Then the next bit is written into the first row of the second column, and so on.
Step 3 (1003)—Row-column permutation: The entries of the n×m are permuted. The relation between input and output interleaving indexes is determined from the following relations: original bit position (i, j) where i=0, 1, . . . , m−1 and j=0, 1, . . . , m−1.
Interleaved position (I, J) is given by:

J=(j*n_j+i*n_i)% n 

I=(i*m_i+J*m_j)% m 

where m_i, m_j, n_i, and n_j are selected based the values of m and n, under the constraint that

m_i,m_j,n_i,n_j>2

GCD(m_i,m)=GCD(m_j,m)=GCD(n_i,n)=GCD(n_j,n)=1

FIG. 12 is an example of row-column permutation that displays the spreading behavior of the interleaver for n=8, m=10, n_j=5, n_i=3, m_i=3 and m_j=7.
Step 4 (1004)—Reading from permuted matrix: Bits are read from the permutation matrix in the same order in which they were written.
In one embodiment, two interleaving modes are defined, depending on how the reading process transitions from one buffer to the next.
In the sub-zero crossing mode, all buffers—except the last one—are discarded immediately after the last bit in the buffer is read. If the last bit of the last buffer has been read and there is still a need for more bits to generate symbol frames, reading continues again from the beginning of the last buffer. In other words, the last buffer is read circularly until all bits required for symbol frame generation are available.
In the zero-crossing mode, a buffer is discarded when the number of bits read from it is NZC. In this mode, all buffers are read in a circular fashion if necessary.
In one embodiment, the sub-zero crossing mode is always used for the PHY header and in any case where the number of repetitions R is greater than one. For the case of one repetition (R=1), the mode may be chosen based in one of the following ways:
1) One possibility is to choose the mode based on accumulated noise statistics, which are either fed back from the receiver explicitly or implicitly determined by the transmitter. In this case, a bit is transmitted in the PHY header to indicate which mode is used for the data.
2) Alternatively, the zero crossing mode is used only when the number of OFDM symbols generated is an integer multiple of the minimum number of OFDM symbols spanning the zero crossing period. In this mode, no signaling of the mode is done since the receiver can obtain by the mode based on the known value of the number of symbols.
Another alternative is the first symbol frame starts reading at the first (leftmost) bit of the symbol stream. Except for the last buffer, every other buffer is discarded after its bits have been read. If the last bit of the last buffer has been read and there is still a need for more bits to generate symbol frames, reading continues again from the beginning of the last buffer. In other words, the last buffer is circularly read till all bits required for symbol frame generation are available.
Interleaving Depth for Various Bands
The number of tones in one zero-crossing interval for the various bands under consideration is given Table 10 for one embodiment.












TABLE 10









No. OF 
NO. OF 




No. OF 
SYMBOLS/
TONES/




TONES/
ZERO 
ZERO 



BAND
SYMBOL
CROSSING
CROSSING





















Cenelec-A
36
14
504



Cenelec-B
17
14
238



Cenelec-(C + D)
13
14
182



FCC
107
23
2461










The interleaving depth is limited according to the following rules:

    
    
        1) For the Cenelec bands, the maximum memory size of 2K bits is not relevant. The maximum interleaving depth is 10 ms times the number of repetitions used.
        2) For the FCC band, the interleaving depth depends on the modulation scheme and the number of repetitions. With (QPSK+repetition-2), note that 2461 bits are contained in 10 ms and a 2K bit interleaver spans more than 80% of this. Simulation results show that a 2K bit interleaver handles most realistic cases without loss of performance.

Handling of Pad Bits and Other Corner Cases

    
    


With block interleavers, one corner case that should be handled is that the number of bits generated may not fit (i) an integer number of OFDM symbol frames, or (ii) desired depth in interleaving. Both of these cases are handled by making the last buffer circular.
Thus, if 300 bits are required for the last symbol frame and only 220 are available, then 80 more bits are read from the beginning of the last symbol frame. Thus, the first mentioned problem is solved and an integer number of OFDM symbol frames can always be generated.
Secondly, suppose the FEC output bits only span 34 ms (with repetitions), and these are divided into four blocks spanning {10, 10, 10, 4} ms respectively. The last block only contains 4 ms of data. If the noise conditions are estimated to be benign, the transmitter may just transmit 34 ms of OFDM symbol frames, in which case the last block is only interleaved over 4 ms. Otherwise, the transmitter may transmit OFDM symbols spanning 40 ms in which case the last block is circularly repeated as necessary to generate 40 ms. Thus, this structure gives the transmitter the flexibility to adjust the overhead to the channel conditions without requiring any additional signaling, as described below.
Signaling Required
Note that the receiver can identify the pad bits at all stages of processing, given only (i) the input number of bits NFEC, which in turn is uniquely determined by the payload length and code rate, and (ii) the number of OFDM symbol frames. These two quantities need to be signaled anyway, so there is no additional signaling required to achieve the necessary padding.
Memory Requirement
The NFEC input bits are partitioned into one or more blocks. Both the interleaver and deinterleaver only require memory allotted for one block of bits (at most 2K bits in length), even if this block is repeated multiple times.
Specifically, at the receiver, every time an LLR is generated, the deinterleaver for the corresponding repeat is calculated and the LLR is accumulated at the deinterleaver output.
Of course, since processing of one block of bits/LLRs will take time, ping-pong buffers may be required for two successive blocks.
Performance Results
In this section, we provide simulation results to verify performance of the proposed interleaver. Table 11 shows the simulation parameters used for performance comparison.










TABLE 11







BAND
FCC BAND









MCS
Coherent QPSK with




repetition 1 and repetition 2



Packet size
~2 kbyte



# active tones
150 active tones with 3.125




kHz tone spacing



Interleaver
10 ms block interleaver, and




2048-bit block interleaver



Channel model
Time and frequency erasure




channel










In one embodiment, a simulation assumes a periodic noise model wherein a fraction of the band gets erased periodically in some fraction of the 10 ms duration between zero crossings. This is more realistic than assuming that the entire 468.75 kHz gets erased during the impulsive noise burst.
It may be questioned whether this is a realistic model instead of whole-symbol clipping. The main questions are what is the ADC dynamic range and whether an impulsive will result in the entire symbol clipping and causing the noise to spread out over the entire band. This depends on

    
    
        (i) the energy of the impulsive noise relative to the signal;
        (ii) the power of the impulsive noise when it is ON;
        (iii) the automatic gain control (AGC) algorithm used; and
        (iv) the dynamic range of the analog-to-digital converter.
    
    


If the impulsive noise is both high energy and high power (i.e., if its duty cycle is sufficiently high), the average energy is effectively dominated by the impulsive noise. Thus, any reasonable AGC algorithm will backoff so that the impulsive noise does not clip in the ADC samples. Subsequently, it is straightforward to identify the band in which the impulsive noise dominates and erase it. Note that the gains of doing so would be significant in cases where the impulsive noise is narrow in frequency.
Even in cases where the average energy is not dominated by the impulsive noise energy, the following conditions ensure that entire symbols are often not erased in the presence of narrowband impulsive noise

    
    
        1) With narrowband impulsive noise, the peak-to-average power ratio (PAPR) is not as high as the desired OFDM signal. Further, since the impulse noise occurs at every zero crossing, the AGC could backoff sufficiently at the A/D converter, using an intelligent AGC algorithms, to ensure that the impulse noise is not clipped.
        2) When the impulse noise power is 10-15 dB higher than the desired signal power, there is not much increased backoff at the ADC since an OFDM signal needs its own backoff of 10-15 dB at the ADC anyway. Hence, the increased backoff due to low PAPR impulse noise is not significant.
        3) Even if a narrow burst of impulsive noise is clipping at say 12 dB above the signal's rms value, the effective noise is frequency domain is small enough (its energy is divided by the ratio of the impulsive noise burst duration to the OFDM symbol duration) that the entire symbol is not completely erased.
    
    


Consequently, we recommend that decisions not be based purely on erasure models, in particular for the FCC band which is broad enough to ensure many impulsive noise bursts affect only a fraction of the tones.
Under the assumptions listed above, and with coherent QPSK with repetition 2 in 20% time erasure and 20% frequency erasure channel, the performance difference between the 2048-bit interleaver and the 10 ms interleaver at 1 e-2 FER is only 0.1 dB.
Under the assumptions listed above, and with coherent QPSK with repetition 2 in 20% time erasure and 50% frequency erasure channel, severe portions of the frequency bands are erased. In this channel, there is a 0.6 dB performance difference between two interleavers.
Under the assumptions listed above, and with coherent QPSK in 20% time erasure and 20% frequency erasure channel, the performance difference between 10 ms interleaver and the proposed interleaver is 0.8 dB.
Additional points to consider include:

    
    
        1) If it turns out that indeed multiple symbols are wholly erased, the repetition ratio used would increase. This, in turn, makes the effective interleaver depth wider. Specifically, with 8 repetitions, the interleaver depth discussed here (corresponding to 2048*8 bits) is almost equal to 10 ms anyway.
        2) In the above case, note that much greater gains can be obtained by using time-domain link adaptation.
    
    


Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions, and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

## Claims

1. A transmitter for encoding a physical (PHY) frame, the transmitter comprising:
a header generation circuit for generating a header with a set of common header bits and a set of embedded header bits;
a header encoder circuit comprising a forward error correction encoder for providing the header with a common header segment using the common header bits; and
a payload encoder circuit comprising a forward error correction encoder for providing a data segment that includes a payload segment and an embedded header segment generated from the set of embedded header bits;
wherein the common header segment includes data indicating the length of the embedded header segment.

2. The transmitter of claim 1, wherein the common header segment includes data identifying the length of the payload segment.

3. The transmitter of claim 1, wherein the embedded header segment and the payload segment are encoded using a same modulation and coding scheme.

4. The transmitter of claim 1, wherein the embedded header segment includes separate cyclic redundancy check (CRC) data.

5. The transmitter of claim 1, wherein the embedded header segment signals a source ID and a domain ID.

6. The transmitter of claim 4, wherein the CRC data comprises a plurality of bits including a first K bits configured to be masked to provide a destination ID.

7. The transmitter of claim 4, wherein the CRC data comprises a plurality of bits including a first K bits configured to be masked to provide a broadcast group ID.

8. The transmitter of claim 1, wherein the common header segment corresponds to a physical layer and the embedded header segment corresponds to a data link layer.

9. The transmitter of claim 8, wherein the embedded header segment comprises a media access control (MAC) header.

10. The transmitter of claim 1, wherein the length of the embedded header segment is indicated by two or more bits in the common header segment.

11. A method for encoding a physical (PHY) frame, the method comprising:
generating a header with a set of common header bits and a set of embedded header bits by using a forward error correction (FEC) encoder of a header encoder circuit to provide a common header segment generated from the common header bits and using a FEC encoder of a payload encoder circuit to provide a data segment that includes a payload segment and an embedded header segment generated from the embedded header bits, wherein the common header segment and the embedded header segment make up the header, and wherein the common header segment includes data indicating the length of the embedded header segment.

12. The method of claim 11, wherein the common header segment includes data identifying the length of the payload segment.

13. The method of claim 11, wherein the embedded header segment and the payload segment are encoded by the payload encoder circuit using the same modulation and coding scheme.

14. The method of claim 11, wherein the embedded header segment includes cyclic redundancy check (CRC) information.

15. The method of claim 11, wherein the CRC information comprises a plurality of bits including a first K bits configured to be masked to provide a destination ID.

16. The method of claim 11, wherein the CRC information comprises a plurality of bits including a first K bits configured to be masked to provide a broadcast group ID.

17. The method of claim 11, wherein the common header segment corresponds to a physical layer and the embedded header segment corresponds to a data link layer.

18. The method of claim 17, wherein the embedded header segment comprises a media access control (MAC) header.

19. The method of claim 11, wherein the embedded header segment signals a source ID and a domain ID.

20. The method of claim 11, wherein the length of the embedded header segment is indicated by two or more bits in the common header segment.