[0068] Example 1
[0069] In this PLC system, two frequency bands are supported, namely frequency band 0 and frequency band 1. Band 0 has a total of 512 sub-carriers, numbered from 0 to 511, the maximum number of available sub-carriers supported is 411, and numbered from 80 to 490, all used, without shielding one or several sub-carriers in the middle; supported by band 1 The number of sub-carriers is 131, numbered from 100 to 230, all of which are used, without shielding one or several sub-carriers in the middle. Such as Image 6 Shown is the physical layer link processing process of a PLC system, in which the processing is divided into two parts: frame control and load data. The frame control part adopts a fixed diversity copy mode each time data is transmitted according to the frequency band used (0 or 1), while the diversity copy mode of the payload data part can be changed according to the current channel quality. Diversity copy parameters will be given in the frame control.
[0070] In this PLC system, the load data will enter the diversity copy module after channel interleaving. The system has specially set up the diversity copy mode parameter table for diversity copy, called TMI value. Among them, the parameter items included in the diversity copy mode include physical block type (number of bytes), number of diversity times, debugging mode, code rate, and number of physical blocks. Each TMI value corresponds to a diversity copy mode. The physical block type is the MAC frame segmentation obtained by the physical layer from the upper layer, and supports several byte types such as PB16, PB72, PB136, and PB520, namely Image 6 The number of input bytes of medium payload data; the system adopts Turbo encoding, supports two code rates of 1/2 and 16/18. After channel encoding, the number of output information bits is the number of bits to be copied for diversity copy; the modulation method determines each The number of bits carried on each subcarrier (BPC); the number of diversity (CopyNum) represents the number of times the information sequence output from the channel interleaving needs to be copied. Table 1 below shows several examples of TMI values.
[0071] Table 1 Example of diversity copy mode
[0072]
[0073] According to the requirements of this scheme, two tasks need to be completed in the diversity copy module. One is to segment the information sequence entering the module, that is, to consider the segment length (BitsPerGroup) mentioned in the scheme; Segment group G i Copy a certain number of times according to CopyNum. In addition, it is also necessary to perform cyclic shift for each diversity and perform intra-block interleaving for each copy block. Cyclic shift refers to a copy of all segment groups (I.e. a certain diversity) After each round of mapping is completed, it needs to perform cyclic shift according to the rules to ensure that each G i Each copy of G is mapped to different frequency bands to form frequency diversity; intra-block interleaving means that each G i Every copy of When mapping to subcarriers, interleaving must be performed in the copy block first. Therefore, a certain number of interleavers are required. The number of interleavers (InterNum) set by the system is an important parameter. The relationship between the number of interleavers required, the number of copies, and the number of interleavers (InterNumPerGroup) required for each segment group is shown in Table 2.
[0074] In addition, some of the parameters involved in segmentation are as follows: DataBitsLen: data bits output by channel interleaving, including information and check bits; ValidCarrierNum: number of effective subcarriers, the number of subcarriers supported by the communication frequency band (0 or 1); UsedcarrierNum : The number of sub-carriers actually used in the diversity copy. When UsedcarrierNum is not equal to ValidCarrierNum, select the lower numbered sub-carrier to use; BPC: the number of bits modulated by each sub-carrier; TotalGroupNum: the total number of segment groups; PadBitsLastGroup: after data segmentation is completed , The number of bits to be filled in the last segment group.
[0075] The following will take the diversity copy mode of the load data part of the PLC system as an example to introduce in detail the channel quality assessment and the specific application of the diversity data merging method proposed in this solution.
[0076] First, the physical layer diversity copy process of the payload data part takes frequency band 0 and TMI=0 as an example. The specific steps are as follows, the process is as follows: Figure 7 Shown.
[0077] Step 1: The system sets the communication frequency band to frequency band 0, and the transmitter sets the diversity copy mode of the payload part to TMI=0, and puts the TMI value into the frame control part. The frame control part adopts QPSK modulation, 4 OFDM symbols.
[0078] Step 2: The payload data is processed by scrambling, Turbo coding, and channel interleaving, and then input to the diversity copy module. According to TMI=0, PB520 is supported, the number is 1 to 4 physical blocks, and the physical block is used as a unit for processing in each module. 520Byte×8=4160bit. After Turbo encoding with a code rate of 1/2, the number of bits in a single physical block that enters the diversity copy is DataBitsLen=4160bit×2=8320bit.
[0079] Step 3: In the diversity copy module, the input data is segmented in units of physical blocks. According to frequency band 0, TMI=0 and Table 2, ValidCarrierNum=411, CopyNum=4, BPC=2bit, InterNum=8. Then:
[0080]
[0081] The agreement stipulates the use of low-frequency numbered subcarriers, so the actual subcarrier numbers used are from 80 to 487.
[0082]
[0083] In this way, physical blocks can be segmented according to BitsPerGroup:
[0084]
[0085] When DataBitsLen and BitsPerGroup cannot form an integer multiple, the last segment group G needs to be filled with the reference signal sequence:
[0086] PadBitsLastGroup=BitsPerGroup-mod(DataBitsLen,BitsPerGroup)
[0087] =204-8320%204=44bit
[0088] Step 4: In the diversity copy module, copy the segmented physical blocks with each segment group G as a unit respectively CopyNum=4 times. A physical block copied can be expressed as:
[0089] Step 5: In the diversity copy module, use copies from different segments As the unit (called the Kth diversity), the cyclic shift is performed to complete the CopyNum round shift, and each round of shift is performed with BitsPerGroup as the step size. The first round of GroupShiftNum[1]=0, indicating the first diversity No shift is required; in the same way, GroupShiftNum[2]=0, GroupShiftNum[3]=0, GroupShiftNum[4]=0.
[0090] Step 6: In the diversity copy module, for each diversity, a single copy block Interleaving is performed in units, each copy block uses InterNumPerGroup interleavers, the interleaver is denoted as I k. For the first diversity, each copy block uses 2 interleavers, and the result after interleaving is The second diversity, the result after interleaving is The third diversity, the result after interleaving is In the fourth diversity, the result after interleaving is A total of 8 interleavers are used. After the interleaving is completed, the data is input to the constellation point mapping module.
[0091] Step 7: In the constellation point mapping module, the input data is sequentially mapped to the subcarriers according to each group set as a unit. According to step 3, the subcarriers used to transmit useful information for each OFDM symbol are 80 to 487. After each symbol is mapped, there are 488 to 490 empty subcarriers remaining in the effective frequency band, BPC=2bit, these three empty subcarriers The reference signal sequence of 2bit×3=6bit will be filled; the total number of bits in a physical block after diversity copy is
[0092] (DataBitsLen+PadBitsPerGroup)×CopyNum=(8320+44)×4=33456bit,
[0093] A total of 4 episodes just filled up
[0094] 33456/(BPC×UsedCarriedNum)=33456/(2×408)=41 OFDM symbols. The time-frequency transmission resource occupancy after the data is mapped to the constellation point is as follows: Figure 8 Shown.
[0095] Step 8: After the data is mapped to the constellation points, it is input to the subsequent module, processed by IFFT transformation, CP and windowing, etc., and then transmitted to the power line channel for transmission.
[0096] Step 9: At the receiving end, the received data passes through physical layer processes such as CP removal and FFT transformation before reaching the demodulation module. The receiving end first analyzes the TMI value according to the diversity copy mode adopted by the frame control data in frequency band 0, that is, the diversity copy mode adopted by the payload data.
[0097] Step 10: In the demodulation module, demodulate the payload data part according to frequency band 0 and TMI=0. Take out the useful information of the 4th diversity in each physical block in turn, and obtain the reference signal sequence at the subcarriers 487 to 490 at the high frequency of each OFDM symbol. Finally, the demodulated data is sent to the channel quality evaluation module.
[0098] Step 11: In the CQI module, calculate the error rate of the reference signal sequence at 3 sub-carriers at the high-frequency positions of 41 OFDM respectively, and determine the OFDM symbols that should be reserved, and calculate the MRC rights for the copy blocks in the reserved symbols coefficient. The error rate of the reference signal sequence in each OFDM symbol is recorded as Sberi (i represents the i-th symbol):
[0099] Sberi=ErrorBitsNum/TotalBitsNum
[0100] We can get {Sber1,Sber2,Λ,Sber41}; then, compare Sberi with the threshold error rate, and keep the symbols smaller than the threshold; finally, calculate the weight coefficient corresponding to the copy block in each symbol according to the error rate : Where (λ>1). Input all the retained diversity and corresponding weight coefficients into the diversity combination module.
[0101] Step 12: In the diversity combining module, group G from the same segment i All copies of the block are merged in accordance with MRC. We assume that the channel quality of all 41 OFDM symbols is qualified and all the diversity is retained. Then, the combined segment groups can be expressed as:
[0102]
[0103] After the segments are combined, all the segments are merged into one physical block of data.
[0104] Step 13: The combined data is input to the subsequent module for processing such as channel de-interleaving, Turbo decoding, and descrambling.
