doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 1 Generalized Puncturing to Eliminate Pad Bits in MIMO-OFDM n Ravi Mahadevappa, Stephan ten Brink, Realtek Semiconductors, Irvine, CA

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 2 Pad Bits in a More Pad Bits in n Ways to Eliminate Pad Bits Expected Gains by Eliminating Pad Bits Generalized Puncturing Overview

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 3 Excerpt from a PHY Standard Pad Bits in a pad bits are zeros and ignored at receiver …

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 4 In a, max. number of coded bits per OFDM symbol is N CBPS = 48*6 = 288 In a possible n system, owing to higher order modulation (e.g. 256QAM) and multiple transmit antennas (e.g. N T =3), the number of bits per OFDM symbol will be several times larger than in a –Coded bits per OFDM symbol: N CBPS = 48*M*N T Say, max. 256QAM, M=8 bits per symbol Max. number of transmit antennas N T =4 N CBPS = 1536 bits –Other example: M=6 (64QAM), N T =3, N CBPS = 864 Moreover, space/time codes might require the number of OFDM symbols to be an integer multiple of 2 (or 3, or 4), and thus, potentially more pad bits needed Another possible reason for more pad bits: Interleaving over several OFDM symbols (e.g. over 6 OFDM symbols) More Pad Bits in n

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 5 Instead of padding N PAD useless zero-bits, transmit useful information (code redundancy) –Repeat a fragment of length N PAD of the message Disadvantage: requires buffering, uneven distribution of redundancy –Adapt (i.e. reduce) the code rate R=K/N to new code rate R n =K/(N+N PAD ) Evenly distribute the added redundancy, all message bits benefit For convolutional codes, use a simple, generalized puncturing scheme to implement the new code rate R n From code rate R and message length (conveyed in SIGNAL-field of packet preamble), new code rate R n can be computed by receiver (from number of pad bits N PAD ); no need for additional signaling of new code rate Ways to Eliminate Pad Bits

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 6 Typical Example: –Message length (info bits) N Li = 6000bits –Code rate R=3/4, coded message length N Lc =8000 –64QAM, M=6, N T =3, N CBPS = 48*M*N T = 864 –N PAD = N CBPS - (N LC modulo N CBPS ) = 640 –New code rate R n = 6000/( ) = … Expected Gains by Eliminating Pad Bits

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 7 From rate 3/4 to rate by generalized puncturing For the 3x4 example, gain is about 1.2dB, simply by using coded bits instead of zero pad bits Expected Gains by Eliminating Pad Bits

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 8 Let the mother code rate be R m (in a: R m =1/2, convolutional code) R is the code rate originally selected by the transmitter –drawn from discrete set of code rates, e.g. R=1/2, 2/3, 3/4, 7/8, for protection/data rate trade-off For R m <=R n <=R: –some of the punctured bits need to be “unpunctured” to reduce the code rate from R to R n For R n <R m : –All punctured bits need to be unpunctured; moreover, some bits need to be repeated Generalized Puncturing

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 9 One very simple implementation (out of many): –Coded message length n (encoded using mother code rate R m ) –New coded message length n_new=n+n_pad Mother code rate Rm new code rate Rn=Rm*n/n_new –Routine returns a repetition/puncturing pattern specifying number of repetitions (or ‘0’, if to puncture) of the bits of the original coded message of length n (indices from 1 to n) –MATLAB: function rep_pct_pattern = gen_puncture(n_new,n) new_seq = ceil((1:n)*n_new/n); rep_pct_pattern = diff([0 new_seq]); A Generalized Puncturing Scheme

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 10 Let the mother code rate be R m =1/2, with coded outputs (c 1,c 2 ) per input bit Rate 3/4, puncturing as usual: c 1 : … c 2 : … Rate … = 6000/8640: c 1 : … c 2 : … Rate … (below mother code rate R m, repetitions needed): c 1 : … c 2 : … Generalized Puncturing Examples

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 11 To make the packet length a multiple of the length of a single OFDM symbol N CBPS, pad bits (“dummy bits”) are used in a With increasing data rate in MIMO-OFDM n, there will be more pad bits on average Replacing pad bits by coded (redundant) bits improves performance without using extra resources (bandwidth); it comes for free A simple generalized puncturing scheme was presented to perform this task Summary

doc.: IEEE /298r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 12 Some References [1]IEEE Std a-1999, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, High-speed Physical Layer in the 5 GHz Band