We shall consider how to combine the proposed modifications. - PowerPoint PPT Presentation

November 1998 doc.: IEEE 802.11-98/369a IEEE P802.11 Wireless LANs TGa Preamble Improvement Date: November 11, 1998 Author: Tal Kaitz and Naftali Chayat BreezeCOM Atidim Technology Park, Tel Aviv 61131 Israel Phone: 972 –3-6456262 Fax: 972-3-6546290 e-Mail: {Talk, Naftalic}@ Breezecom.co.il Preamble Improvement for Tga • We shall consider the two issues: • An improvement to the channel estimation section. • Two methods of increasing the robustness of the rate- signaling field. • We shall consider how to combine the proposed modifications. Submission page 1 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a Current Preamble Structure Fine Frequency Offset 8.0 µ s µ s 0.8 3.2 µ s µ s 0.8 t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 Data GI T1 T2 AGC Channel Estimation Coarse Frequency Offset SIGNAL and Timing 17.6 µ s Figure 1 Functions: • AGC tuning: t1…t3. • Coarse frequency estimation: t5 and t6. • Fine frequency: t6 and t9 • Channel estimation by T1 and T2. • Rate signaling by t11 and t12. Submission page 2 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a Channel Estimation Improvement • The functions of fine frequency estimation and channel estimation can be unified to allow a more efficient structure as shown in figure 2. 0.8+3*3.2=10.4 µ s µ s 3.2 µ s 0.8 µ s 0.8 t1 t2 t3 t4 t5 t6 t7 GI T1 T2 T3 Rate signalling AGC Coarse Frequency Offset Channel estimation and fine frequency offset SIGNAL and Timing 17.6 µ s Figure 2 • The channel estimation now consists of 3 long sequences. • Averaging over 3 results in a more accurate estimation Submission page 3 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a • Fine frequency estimation is performed by comparing the phase of the T1 sequence to that of T3 by means of a “dot product” • Slightly better frequency estimation SNR. Simulation results Trms =150nSec 100Bytes Performance in fading channels Trms=150nsec 0 10 Legend : Black: New proposal -1 per 10 Blue: Current proposal -2 10 10 11 12 13 14 15 16 17 18 EbNo A performance gain of about 1dB is apparent. Submission page 4 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a Rate Signaling Improvement • Rate signalling is performed by QPSK modulation of the short sequences t11 and t12. • Basic requirement: signalling scheme should as reliable as the lowest rate data (6Mb/s BPSK OFDM). • This is the case for AWGN channels: Each rate field bit carries the energy of 3 data bits. Accounting for 5dB coding gain for data bits we have the same probability of error. • • However : Not the case for severe multipath conditions: Error rate of rate field and 6Mb/s data (100bytes frame size) 0.4 0.35 Legend : 0.3 Green: Rate field error rate 0.25 Error rate Blue: Data frame rate 0.2 • Problem: Interference from 0.15 adjacent symbols due to long 0.1 impulse response. 0.05 • We shall consider two solutions. 0 200 250 300 350 400 450 500 550 600 Trms [nSec] Submission page 5 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a 1. Adding a dedicated BPSK –OFDM symbol 2. Modifying phase assignment to QPSK symbols. 1. Adding a dedicated BPSK-OFDM symbol • 24 bit encoded and modulated as in the 6Mb/s mode. • Bit assignment: 2 bits random scrambling 4 bits rate 12 bits duration field 6 bits CRC • The duration field allows receiving units to assert a channel busy condition for the duration of the packet even if the unit is incapable to receive the PLCP header. Submission page 6 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a • A “tail-bite” encoding mode is used in which trellis termination is achieved without incurring any overhead. This is performed by initiating the encoder registers with the last bits of the block. Decoding is performed by cyclically pushing the data stream into the VA. Extra bits should be entered to recover from unknown initial and final state. • The CRC is extended hamming code (5 check bits +parity) capable of detecting up to 3 error bits. Submission page 7 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a 2. Better phase assignments • Most of the distortion is from preceding symbols. • Basic idea: Assign the phases {0,0} the 6Mb/s case and decode by comparing the phase of t10 to that of t11 t12. • Channel effects will be identical in t10 and t11 and t12 and will cancel out. • For other low rate assign the phases {0, exp(pi*j*n/2)} How to combine ? 6Mb/s and 12Mb/s data and rate signaling errors Legend 0.14 6Mb/s : Black =rate error 0.12 We have 4 options: Blue = data error 0.1 A. New channel 12Mb/s : Green =rate error 0.08 estimation and a dedicated error rate Red = data error rate symbol: 0.06 0.04 0.02 0 200 250 300 350 400 450 500 550 600 Submission Trms nsec page 8 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a 0.8+3*3.2=10.4 µ s 3.2 µ s µ s 0.8 t1 t2 t7 Rate signalling GI T1 T2 T3 Channel estimation and fine frequency offset SIGNAL µ s 20 Overall length 20uSec. Pros: • Robustness both in channel estimation and rate-signalling. • Duration field decodable by all stations Cons: High complexity and overhead A. New channel estimation and QPSK rate signalling Note that an extra guard interval (t8) should be added. Submission page 9 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a 0.8+3*3.2=10.4 µ s 3.2 µ s µ s 0.8 t8 t1 t2 t7 t9 t10 GI T1 T2 T3 SIGNAL Channel estimation and fine frequency offset 18.4 µ s \\ Overall length 18.4uSec cons: Less robust rate signalling B. Current channel estimation with a dedicated OFDM symbol Overall length is 20uSec C. Current channel estimation with QPSK symbols. Overall length is 17.6uSec Conclusions Submission page 10 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a • Improvement to channel estimation section • Two method of increasing rate field robustness. • Several ways of combining the two elements Submission page 11 Tal Kaitz and Naftali Chayat, BreezeCOM

November 1998 doc.: IEEE 802.11-98/369a Submission page 12 Tal Kaitz and Naftali Chayat, BreezeCOM