On the Efficiency of TGnSync Preambles David Tung, dtung@ralinktech.com Tom Pare, tpare@ralinktech.com Ralink Technology

Overview A short review of legacy 802.11 preambles shows that it is important to keep the preamble design as efficient as possible. We developed a method to quantify the performance impact of TGn Sync’s longer preamble design. Compared to WWiSE’s greenfield preamble design, TGn Sync’s preambles degrade performance as high as 6~14.7 dB for 1536 bytes DATA packet. We conclude that greenfield preamble is necessary for all TGn proposals.

Legacy Preambles

Proposed 11n Preambles

Preambles Review Legacy 802.11 preambles provide low overhead to achieve ~50% efficiency (throughput/PHY rate) under typical TCP/IP traffic. One of the biggest challenges in 11n preambles design is to keep it as efficient (short) as possible. Aggregation techniques will be helpful, however penalty will still be high in many cases without careful optimization. Long 11n preamble will show major penalty with smaller data packets or higher number of TX streams.

Evaluation Method Calculate throughput using different preamble lengths with bounded overheads. Given a preamble length (P us), symbol length (S us) and PHY rate (R Mbps) find the corresponding throughput (T Mbps) with a selected overhead (H us) and packet Length (L bytes) , the formula of throughput is simply: T = L*8/[L*8/R+P+H]S, where [.]S is a rounding up to multiple of S. Find the corresponding PHY rates for the same throughput with different preamble lengths.

Evaluation Method (cont’d) Convert the difference of PHY rates (with the same throughput) into penalty in dB. Assume the PHY rates are X and Y (X > Y), assume rate X with B bits per sub-carrier. The performance difference (D db) is estimated to be : D = (X-Y)/X*B*3. For example performance differences of 54 Mbps and 48, 36, 24, 18, 12, 9, 6 Mbps are estimated to be 2 (1), 6 (5), 10 (9) , 12 (12), 14 (14), 15 (16), 16 (17) dB, where (.) are 11a spec.

Penalty with 2 TX Streams

Penalty with 4 TX Streams

TGnSync Penalties Table

Conclusions The performance penalties of longer TGn Sync preambles are as high as 6~14.7 dB for 1536 bytes DATA packet. Greenfield preamble is a must for all TGn proposals.

Backups

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