Preamble Design for 802.16a/b SISO and MIMO OFDM Systems IEEE 802.16 Presentation Submission Template (Rev. 8.21) Document Number: IEEE 802.16abc-01/55 Date Submitted: 2001-11-07 Source: Apurva N. Mody Voice: 404-894-9370 Georgia Institute of Technology Fax: 404-894-7883 250, 14th Street NW, #549 E-mail: [email protected] Atlanta, GA 30318. Venue: IEEE 802.16, Meeting 16, Austin, TX, U. S. A. Base Document: Purpose: For posting on the 802.16 website, presentation to the preamble ad-hoc and TG3/4. Notice: This document has been prepared to assist IEEE 802.16. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. 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Please notify the Chair <mailto:[email protected]> as early as possible, in written or electronic form, of any patents (granted or under application) that may cover technology that is under consideration by or has been approved by IEEE 802.16. The Chair will disclose this notification via the IEEE 802.16 web site <http://ieee802.org/16/ipr/patents/notices>. PPrreeaammbbllee DDeessiiggnn ffoorr 880022..1166 SSIISSOO aanndd MMIIMMOO OOFFDDMM SSyysstteemmss ((TTGG 33//44)) AAppuurrvvaa NN.. MMooddyy GGeeoorrggiiaa IInnssttiittuuttee ooff TTeecchhnnoollooggyy AAttllaannttaa,, GGeeoorrggiiaa,, UU.. SS.. AA.. aappuurrvvaa@@eeccee..ggaatteecchh..eedduu Efficient Preamble Design for OFDM Systems Good correlation properties, • Low PAPR, • Suitable for parameter estimation, • Suitable for frequency offset estimation over • a wide range, Low computational complexity and low • overhead. Use of Short Periodic Sequences Short sequences have following advantages • (cid:1) They help in rapid synchronization because of shorter length, (cid:1) They increase the frequency offset estimation range, (cid:1) If length of the channel is less than the short sequences, they can be used to estimate the channel. Disadvantages of the short sequences • (cid:1) They increase the system overhead, (cid:1) Susceptible to multipath and noise. Length of the Sequence Needed for Channel Estimation Two back to back similar sequences of length greater • than or equal to the length of the channel are sufficient for any DFT based channel estimation. [4 (Tellambura), 5 (Manton)] S (GT) S (GT) 1 1 T = Sample time, G = Number of taps in the guard N = OFDM Blocksize Linear convolution becomes a circular convolution • and DFT based techniques can be applied Expected Channel Lengths for the Proposed System Number of taps BW=3.5 MHz BW=7 MHz f =4.0832 MHz f =8.1664 MHz s s SUI-4 17 34 SUI-5 42 83 SUI-6 83 164 Note: SUI6 channel is highly dispersive. Effect of Insufficient Guard Interval in OFDM Insufficient guard interval (cid:1) causes ISI, Inter Carrier Tone Interferece (ICI ) and Intra 1 Carrier Interference (ICI ) k+1 Y i-1 Y i Y i+1 ISI 2 k+1 k+1 k+1 Ways to mitigate highly ICI 1 dispersive channels k Y i-1 Y i Y i+1 k k k ICI Blocksize of the OFDM can be 2 (cid:1) increased (256->512) such k-1 Y i-1 Y i Y i+1 k-1 k-1 k-1 that it permits the use of a Time larger guard interval (64- >128). i-1 i i+1 Impulse response shortening (cid:1) techniques can be used to combat insufficient guard interval (SUI6). One OFDM Symbol Long Preambles that Allow for Greater Frequency Estimation Range SCHEME 1. Frequency offset SCHEME 2. Frequency offset estimation range = 4/(NT) or 4 estimation range = N /(NT) or N s s subcarrier spacings. Better frequency subcarrier spacings. Larger frequency offset estimates. WILL WORK FOR estimation error. WILL WORK FOR SUI6 IF N=512. SUI6 IF N=512. Sequence Construction SCHEME 1 • (cid:1) S represents a sequence in the frequency domain. 1 (cid:1) For NT/4 = 64, S =[0 0 0 0 0 0 0 1 1 1 1 -1 -1 -1 -1 -1 1 1 -1 -1 1 -1 -1 1 1 -1 1 -1 -1 1 1 -1 1 0 -1 1 -1 1 -1 1 1 1 -1 1 -1 -1 1 -1 1 1 -1 -1 -1 1 -1 1 -1 -1 1 0 0 0 0 0 0] — PAPR=3.15 dB. (cid:1) An NT/4 point IFFT is taken to form s and the resultant 1 sequence is used to construct the preamble. (cid:1) The preamble is constructed as [s s s s ] 1 1 1 1 in the time domain and a suitable guard interval is inserted as a cyclic prefix. Sequence Construction (Contnd.) SCHEME 2 • (cid:1) S is as in Scheme 1, S is an N/Ns point sequence in the 1 2 frequency domain. (cid:1) For Ns = 8, S =[0 0 0 0 -1 1 -1 -1 1 -1 -1 1 1 1 1 -1 0 -1 1 1 1 -1 1 1 1 -1 2 1 1 1 0 0 0 ] — PAPR=2.355d B. (cid:1) For Ns = 16, S =[0 0 -1 -1 1 1 1 1 0 1 -1 1 -1 -1 1 0 ] — PAPR=1.76 2 dB. (cid:1) N/Ns point IFFT is taken to obtain s 2 (cid:1) The preamble is constructed as [s . s s s s .. s ] 2 2 1 1 2 2 and a suitable guard interval is inserted as a cyclic prefix.