Sept 2003 doc.: IEEE 15-03-0336-00-003a Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Techniques for MB-OFDM improvement Date Submitted: 5 September 2003 Source: Mitsuhiro Suzuki, Chihiro Fujita, Michio Yotsuya, Kazuhisa Takamura, Takashi Usui Bob Huang Company: Sony Corporation Sony Electronics of America Address: 6-7-35 Kitashinagawa Shinagawa-ku,Tokyo. Japan 141-0001 One Sony Drive TA-1 Voice: +81-3-6409-3201, FAX: +81-3-6409-3203 Park Ridge, NJ 07656 E-Mail: suzuki@wcs.sony.co.jp, chihiro@ wcs.sony.co.jp, V: 201-358-4409 yotuya@ wcs.sony.co.jp, takamura@wcs.sony.co.jp, F: 201-930-6397 usui@ wcs.sony.co.jp EMail: robert.huang@am.sony.com Re: none Abstract: This presentation introduces the unique techniques for MB-OFDM , ranging, null prefix, preamble waveform, coding, tracking Purpose: Technical contribution to MB-OFDM proposal. Notice: This document has been prepared to assist the IEEE P802.15. 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. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15. Submission Slide 1 Suzuki, et al, Sony Corp., Sony Electronics

Techniques for MB-OFDM improvement Sept 2003 Ubiquitous Technology Labs Sony Corporation

Contents 1. Ranging Techniques -Clocking burst transmission -Coherent sub-banding -Frequency domain smoothing 2. Modified Tx preamble waveform 3. Null prefix for OFDM 4. Coding scheme 5. Tracking method without pilot

Clocking Burst Transmission (How to measure the turn around time)

Clocking Transmission Mechanism Define clocking period T which is longer than 2x(Max propagation delay) Device A start to clock. Device A transmit signal to Device B at the clocking timing. Device B detect the signal arrival time from A. Device B start to clock from the arrival time. Device B transmit to Device A at the clocking timing. Device A detect the signal arrival time from B. Time lag between the arrival time and the last clocking timing is 2x(Propagation delay)

In MB-OFDM case, T which is same as time slot length ( 312 In MB-OFDM case, T which is same as time slot length ( 312.5[ns] ) is convenient. Physical layer system closed (with no MAC help ) No negotiation is necessary.

(How to improve measuring precision) Coherent sub-banding (How to improve measuring precision)

Current problem for ranging Frequency characteristics of each band can be obtained. However, no phase coherency between each band. Ranging resolution is determined by each bandwidth (528[MHz]).

Solution for increase measured bandwidth 1. Define reference time of t0 during the burst. ( It is the best that t0 is set to head of the channel estimation part.) 2. Define phase of each carrier set to 0[deg]. Phase coherency between each band is achieved. (even if the difference between t0 of Tx and t’0 of Rx exists.)

Setting phase of each carrier Fortunately, every frequency can be synthesized from one oscillator (4224[MHz]). All carrier’s phase is set to 0[deg] at the time of 264[MHz] phase 0[deg]

Frequency domain smoothing (How to reduce complexity to calculate time response from frequency response )

Current problem of calculate time response Large size FFT is needed ( In 3 band case, 128x3=384 )

Frequency domain smoothing and decimation For the purpose of ranging, time response around time 0 is important. Therefore, smoothing and decimation in frequency domain is allowed. FFT size can be reduced.

Conclusion for ranging techniques. Turn around time can be measured only by physical layer implementation by clocking burst transmission. 1.5[GHz] bandwidth channel response measuring is possible by coherent sub-banding (3 band case) Calculation complexity can be reduced by frequency domain smoothing.

Tx preamble waveform improvement

Current preamble (time domain) Hierarchical code is convenient to reduce complexity Preamble No.1 A little bit bad auto-correlation Auto-correlation

Current preamble (frequency domain) Power spectrum shape is not flat. (Tx power have to be reduced 5[dB]!!! in FCC rule point of view) Preamble part power spectrum Power spectrum Shape is different from OFDM part. How to shape the spectrum into 500[MHz] with 528 Mchip/s chip rate ? OFDM part power spectrum

Modification of Tx Preamble ( original preamble ) force to set amplitude = 0 force to set amplitude = 1 keeping phase information Preamble at Tx side is regarded as OFDM signal which symbols are in frequency domain. Power Spectrum shape is (of course) same as OFDM. Power spectrum is (of course) flat. Modified power spectrum

Preamble for Tx(red) and Rx(blue) In Rx side, original preamble is used Tx preamble is no longer binary… ( still real number) Original binary preamble is used at the receiver not to increase correlation complexity Preamble for Tx(red) and Rx(blue) Correlation characteristics improves. ( approx. 3[dB] ) Cross-correlation

Conclusion for Tx preamble improvement Preamble at Tx: amplitude modification in frequency domain. same power spectrum as OFDM no-necessity to care about extra spectrum shaping flat power spectrum and allowed max Tx power. Preamble at Rx: same as original preamble not increase correlation complexity better correlation performance.

OFDM with null prefix

Conventional cyclic prefix GI Cyclic prefix is to avoid ICI ( Inter-subCarrier-Interference)

Proposed null prefix Null prefix (or postfix) OFDM can also avoid ICI by cyclic adding at the receiver.

Necessary processing for null prefix Need detecting multi-path time dispersion ( TMP ) ( This may be done during CCA? ) Need ADC during TMP longer period. Need cyclic adding during TMP period.

Null prefix case, the receiver will pick up same noise Picked up noise Null prefix case, the receiver will pick up same noise or (TEM+TMP)/TEM larger noise. (Depend on multi-path time dispersion: 0 < TMP < TGI ) However,,,

Tx power can be increased (prefix energy consumption aspect ) UWB Tx power is specified by power density. Null prefix OFDM can have (TEM+TGI)/TEM higher Tx power because it does not spend the energy for “prefix”. TGI>TMP (typical): S/N at receiver improves. TGI=TMP (worst) : S/N is same as cyclic prefix.

Tx spectrum shape will be flat Cyclic prefix generates ripple in spectrum. Null prefix generates no ripple in spectrum.

Tx power can be increased (power density in MHz aspect) UWB Tx power is specified by power density in MHz Null prefix OFDM is allowed (TEM+TGI)/TEM higher Tx power keeping FCC spectrum regulatory. Null prefix has higher allowable Tx power when sub-carrier spacing is not less than 1[MHz]

Conclusion for null prefix Null prefix improves from 2[dB] to 1[dB] link performance. Null prefix can be applied to modified Tx preamble

Coding scheme

Coding scheme Decoder power consumption can not be neglected. Decoding latency is not so critical in payload part. K = 7 convolutional coding (current ) low latency, but high power consumption not so good BER/PER K = 4 CC + RS(255,239) concatenation (proposed) K = 4 gate count is 1/8 smaller than K=7 K = 4 power consumption is 1/8 smaller than K=7 RS(255,239) gate count is comparable to K=7 RS(255,239) power consumption is 1/8 smaller than K=7

Performance comparison To be prepared

Conclusion for coding scheme. K=4 CC + RS concatenation a few dB better BER/PER performance than K=7 gate count is comparable to K=7 power consumption is ¼ lower than K=7 All of considerable coding scheme should be studied. Low power consumption coding scheme is desired as mandatory.

Tracking method without pilot

Synchronization situation Fortunately, sub-carrier of OFDM and center frequency are synchronized. Modulation timing error and carrier phase error are synchronized Timing error ( carrier phase error ) is caused by reference frequency difference between Tx and Rx.

Rotation in frequency domain OFDM signal is described. OFDM with timing error is described. Timing error is observed rotation in symbol on each sub-carrier. The phase value of rotation is represented by center frequency, because bandwidth is small compared to center frequency.

Tracking method block diagram Timing error detection part (red) is very conventional Pilot is not necessary

Conclusion for tracking using pilot More precise timing error detection by using many (122) sub-carrier information. (current number of pilot symbol is 12) 0.5[dB] (=112/100) link performance will be improved because of no energy lost for pilot. Coding rate for high bit rate (e.g. 480[Mbps]@r=3/4 ) can be decreased and improve Eb/No performance. -More link performance will be improved. -SOP performance will be improved Pilot is not necessary ( The idea is under qualitative consideration. Quantitative value by simulation is needed. )

Total conclusion This document introduced 1. Ranging Techniques -Clocking burst transmission -Coherent sub-banding -Frequency domain smoothing 2. Modified Tx preamble waveform 3. Null prefix for OFDM 4. Coding scheme 5.Tracking method without pilot These achieve better performance in MB-OFDM system

THE END OF SLIDES Thank you!