Hopping localized transmission to improve UL transmit power

Hopping localized transmission to improve UL transmit power Document Number: S 80216 m-08/171 Date Submitted: 2008 -03 -10 Source: Tom Harel Yuval Lomnitz Voice: E-mail: +972 -3 -9207175 Tom. Harel@intel. com Yuval. Lomnitz@intel. com Intel Corporation Venue: IEEE session #54, Orlando, FL. Base Contribution: None. Purpose: For discussion Notice: This document does not represent the agreed views of the IEEE 802. 16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802. 16. Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: <http: //standards. ieee. org/guides/bylaws/sect 6 -7. html#6> and <http: //standards. ieee. org/guides/opman/sect 6. html#6. 3>. Further information is located at <http: //standards. ieee. org/board/pat-material. html> and <http: //standards. ieee. org/board/pat >. 1

Introduction • This contribution proposes a way to improve UL link budget by improvement of the TX power • The information is provided for discussion only, as preparation for UL symbol structure discussions expected in next IEEE session 2

Link budget issue in the uplink • From 802. 16 m SRD, section 7. 4, Cell coverage: – “the link budget of the limiting link (e. g. DL MAP, UL bearer) of IEEE 802. 16 m shall be improved by at least 3 d. B compared to the Wireless. MAN-OFDMA Reference System. ” • One of the factors affecting UL link budget is the transmit power • Mobile TX power is limited due to the following factors: – High PAPR - large variation of the of OFDM signal envelope – Non-linear “practical” power amplifier – Constraints • Out of band emission is limited by spectral mask (varies by regulation) • Minimum EVM is needed (in-band noise limitation), dending on MCS • PA may have power consumption limitation (in addition to peak power limitation) • 802. 16 e OFDMA uplink performance is limited with respect to the downlink (TX power 23 d. Bm vs. 46 d. Bm, while maximum subchannelization gain ~12 d. B) • Maximum TX power (of lowest rate) is limited by spectral mask requirement (since EVM requirement loosens for low rates) 3

PAPR reduction methods • • • PAPR reduction techniques improve peak power The actual performance gain from PAPR reduction methods like tonereservation, tone-injection etc. is very small Reasons: – Improving the peak power doesn’t have a 1: 1 impact on the maximum TX power: • It has small effect on OOB and in-band distortion since most of them created by nonpeak signal • EVM and OOB improvement relates in a ratio of approx 1: 3 to TX power improvement (in d. B) • For example ideally limiting the OFDM amplitude to 7 d. B has ~0. 5 d. B gain in TX power (depending on model and mask) – These methods insert some overhead or loss in performance that balances some of the gain • • Clipping & filtering is an effective method to be applied in the transmitter and no standardization is needed for it, except correct definition of the EVM levels We propose to further improve the maximum TX power not by changing the signal amplitude distribution but by different use of the spectrum => “PAPR reduction” methods evaluation 4

Facing the spectral mask – localized-OFDMA • Non-linear PA causes spectral expansion of the transmitted signal. Narrower signal’s spectrum will cause narrower expansion. • We suggest to allocate narrow localized chunk of subcarriers for power limited users • This simple mechanism has very good performance, although it doesn’t change the signal’s PAPR. Original OFDM signal with OOB Narrow band signal with same spectral density Narrow band signal amplified to meet spectral mask requirement 5

Localized-OFDMA The following results show the gain obtained with actual OFDM signal and the following parameters: 23. 3 d. Bm PA model: RAPP-3 Mask: FCC & HUMAN d +7 => Spectral efficiency of localized OFDMA 30. 5 d. Bm Narrow-band OFDM signal, band center B Wide-band OFDM signal +2 d. B OFDM parameters: 10 Mhz, FFT 1024, wideband=PUSC 3 subchannels, narrowband = 72 subcarriers 25. 2 d. Bm Narrow-band OFDM signal, band edge 6

Adding frequency diversity by hopping • For high mobility user the frequency diversity gain in MIMO 2 x 2 is ~6 d. B (PUSC versus AMC) • In localized transmission we lose this diversity gain • To combine the frequency diversity of UL-PUSC with power advantage of localized OFDMA, fast frequency hoping should be applied (e. g. hop duration of 2 symbols), therefore we propose hopping localized transmission • On the other hand hopping localized requires continuous chunk of spectrum to be allocated to a single user which poses a limit on other users. • Therefore we propose to limit this type of allocation to cell-edge (power and throughput limited) users 7

Hopping localized allocations • We propose that a mix of three allocation types will be supported by the UL symbol structure: – Power limited diversity users: hoping localized (HL) allocation – Closed loop (low mobility) users: constant localized allocation (“AMC”) – High throughput diversity users: distributed allocation (similar to UL-”PUSC”) • The power boosting in HL allocation can be a function of the location in the band (maximum power can be applied to ~80% of the band, lower power in the edges) (See slide Localized gain as function of location in the band) 8

Dwell time tradeoffs • The basic allocation unit is a time-frequency rectangle. It’s size is affected by: – Large number of sub-carriers reduces the maximum sub-channelization gain, therefore span maximum time (e. g. 2 subframes) minimum frequency – Given the frequency width, the tradeoff on dwell time: • Small dwell time => more hops, more diversity • Large dwell time => higher pilot efficiency • Recommended parameters: – A hop per 2 symbols yields an optimum point between pilot loss and diversity loss, assuming TTI=2 subframes – Having 6 hops within a frame yields reasonable frequency diversity (assuming interleaving over time). – Assuming UL transmission may span TTI=2 subframes, we assume 3 hops per subframe, i. e. hop every 2 symbols – This yields a tile of e. g. 9 x 2, 12 x 2 or 18 x 2 which has reasonable pilot efficiency 9

Backup 10

Methodology of transmission methods comparison • Methodology – – Generate sample signals Compress them in PA model Measure spectrum and EVM Estimate performance and compare different methods: • Maximum TX power • EVM dependence on TX power • SE versus link margin 11

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“PAPR reduction” methods evaluation EVM comparison 50 45 40 EVM [d. B] 35 30 OFDM, Max effective TX power=23. 75 d. Bm Clip&filt@TD, #iter=5, rejection=15 d. B, filt length=105, clip thres=4. 0 d. B, Max effective TX power=25. 14 d. Bm Tone-resevation, Nreserved=140, clip thres=4. 0 d. B, Max effective TX power=24. 65 d. Bm Tone-resevation w distortion, Nreserved=140, clip thres=4. 0 d. B, Dstrt ratio=0. 05, Max effective TX power=25. 09 d. Bm 25 Tone-injection, offset=2. 828427 e+000, clip thres=2. 5 d. B, clip gain=1. 2, Max effective TX power=24. 26 d. Bm 20 15 17 18 19 20 21 22 Effective Pout [d. Bm] 23 24 25 26 13

“PAPR reduction” methods evaluation SE [b/s/hz] Spectral efficiency vs. link loss Link loss [d. B] 14

Spectral efficiency of localized OFDMA 15

Localized gain as function of location in the band center edge Signal bandwidth 16

Power limited PA • Our results are with PA peak power limit (or fixed Vcc). • Another option is to consider PA with a current limitation (modify Vcc to meet same power). • In this case all differences in transmit powers are approximately halved (in d. B) 1 Increase signal power while keeping constant Vcc, therefore reduce backoff Slope =2 2 Scale signal power and Vcc together to keep constant backoff, that is keep constant proportion between signal and out-of-band emission. This can be associated with relative spectral mask. Slope =1 3 Scale signal power while keeping the out of band emission power approximately constant. Slope = 0. 8 17