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On the Multiple Access Schemes for IEEE 802

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1 On the Multiple Access Schemes for IEEE 802
On the Multiple Access Schemes for IEEE m: Comparison of SC-FDMA and OFDMA Document Number: C802.16m-08/045r1 Date Submitted: Jan 24, 2008 Source: Yang-Seok Choi Intel corp Rongzhen Yang Intel corp Jiacheng Wang Intel corp Tom Harel Intel corp Yuval Lomnitz Intel corp Hujun Yin Intel corp Venue: TGm Call for contribution on SDD, Levi, Finland Base Contribution: C80216m-08/045r1 Purpose: For discussion of comparison between OFDMA and SC-FDMA, and approval of OFDMA system by IEEE Working Group Notice: This document does not represent the agreed views of the IEEE 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 Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: <http://standards.ieee.org/guides/bylaws/sect6-7.html#6> and <http://standards.ieee.org/guides/opman/sect6.html#6.3>. Further information is located at <http://standards.ieee.org/board/pat/pat-material.html> and <http://standards.ieee.org/board/pat >.

2 SC-FDMA structure and Link level comparison

3 Signal at each subcarrier is a linear combination of all M symbols
SC-FDMA TX Structure Spreading by DFT Signal at each subcarrier is a linear combination of all M symbols ~2 dB gain in PAPR Signal at each subcarrier is a linear combination of all M symbols Low PAPR Low PAPR High PAPR Duality

4 OFDMA in SISO Channel Matrix : Received signal after FFT
The channel matrix H is “orthogonal” even in frequency selective channel No inter-carrier interference No ISI due to CP One-tap linear equalizer is sufficient Channel Matrix :

5 SC-FDMA in SISO Spreading Matrix : Channel Matrix :
Received signal after FFT The channel matrix is NOT “orthogonal” in frequency selective channel Inter-subcarrier interference due to the spreading matrix No ISI due to CP Channel Matrix :

6 SC-FDMA in SISO (cont’d)
MMSE Equalizer One tap equalizer followed by De-spreading Equalizer output :

7 SC-FDMA in SISO (cont’d)
Post-MMSE SINR OFDMA: SC-FDMA : From above

8 SC-FDMA in SISO (cont’d)
Note is a harmonic mean of Thus, where the equality holds if and only if is constant regardless of l (i.e. flat fading) is constant irrespective of m  “Steeper PER curve”, “Diversity gain” As delay spread increases,  “PER curve moves to right” Longer delay spread at Cell edge As M increases, becomes smaller in frequency selective channel Note As delay spread and/or M increase, the loss of SC-FDMA in link level will be more evident

9 Link-Level Simulation Results
In frequency-selective fading, the loss in LL is noticeable due to loss of orthogonality Delay spread=CP, rms delay spread=CP/4, exponential decaying delay profile ½ convolution code, Distributed mapping SC-FDMA 3 dB loss OFDMA

10 MIMO Use of maximum likelihood detector (MLD) receiver In large eigen-value spread channel, MMSE does not provide MIMO gain fast MLD for 2x2 and 4x4 MIMO available Virtual MIMO, MU-MIMO in UL Significant Gain Other fast MLD

11 KxK MIMO (cont’d) OFDMA : block diagonal matrix
No inter-carrier interference —KxK MLD per subcarrier SC-FDMA: Not a block diagonal matrix Inter-carrier interference Can’t apply per-subcarrier MLD — Need KMxKM MLD (not feasible) K x K matrix

12 Large PAPR in Frequency Domain
After spreading : x can be modeled as Gaussian random variable means high PAPR in frequency domain SC-FDMA has larger PAPR in frequency domain Out-of-band emission Though Average ICI power and OOBE are the same as in OFDMA, Larger fluctuation of instantaneous OOB Emission causes worse interference to adjacent carrier In-band fluctuation Larger ICI power variance in time-varying channel

13 ICI ICI component at k-th subcarrier : ICI power :
4th order moment (assuming flat channel) Variance of ICI power OFDMA SC-FDMA QPSK 1 2 16QAM 1.32 64QAM 1.381

14 ICI (cont’d) Normalized variance of ICI power
SC-FDMA exhibits higher fluctuation of ICI power

15 TX power improvement of SC-FDMA

16 Simulation assumptions
QPSK modulation WiMAX frequency assignment (BW=10 MHz, Nfft=1024, Nused=841, SamplingFactor=28/25) PA model: Rapp-3, saturation power 31dBm Spectral mask FCC BRS (absolute) and ETSI Mobile (relative) WiMAX UL permutation: Distributed (WiMAX-I PUSC), 3 subchannels Localized (WiMAX-I AMC) SC-FDMA modes Distributed diversity mode Localized (adopted in LTE) TX power shown is the maximum TX power that can be attained with the above PA parameters while obeying FCC masks

17 Transmit power and consumed power
Higher TX power with same 1dB compression point, implies higher power consumption Thus, even if with same PA settings, a certain TX power improvement is shown, it is not always feasible due to power consumption A possible fair normalization is to keep the consumed power constant (by changing Vcc of the PA), and measure the improvement in TX power for the same consumed power With Class-AB amplifier the consumed power is approximately proportional to Therefore if results show N-dB improvement, there is also N/2-dB penalty in consumed power In order to normalize to the same consumed power while keeping a constant backoff, the improvement is halved (i.e. the improvement will be N/2-dB) Maximize Coverage area and Battery life

18 PUSC (distributed OFDMA)
TX power improvement PUSC (distributed OFDMA) 23.3 dBm Distributed SC-FDMA 25.7 dBm AMC (localized OFDMA) 25.2 dBm Localized SC-FDMA 26.1 dBm

19 TX power improvement (contd.)
Subcarrier Mapping Gain of SC-FDMA over OFDMA under same PA size Gain of SC-FDMA over OFDMA under same power consumption and backoff Distributed 2.4 dB 1.2 dB edge 0.9 dB 0.45 dB center 0.04 dB 0.02 dB No difference in maximum TX power if resource is allocated at band center 30.53 dBm 30.57 dBm Localized SC-FDMA and OFDMA Centered in band

20 Modeling PAPR/CM not accurate metric Rather, Indirect method
Need to consider OOBE, EVM requirement, power consumption, and Multipath Effect together Rather, Pass to RF filter PA Check OOBE and EVM requirement Adjust Tx power Channel Check PER/Coverage

21 Block Diagram of Joint Simulation
Joint simulation (PA model+link level) : captures the effects of non-linear distortion and subchannelization gain

22 Path loss Received signal power
1dB compression point backoff Tx antenna gain Rx antenna gain Path loss With 90% availability of shadow fading ( 8dB standard deviation) Path loss : Urban Macro

23 Simulation conditions
1dB compression: 31dBm (assume the same PA size) Power amplifier backoff: depend on DFT size, subcarrier location, etc. Tx antenna gain: 0dBi Rx antenna gain (include cable loss): 15dBi Carrier frequency: 2.5 GHz System bandwidth: 10MHz Noise figure: 5dB FFT size: 1024 ½ CTC QPSK Channel Model: Urban Macro-cell in 16m EVM document 1x1 SISO/2x2 MIMO (SM, vertical coding) Antenna spacing: Tx = 0.5 lambda, Rx = 4.0 lambda Packet length = 120 bytes SC-FDMA, localized, 32/64/128 DFT OFDMA, localized, 32/64/128 used sub-carriers Band edge and center Rapp power amplifier model, p=2.0 8 times over-sampling 193-order low-pass filter, cut-off frequency = 0.9 Equalization: MMSE (MLD for 2x2 MIMO, OFDMA) EVM noise and Backoff tradeoff not optimized Non-ideal channel estimation non-linear distortion effect (i.e. EVM noise) is not included

24 OFDMA vs. SC-FDMA: 1x1 SISO, band edge
Equalizer loss in SC-FDMA is more dominant than the effect of larger EVM noise and backoff in OFDMA SC-FDMA has higher power consumption due to smaller backoff under identical PA

25 OFDMA vs. SC-FDMA: PER CDF @ SNR = 5dB

26 OFDMA vs. SC-FDMA: 2x2 MIMO, band edge
MMSE : with larger M, SC-FDMA is better In correlated MIMO channel the Interstream interference effect is more dominant than the equalizer loss in SC-FDMA The crossing point moves to higher SNR. At higher SNR, choose higher MCS with lower M MLD : the gain of MLD is noticeable In cell edge, STBC will be chosen highly likely. In this case, OFDMA will be better than SC-FDMA as in SISO case

27 OFDMA vs. SC-FDMA: 2x2 MIMO, band edge
MMSE, 16QAM ½ OFDMA outperforms

28 OFDMA vs. SC-FDMA: 1x1 SISO, band center

29 OFDMA vs. SC-FDMA: 2x2 MIMO, band center
Higher EVM noise Need to optimize tradeoff of Backoff and EVM noise

30 Duality OFDMA SC-FDMA PAPR High in Time Low in Frequency Low in Time
High in Frequency Spreading Data spread in Time Data localized in Frequency Data spread in Frequency Data localized in Time

31 Duality (cont’d) - PAPR
PAPR in TD High in OFDMA Smaller Tx power : Due to higher PAPR, more back-off needed. However, by scheduling the resource at the center of band, no difference compared with SC-FDMA is observed Higher EVM noise : Due to higher PAPR, more non-linear distortion observed. However, in cell edge the thermal noise is dominant. In addition, the operating SNR of OFDMA is lower due to no equalizer loss and advanced receiver such as MLD. Thus, the impact of EVM noise is negligible. Low in SC-FDMA Larger Tx power PAPR in FD Low in OFDMA Smaller fluctuation of OOBE Smaller fluctuation of ICI power High in SC-FDMA Higher instantaneous OOBE Larger variance of out-of-band power : More ACI to neighboring systems Higher instantaneous ICI power in time varying channel Larger variance of ICI power

32 Duality (cont’d) - Spreading
Spreading in TD Data spread in TD in OFDMA More robust to impulse noise and nonlinear distortion Data localized in TD in SC-FDMA More susceptible to impulse noise and nonlinear distortion Spreading in FD Data localized in FD in OFDMA Less frequency diversity Data spread in FD in SC-FDMA More frequency diversity Steeper PER curve Equalizer loss PER curve moves to right

33 Pilot design OFDMA SCFDMA
Two dimensional pilot allocation : Time and Frequency More flexible and potentially lower pilot overhead SCFDMA Only Time domain : pilot dedicated symbol Does not allow mix of data and pilot subcarrier Can’t optimize the pilot design

34 TDD Duplex Scheme OFDMA can be straightforwardly used to exploit the TDD reciprocal DL/UL channel properties By applying the DL common pilots and UL dedicated pilots or sounding symbol Enable many channel-aware transmission techniques and allow the implementation based enhancement Such as beam-formed MIMO, SDMA SC-FDMA makes it difficult to explore the TDD application/advantage if not possible

35 Conclusions and Remarks
SC-FDMA exhibits 0 to 1.2 dB gain in max TX power owing to smaller PAPR However, by proper scheduling the resource, OFDMA shows no degradation The typical Tone Reservation/clipping algorithms can achieve similar PAPR of SC-FDMA SC-FDMA exhibits equalizer loss in frequency selective channel Especially when the delay spread is large and/or the number of subcarriers is large Note that in cell edge the delay spread is larger No future proof for Higher order MIMO in SC-FDMA Practical MLD for MIMO is not feasible even in 2x2 MIMO Higher order MIMO is not possible Practical MLD receiver in OFDMA significantly outperforms SC-FDMA receiver The limitation of SC-FDMA to evolve for future UL MIMO Capability is clear In SC-FDMA asymmetric resource allocation in UL/DL OFDMA can be used to exploit the TDD reciprocal DL/UL channel properties SC-FDMA should be ruled out for 16m Multiple access discussion Adopt OFDMA system in both UL and DL

36 Back-up

37 High code rate


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