Submission

doc.: IEEE 802.11-14/1446r0November 2014

Leif Wilhelmsson, EricssonSlide 1

Analysis of frequency and power requirements for UL-OFDMA

Date: 2014-11-03

Name Affiliations Address Phone email Leif Wilhelmsson

Ericsson AB Mobilvägen 1, 22362 Lund, Sweden

+46 706 216956

Leif.r.wilhelmsson@ericsson.com

Naveed Butt Ericsson AB

Authors:

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

Abstract

• Frequency errors and near-far effects may severely degrade performance for UL-OFDMA

• We provide some analytical results that can be used to analyze requirements without lengthy simulations in case of UL-OFDMA

• We also address the issue with varying received powers and present an approach to analyze requirements on power control

Slide 2

November 2014

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

Outline

• Motivation/Related work

• Recap of single user case

• SNR loss due to frequency and power miss-alignment for UL-OFDMA

• Some simulation results and discussion on requirements for frequency and power alignment

• Conclusions

Slide 3

November 2014

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

Motivation/Related work

• UL-OFDMA sets requirements on frequency synchronization and power alignment for the signals sent by the STAs

• [3] and [4] discuss frequency synchronization requirements and compare with what is used in LTE

• [5] proposes an ACK procedure, but do not discuss corresponding requirements for this to be feasible

• [6] addresses the same problem but for UL MU-MIMO, by using complete link simulations

• Here we try to assess requirements for UL-OFDMA analytically for increased insight and fast evaluation

Slide 4

November 2014

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

ICI in Single-User Case

• For relatively small CFO errors, the SNR degradation due to ICI is well-approximated by the commonly used expression derived in [1]

• Here is the CFO error, is the subcarrier spacing, and D represents the amount by which the original SNR, , has to be increased to maintain the same symbol error rate (SER) as in the CFO-free case.

Slide 5

November 2014

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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

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Relative Frequency Error (% of carrier spacing)

SN

R D

egra

datio

n (d

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64-QAM

16-QAM

QPSK

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

ICI in OFDMA UL

• Consider two users, u1 and u2, transmitting over blocks of ideally orthogonal subcarriers with powers P1 and P2, respectively. User 1 is assigned N1 subcarriers, and user 2 is assigned N2 subcarriers, while the two users suffer from CFO errors fe1 and fe2, respectively.

• With the channel normalized to unity, we find the power of the ICI from all the subcarriers of u1 onto the k-th subcarrier of u2, by summing up over the subcarriers of u1

• Where we have also included the possibility of having g guard sub-carriers between the subcarrier blocks of the two users, and k = 1,…, N2.

Slide 6

November 2014

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

ICI in OFDMA UL

• The total ICI on the N2 subcarriers of u2 may then be expressed as

• Finally, the SNR degradation for user 2 due to the CFO-related MUI from user 1 is calculated as

Slide 7

November 2014

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Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, EricssonSlide 8

November 2014

Case: 64-QAMN1=N2 =2660,000 Monte-Carlo Simulations

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Relative Frequency Error (% of carrier spacing)

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Comparison of simulated results (dashed) with analytical results (solid) based on (2). Also included, for comparison, is the analytical result based on (1) (diamond). The bottom three curves show power ratios ( ) of 0 dB (bottom), 5 dB (middle) and 10 dB (top).

Comparison: OFDM formula vs. OFDMA

Submission

doc.: IEEE 802.11-14/1446r0

Ex: Degradation vs. frequency offset at AP

Slide 9

November 2014

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 30

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Relative Frequency Error (% of carrier spacing)

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Dc, PR = 10dB

Dc, PR = 5dB

Dc, PR = 0dB

Dc, PR = -5dB

Dc, PR = -10dB

Ds

• Illustration of degradation in UL-OFDMA. Desired signal has relative frequency error of minus 2%, aggressor has relative frequency error of 3% relative APs nominal frequency.

• Self-ICI is dominating for equal power, because significant MUI is mainly between the very closest sub-carriers.

• However, with largely different power, MUI can easily dominate Leif Wilhelmsson, Ericsson

Frequency offsetaggressor

Frequency offsetdesired signal

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

Joint power and frequency requirements

Slide 10

November 2014

-10 -5 0 5 10 15 200

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Fre

quen

cy E

rror

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Power Ratio (dB)

0.5dB, 64-QAM

1dB, 64-QAM2dB, 64-QAM

3dB, 64-QAM

0.5dB, 16-QAM

1dB, 16-QAM2dB, 16-QAM

3dB, 16-QAM

0.5dB, QPSK

1dB, QPSK2dB, QPSK

3dB, QPSK

• Degradation due to MUI as a function of power ratio and frequency error• The requirements for MUI seems reasonable, although rough power control or

proper scheduling (selection of users with somewhat similar powers) is required, say within 10 dB

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

What sets the requirements?

Slide 11

November 2014

• Referring to the figures above, the degradation due to self interference is the same as the degradation due to MUI if the power ratio is about 15 dB

• If degradation is set to the same value, say 2 dB, self interference sets the limit (e.g. 2%) if power difference is less than 15 dB, otherwise MUI sets the limit

• Note: This was for 2 “10 MHz” STAs in one 20 MHz channel. Evaluation for more STAs and other sub-channel BWs is straight forward

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Relative Frequency Error (% of carrier spacing)

SN

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64-QAM

16-QAM

QPSK

MUI Self interference

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Frequency Error (% of carrier spacing)

Pow

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0.5dB, 64-QAM1dB, 64-QAM

2dB, 64-QAM

3dB, 64-QAM0.5dB, 16-QAM

1dB, 16-QAM

2dB, 16-QAM

3dB, 16-QAM0.5dB, QPSK

1dB, QPSK

2dB, QPSK3dB, QPSK

16-QAM 2dB degradation

64-QAM 2dB degradation

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

Conclusions

• The derivation (1), based on the single-user OFDM case, is not directly applicable for calculating MUI for the OFDMA UL case as it assumes same frequency error for all sub-carriers and equal power

• Generalization to OFDMA is straight-forward and allows for evaluation of degradation without detailed link level simulations

• When the two users transmit with equal powers, the SNR degradation due to MUI is significantly less than what is obtained from (1)

• Comparing degradation due to MUI and self interference is one means to set requirements on power differences, in addition to e.g. dynamic range in the ADC

Slide 12

November 2014

Submission

doc.: IEEE 802.11-14/1446r0November 2014

Leif Wilhelmsson, EricssonSlide 13

References

1. Pollet, T.; Van Bladel, M.; Moeneclaey, M., "BER sensitivity of OFDM systems to carrier frequency offset and Wiener phase noise," IEEE Transactions on Commun., vol.43, no.2/3/4, pp.191-193, Feb./March/April 1995.

2. Faulkner, M.; Wilhelmsson, L.R.; Svensson, J., "Low-Complex ICI Cancellation for Improving Doppler Performance in OFDM Systems," IEEE VTC-2006 Fall. pp.1,5, 25-28 Sept. 2006.

3. 11-14/0818r0, “Synchronization requirements”

4. 11-13/1388r0, “Uplink multi-user transmission”

5. 11-14/1211r0, “Ack procedure for OFDMA”

6. 11-09-1036-00 “Uplink MU-MIMO Sensitivity to Power Differences and Synchronization Errors”

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

ICI in OFDMA UL

• Consider two users, u1 and u2, transmitting over blocks of ideally orthogonal subcarriers with powers P1 and P2, respectively. User 1 is assigned N1 subcarriers, and user 2 is assigned N2 subcarriers, while the two users suffer from CFO errors fe1 and fe2, respectively.

• In this case, we use the result derived in [2], stating that that if the channel is assumed to vary in a linear fashion during the OFDM symbol, then the ICI on subcarrier K caused by the symbol sent on subcarrier K+L is given by

• Here and are the channel change on subcarrier K+L during the information part of the OFDM symbol, and the symbol transmitted on subcarrier K+L, respectively.

Slide 14

November 2014

𝑅𝐾 ,𝐾 +𝐿≈𝑆𝐾 +𝐿𝐻𝐾 +𝐿′ 1

𝑗2𝜋 𝐿

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

ICI in OFDMA UL

• Since the affected channel due to the frequency offset may be written as

• Where the approximation holds for small , we see that the channel change is approximately

• Which leads to

• The formula for ICI follows from the normalizations

Slide 15

November 2014

𝐻𝐾+𝐿𝑒𝑗 2𝜋 𝑓 𝑒/Δ 𝑓 ≈𝐻𝐾 +𝐿 (1+

𝑗2𝜋 𝑓 𝑒Δ 𝑓 )=𝐻𝐾+𝐿+𝐻𝐾+𝐿

𝑗 2𝜋 𝑓 𝑒

Δ 𝑓

𝐻 ′𝐾 +𝐿=𝐻𝐾 +𝐿

𝑗2𝜋 𝑓 𝑒Δ 𝑓

𝑅𝐾 ,𝐾 +𝐿≈𝑆𝐾 +𝐿𝐻𝐾 +𝐿❑ 𝑓 𝑒 / Δ 𝑓

𝐿

𝐸 [𝑆𝐾 +𝐿]=1 ;𝐻𝐾 +𝐿❑ =1

Submission

doc.: IEEE 802.11-14/1446r0

Leif Wilhelmsson, Ericsson

ICI in OFDMA UL

• Additionally, the ICI suffered by u2 from its own carriers may be calculated as

• Using Ms the total degradation of u2 due to fe2 may be evaluated along the lines of [1] as

Slide 16

November 2014

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