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Analysis of frequency and power requirements for UL-OFDMA

Analysis of frequency and power requirements for UL-OFDMA. Date: 2014-11-03. Authors:. Abstract. Frequency errors and near-far effects may severely degrade performance for UL-OFDMA

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Analysis of frequency and power requirements for UL-OFDMA

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  1. Analysis of frequency and power requirements for UL-OFDMA • Date:2014-11-03 Authors: Leif Wilhelmsson, Ericsson

  2. 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 Leif Wilhelmsson, Ericsson

  3. 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 Leif Wilhelmsson, Ericsson

  4. 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 Leif Wilhelmsson, Ericsson

  5. 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. (1) Leif Wilhelmsson, Ericsson

  6. 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. Leif Wilhelmsson, Ericsson

  7. 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 (2) Leif Wilhelmsson, Ericsson

  8. Comparison: OFDM formula vs. OFDMA Case: 64-QAM N1=N2 =26 60,000 Monte-Carlo Simulations 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). Leif Wilhelmsson, Ericsson

  9. Ex: Degradation vs. frequency offset at AP Frequency offset aggressor Frequency offset desired signal • 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

  10. Joint power and frequency requirements • 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 Leif Wilhelmsson, Ericsson

  11. What sets the requirements? Self interference MUI 16-QAM 2dB degradation 64-QAM 2dB degradation • 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 Leif Wilhelmsson, Ericsson

  12. 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 Leif Wilhelmsson, Ericsson

  13. References • 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. • 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. • 11-14/0818r0, “Synchronization requirements” • 11-13/1388r0, “Uplink multi-user transmission” • 11-14/1211r0, “Ack procedure for OFDMA” • 11-09-1036-00 “Uplink MU-MIMO Sensitivity to Power Differences and Synchronization Errors” Leif Wilhelmsson, Ericsson

  14. 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. Leif Wilhelmsson, Ericsson

  15. 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 Leif Wilhelmsson, Ericsson

  16. 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 (3) Leif Wilhelmsson, Ericsson

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