© 2004 Texas Instruments Incorporated 1 C30-20050516-050 Title: Comments on the CDMA EBM and OFDM EBM Implementation and Performance Issues as Described

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© 2004 Texas Instruments Incorporated

C30-20050516-050

Title: Comments on the CDMA EBM and OFDM EBM Implementation and Performance Issues as Described in C30-20050516-048

Source: Aris Papasakellariou, Texas Instruments Inc.214-480-4572, [email protected]

Date: May 16, 2005

Recommendation: Review and Adopt

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This document has been prepared by Texas Instruments Incorporated to assist the development of specifications by 3GPP2. It is proposed to the Committee as a basis for discussion and is not to be construed as a binding proposal on Texas Instruments Incorporated. Texas Instruments Incorporated specifically reserves the right to amend or modify the material contained herein and nothing herein shall be construed as conferring or offering licenses or rights with respect to any intellectual property of Texas Instruments Incorporated other than provided in the copyright statement above.

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CDMA EBM and OFDM EBM Comparisons

In C30-20050516-048 ([1]) the following were suggested:

Claim: Due to the absence of CP, CDMA-EBM performance is vulnerable to high delays channels (slide 3)

Fact: Performance sensitivity to high delay channels has nothing to do with the existence or not of a CP. The reason is the much reduced number of chips available for SIC in the case of severe repeater power and delays.

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CDMA EBM and OFDM EBM Comparisons cont.

Claim: SIC is vulnerable to rank deficiency problem (slide 3)

Fact: There is no fundamental rank deficiency problem.

— Original CDMA EBM can comfortably provide the same robustness as OFDM EBM with 112 pilot chips and higher robustness with 128 chips. The performance impact due to the increased overhead is only a small dB fraction.

— Updated CDMA EBM has a considerably larger ratio of “chips available for channel estimation” over “channel taps to be estimated” than OFDM EBM. For example, for Channels B and D (worst case scenarios) the CDMA EBM processing gain is at least 3 while it is always about 1.2-1.5 for OFDM EBM.

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Claim: It is hard to realize the predicted performance in actual practice (slide 3)

Fact:

— FDE (and DFE) are well understood techniques with available commercial products. FDE is already the preferred solution for improved performance in existing CDMA solutions.

— SIC is also a very well mature and understood technique with straightforward implementation (especially for the original EBM proposal) and without any robustness issues.

Claim: Coverage Comparison Results (slide 6)

Fact: The presented relative coverage results are not supported by link level performance and, in the case of CDMA EBM, are substantially worse than the ones obtained by TI.

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Claim: OFDM EBM is designed for robust performance in harsh environments (slide 7)

Fact: OFDM EBM is most vulnerable in low diversity conditions that are the most important for coverage.

Claim: Original CDMA EBM can only handle delays up to around 48 chips. Updated CDMA EBM can only handle delays of around 20 chips (slide 7)

Fact: Original CDMA EBM with 112 or 128 pilots can handle about the same or larger delays as OFDM EBM. The number of EBM pilot chips is a simple parameter adjusted to account for meaningful repeater delays. Updated CDMA EBM can also handle the same or larger delays as the updated OFDM EBM.

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Claim: Salvaging (sic!) CDMA EBM involves unrealistic requirements on receiver filter calibration and channel model assumptions (slide 7)

Fact:

— During calibration, similar channel estimation windows were used for both CDMA and OFDM EBMs.

— In the worst case scenario that no intelligence is applied in the selection of the channel estimation window, this window can be set to have a fixed value for both EBM proposals (no assumption on channel model).

— The receiver filter calibration for multi-sector SIC is a simple process already employed in KDDI’s suggested channel estimation for DO.

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Claim: Edge effects on CDMA EBM (slides 8-10)

Fact: It has already been shown during calibration round 1 that CDMA EBM equalization over 512 chips (FFT is zero-padded with 112 zeros) experiences no impact due to edge effects.

Claim: Edge effects impact on CDMA EBM performance (slides 11-12)

Fact: In addition to using a fictional scenario of just 2, equal power sectors, the performance impact on CDMA EBM (not coincidently appearing only for the high delay spreads) is due to channel estimation (96 pilot chips not enough for the very long delays considered) and not due to the edge effects.

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Claim: Channel Estimation Issues for FDE – rank deficiency (slide 13)

Fact: Issue was previously addressed.

— CDMA EBM operation does not depend on any assumption regarding the channel.

— Intelligent choice for the channel estimation window can improve the performance - this was used for both CDMA and OFDM EBMs during calibration.

— CDMA EBM does not need a “careful choice of the channel estimation window” any more than OFDM EBM does.

Claim: Receiver Filter Modifications (slide 14-16)

Fact: An improved receiver filter is not needed. It was suggested as a possible source for further performance improvements and this may or may not be applicable (depending on the transmit filter)

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Claim: Receiver Architecture Comparisons – same resources for CDMA EBM and OFDM EBM (slides 17-18)

Fact: While there is commonality in the receiver operations (FFT), the CDMA and OFDM receiver operations are different and re-use of OFDM receiver blocks for CDMA reception, if possible, will probably involve complex control, delay, and memory requirements for the necessary units to communicate. Different receiver chains will most likely be required for CDMA and OFDM despite the commonality of FFT.

Claim: Performance Comparison for Round 2 and Round 4b (slides 19, 21)

Fact: Performance comparison is for a setup (Drop 4, Channel B) too benign to determine coverage. CDMA EBM clearly outperforms OFDM EBM for Drop 1 and Channel A (determining coverage). With ACI whitening, CDMA EBM actually outperforms OFDM EBM for Drop 4 and Channel B for geometries larger than 16 dB (it always outperforms for smaller geometries).

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Claim: Insights from Outer Code Analysis (slide 20)

Fact:

— A typical case of setting a specific target value for a first of two directly related parameters in order to get a desired value for the second.

— The target PER after outer code decoding was set to 0.01% instead of 0.1%. Note that with a 0.01% PER value (a first time appearance!), previously made coverage claims (that always considered 0.1% target PER) are doubtful.

— The considered target PER range of 1%-5% before outer code decoding is very appropriate. Nevertheless, the relative performance of CDMA and OFDM EBMs remains the same at the lower PERs (e.g. 0.5%) implied by a target PER with outer coding of 0.01%.

Claim: Performance Comparison for Round 5a (slides 23, 26)

Fact: Calibration results from Round 5a do not match reported results. Results in slide 26 are contrary to what has been observed in the calibration rounds.

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Claim: Aggregate geometry CDF for SFN (slide 24)

Fact: It is unreasonable to expect that in a 19-cell layout, all site-to-site distances will be equal to or smaller than 2.0 Km (or 1.4 Km). It is even more unreasonable to assume that the same content will be transmitted throughout an entire city (no different content interference). Even for the SFN topology, there is no acceptable justification in considering geometries above 14 – 16 dB (also overly optimistic and only applicable for 1.4 Km site-to-site distance and less than 95% coverage).

Claim: Performance Comparisons for Round 5b (slides 25, 27-31)

Fact: All CDMA EBM performance limitations for this extremely severe repeater power and delay setup are due to the fact that SIC is not provided with adequate pilot chips for pilot estimation (previous comments apply).

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CDMA EBM and OFDM EBM Comparisons

Claim: General claims about OFDM robustness

Fact: It is yet unknown how concurrent deployment of DO and OFDM will affect the performance of either. For example, the colored OFDM interference may have severe implications on DO performance.

Claim: CDMA EBM is not “true CDMA”

Fact: If it walks like a duck and quacks like a duck, it is not an elephant.

— Same transmitter operations as DO

— Same receiver operations as DO

— Same signal spectrum as DO

Documents

© 2004 Texas Instruments Incorporated 1 C30-20050516-050 Title: Comments on the CDMA EBM and OFDM EBM Implementation and Performance Issues as Described