`3GPP TSG-RAN WG1 #28
`Seattle, USA
`19 – 22 August, 2002
`
`Agenda Item :
`
`11.4
`
`Source :
`
`IPWireless
`
`Tdoc R1-02-1166
`
`Title :
`
`Comments on Proposals to Restrict the OFDM Study Item to FDD
`Only
`
`Document for :
`
`Discussion
`
`
`
`Comments on Proposals to Restrict the OFDM Study Item
`to FDD Only.
`
`1 Background and Introduction
`
`At RAN #16 June 2002, a study item for WG1 named “Analysis of OFDM for UTRAN Evolution” [1]
`was approved. The version of the study item description that was approved did not restrict
`analysis of OFDM to either FDD or TDD modes.
`
`Since that time RAN WG1 have been attempting to further clarify the content and scope of the
`study item. Some input papers on this topic eg: [2] have proposed a restriction of applicability of
`the OFDM study item to FDD only and others, although not explicitly stating a restriction, have
`failed to give any consideration to TDD. Some comments voiced on the RAN WG1 email reflector
`have also proposed to limit the scope of the SI to FDD.
`
`We try to clarify within this document some of the arguments against such a restriction of the
`OFDM study item to FDD only and propose a way forward on this issue.
`
`2 Discussion
`
`It is noted that the result of the study item (TR-25.892) is scheduled for submission to RAN plenary
`#19 for information and for final approval at RAN #20. It is felt that the OFDM study item is
`potentially extremely large (probably far bigger than can be thoroughly studied within the given
`time frame), since it is not only a specification of a new modulation format; rather it has
`ramifications over the entire system design, affecting system fundamentals such as frequency
`domain reuse, multiple access type, RF deployment and co-existence scenarios, receiver
`complexity issues, physical channel definition, transport channel processing, power control
`procedures, macro diversity etc…etc…etc…
`
`Bearing this in mind it would seem that careful focus of the study item is required in order to
`produce meaningful results in the given timeframe. The results should therefore maintain a high
`degree of generality as it is not the goal of the study item to define the detail of the OFDM system,
`only its potential and applicability to UTRAN.
`
`3 Drawbacks of an FDD-Only Restriction
`
`It is felt that restriction of the SI to FDD only is not acceptable from the following three standpoints:-
`
`3GPP
`
`NAC1056
`Page 1
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`

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`
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`
`
` Such a restriction was not imposed in the original SI description as was approved by RAN
`[1]. This would imply by deference that the intention of RAN was to include both modes in
`the study.
`In the event that the outcome of an “OFDM-for-FDD” study is positive for OFDM, we are
`then in the situation that in order to assess the benefits (if any) for TDD, a new SI or an
`extension to the old one is most-likely required. This would impart a significant and
`unacceptable delay in terms of the formal standardisation of OFDM for TDD with respect to
`FDD.
` The SI is intended to investigate whether or not OFDM is capable of bringing benefits to
`UTRAN (as per the SI title). If OFDM brings a complexity or performance gain to FDD that
`may have been similarly achieved via the use of the existing TDD mode, then the new
`(OFDM) mode must be introduced only following due consideration of whether it brings an
`overall benefit/incremental gain to UTRAN (UTRAN here being the set of existing FDD and
`TDD modes).
`
` 4
`
` Suggested Way Forward
`
`It has already been agreed that the primary focus of the SI is on downlink only. As such the
`reference OFDM design will utilise an OFDM component on the DL carrier coupled with a WCDMA
`UL. For FDD it is likely therefore that a target spectrum allocation would be the typical 2x5MHz
`deployment, with the DL carrier using only OFDM or at least an element thereof, whilst the UL
`carrier closely (or exactly) resembles the current WCDMA FDD uplink.
`
`However, for TDD, there is of-course the added dimension provided by the time-slotted nature of
`the system. As such, it should be possible to utilise WCDMA DL, OFDM DL and WCDMA UL
`technologies within the same radio frame, the technologies being made orthogonal in the time-
`domain within the same carrier.
`
`Thus, there already exists some element of compatibility between OFDM for FDD and OFDM for
`TDD since DL-only introduction of OFDM is feasible for both; it’s just that for the TDD carrier there
`is some additional flexibility in that appropriate/optimised fractions of the 1x5MHz resource may be
`assigned to WCDMA UL/DL and to OFDM [as for R99/4/5 TDD allows for the amount of deployed
`resource for uplink to be more accurately tailored to service requirements].
`
`In light of the above, it is proposed that a single reference OFDM system is anaylsed and
`compared to the performance and complexities of both FDD and TDD modes.
`
`That is to say that when deriving the reference OFDM system, features should not be added that
`would restrict the use of this reference system to only one of the duplex modes.
`
`Using such a reference system it should be possible to single-out and extract those fundamental
`areas of the system in which it is believed gain may be achieved. As an example; for a simple
`hypothesis that OFDM can bring significantly reduced Eb/No requirements under certain channel
`conditions, the reference system should provide isolated results that demonstrate this gain when
`compared to existing results for the current releases of both FDD and TDD. The gain in this case
`could then be identified as being applicable to neither/either or both modes since the reference
`OFDM system is itself generic and equally applicable to FDD and TDD.
`
`It is clear that when performing capacity analyses, compensation must be made for any differences
`(between FDD and TDD) in the fraction of the resource used for non-OFDM purposes. For
`example, the spectral efficiency of a 100% OFDM DL and 100% WCDMA UL using 2x5MHz (FDD)
`could be compared to a 67% OFDM DL and 33% WCDMA UL using 1x5MHz (TDD) etc…
`However, the baseline OFDM system behind these calculations/simulations is the same for each
`mode.
`
`3GPP
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`NAC1056
`Page 2
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`5 Summary and Conclusions
`
`The issue regarding the proposed restriction of the recent OFDM study item to FDD only has been
`discussed.
`
`It is proposed that the SI is not restricted to FDD only, but that a generic OFDM reference system
`is derived from which results may be extracted that are equally applicable to both modes.
`
`Benefits of this approach are:-
`
` The study item will be properly able to conclude whether OFDM brings anything to UTRAN
`(UTRAN being the set of both FDD and TDD modes)
`
` Very little additional effort is required since alternative reference systems need not be
`derived for FDD and TDD.
`
` TDD is not delayed with respect to FDD in the event that the outcome for OFDM for both
`modes is positive.
`
`6 References
`
` RP-02-0442 “Proposed Study Item: Analysis of OFDM for UTRAN Evolution”, Nortel
`Networks and Wavecom, 3GPP RAN #16, Marco Island, USA, 4th-7th June 2002.
`
` R1-02-0932, “Assumptions and Objectives for ‘Analysis of OFDM for UTRAN Enhancement”
`Study Item”, Nortel Networks, RAN WG1 #27, Oulu, Finland, 2nd-5th July 2002.
`
` R1-02-1023, “Revised Study Item Description for Analysis of OFDM for UTRAN
`Enhancement”, Nortel Networks, RAN WG1 #28, Seattle, USA, 19th-22nd August 2002.
`
` R1-02-1024, “Revised Draft TR on Feasibility Study for Analysis of OFDM for UTRAN
`Enhancement”, Nortel Networks, RAN WG1 #28, Seattle, USA, 19th-22nd August 2002.
`
` R1-02-1114, “Views on the OFDM Study Item Scope”, Telia AB, RAN WG1 #28, Seattle,
`USA, 19th-22nd August 2002.
`
`[1]
`
`[2]
`
`[3]
`
`[4]
`
`[5]
`
`
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`3GPP
`
`NAC1056
`Page 3
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`