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Enhanced Frequency Diversity and Enhanced Frequency Diversity and Scheduling Performance in Scheduling Performance in

Evolved UTRA Evolved UTRA

R1-050888

Agenda Item: 10.5

3GPP TSG RAN WG1 Meeting #42

London, UK, 29 August – 2 September, 2005

Samsung Electronics Co. Ltd.

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Promise for Evolved UTRAPromise for Evolved UTRA

• OFDMA based downlink promise greater spectral efficiency for evolved UTRA

• Gains of OFDM partly stems from its capability to efficiently exploit frequency-diversity and frequency selective scheduling .

• Achievable level of diversity/scheduling gain is dependent upon the channel environment:– For example, no frequency diversity or frequency selective scheduling

gains in a flat-fading channel, a lightly frequency selective channel and smaller bandwidths.

• Can we introduce frequency-diversity in the channel?– Diversity can be artificially introduced in the OFDM channel on “as

needed basis” by simply transmitting a delayed signal from the second antenna (or multiple antennas if available).

• The frequency-selectivity can then be exploited by using frequency-selective scheduling or frequency-diversity. – The frequency-selective scheduling provides gains for low mobility users

and the frequency-diversity provides robustness against fading for high-mobility users.

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Potential of Frequency SelectivityPotential of Frequency Selectivity

• Smaller system bandwidths such as 1.25 and 2.5MHz experience relatively smaller frequency selectivity:– leaving little room for frequency selective scheduling and/or frequency

diversity gains.

• Single path fading or Rician Channel (Line-of-Sight) may also occur in some cases in macro-cell deployments:– resulting in a non-frequency selective channel.

• Small Pico-cell (e.g. Indoor) deployments exhibit typical path delay of the order of 100’s of ns (coherence bandwidth~10MHz):– exhibiting “flat-fading” for most of the system bandwidths in Evolved

UTRA.

• Performance under the above situations could be improved by artificially introducing frequency selectivity in the OFDM channel.

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Injecting Frequency SelectivityInjecting Frequency Selectivity

• A delayed version of the OFDM symbol transmitted from the second Tx antenna:

– Can easily be extended to more than 2-Tx antenna case

• The OFDM symbols after IFFT operation can be processed independently for different channels and/or users.

– For example, a smaller delay value can be used for frequency-selective scheduling (larger coherence bandwidth) while a relatively larger delay value used for frequency-diversity (smaller coherence bandwidth)

OFDM Symbol(N+G samples)

D1

g0g1

ANT1

ANT2

i

ig 12

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Frequency-Selective Scheduling (1)Frequency-Selective Scheduling (1)

• Possible Peaks of 10*log10(K) dB

– K is the number of (equal power) paths and equals to the number of Tx-antennas

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Frequency-Selective Scheduling (2)Frequency-Selective Scheduling (2)

• Low-speed UEs can be scheduled at peaks in the frequency-domain

– Locations of peaks determined by the user channel response and antenna weights (if applied)

– Legend shows (ANT1, ANT2) weights

• In case of flat-fading and correlated channels for many users, the weights can be randomly assigned to users.

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Frequency DiversityFrequency Diversity

• Frequency diversity useful for high Doppler users

• In an already frequency selective channel, adding a delayed signal from the second antenna will increase the total number of independent paths.

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Performance DataPerformance Data

• Simple simulations based on effective SNR comparison:

– The effective SNR is calculated by using the EESM method.

• 5MHz system bandwidth, sampling rate of 7.68MHz, number of occupied subcarriers=301.

• Flat fading and TU channel models with and without Rx diversity

• The number of subbands for frequency-selective scheduling is equal to 15.

– The effective SNR of the served user is calculated as the largest SNR on a subband within a TTI

• For frequency-diversity evaluation, the effective SNR outage is considered.

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Frequency Selective Scheduling Gains Frequency Selective Scheduling Gains (1)(1)

• The flat fading channel represents the case where highest gains can be achieved from frequency selective scheduling enabled by artificial frequency-selectivity via delay diversity.

• With 2-Tx delay diversity, the served user’s mean effective SNR is 2.3dB and 1.9dB larger for no Rx diversity and 2-Rx diversity case respectively.

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Frequency Selective Scheduling Gains Frequency Selective Scheduling Gains (2)(2)

• The scheduling gains from artificial frequency-selectivity are relatively smaller because the TU channel is already a frequency-selective channel.

• With 2-Tx delay diversity, the served user’s mean effective SNR is 0.7dB and 0.5dB larger for no Rx diversity and 2-Rx diversity case respectively.

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Frequency Diversity Gains (1)Frequency Diversity Gains (1)

• Gain from delay diversity expected to be larger even for TU case in smaller bandwidths.

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Frequency Diversity Gains (2)Frequency Diversity Gains (2)

• With 2-Rx diversity, the additional gains from delay diversity, as expected, are relatively smaller.

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Effect on Cyclic PrefixEffect on Cyclic Prefix

• Channels with “single path” fading are in more need of frequency diversity

– Need to use larger delay from the second antenna

– Larger fraction of the CP available without incurring ISI

• Channels with significant multi-path components:

– Small number of additional CP samples available

– Need smaller delay from the second

• No need for any additional CP overhead!

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Pilot and CQI Feedback OverheadPilot and CQI Feedback Overhead

• No additional pilot overhead than that is required in any MIMO scheme with 2-Tx antennas:

– The pilot from the second antenna is orthogonal to the first antenna pilot and is not delayed.

– No impact on channel estimation performance due to increased frequency selectivity on information transmission because the frequency-selectivity on the pilot signal is not affected (no delay for pilot)

• No additional Channel Quality Indication (CQI) feedback overhead relative to that already required for frequency-selective scheduling

– For example, CQI feedback per subband

• The CQI is calculated at the UE by taking into account the delay value used at the second antennas for its transmission.

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Relationship to MIMORelationship to MIMO

• Multiple antennas provide additional degree of freedom

• The approach described in this document exploits the additional degree of freedom for either scheduling gain or diversity gain:

– No bandwidth expansion (spatial multiplexing) gain is achieved due to the fact that a delayed signal carrying the same information is transmitted from the second transmit antenna (or multiple antennas).

• Scheduling (multi-user diversity) gain is particularly useful for weak users in the system while frequency diversity gain is useful for medium to high speed users.

• Spatial multiplexing approaches can be used in conjunction with this approach on orthogonal frequency resource controlled by the scheduler.

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SummarySummary

• A simple approach where a delayed OFDM signal is transmitted from the second antenna can lead to increased frequency-selectivity in the channel.

• The increased frequency-selectivity can then be exploited by using frequency-selective scheduling or frequency-diversity for greater spectral efficiency.

• The approach does not result in any additional pilot, cyclic prefix, or CQI feedback overhead.

• The proposal should be further studied in RAN1 for its potential of providing greater spectral efficiency in the Evolved UTRA system.