Comparison of N1 and N2 Cycling

Document Number: IEEE C80216m-09_0716

Date Submitted: 2009-03-09

Source:Kiran Kuchi, J. Klutto Milleth, Vinod R, Dileep M K, Divagar , Padmanabhan M S, Bhaskar R, Giridhar K Voice:CEWiT, India E-mail:

kkuchi@cewit.org.in, klutto@cewit.org.in

Venue: Call for Comments on IEEE 802.16m-09/0010 IEEE 802.16m Amendment text

Base Contribution: None

Purpose: To discuss and adopt the proposed text in amendment text

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Presentation Outline

CDF of CQI

CQI mismatch analysis for

2D-POD (N2-cycling in frequency+precoder cycling over sub-frames)

1D-POD (N2 cycling in frequency)

N1 Cycling+best-M

Performance of POD (DFT) and 16e code books with correlated antennas

Discussion

N1 Vs N2 Precoder cycling

Pre-coding Vector Table

Let pre-coding vector used in the ith PRU in the mth sub frame be Pi .

N1 Cycling : Ci= C(:, k) where k = floor( i / N1)% 4, N1 = 4, i =0 to 47, m=0 to1

N2 Cycling (1D-POD) : Ci= C(:, k) where k = i % 4, N2 = 1,i =0 to 47, m=0 to 1

N2 Cycling (2D POD) : Ci,m= C(:, k) where k = ( i % 4) + ((i%2)*2 +1)*m , N2 = 1,i =0 to 47, m=0 to 1

jj

C11

1

1

1

1

N1 Vs N2 precoder cycling

Examples of precoder cyling in two subframes

Ci Ci

Ci Ci

Ci Ci

Ci Ci

C0 C1

C1 C2

C2 C3

C3 C0

N1 cycling N2 Cycling (2D-POD)

C0 C0

C1 C1

C2 C2

C3 C3

N2 Cycling (1D-POD)

PRU(i,0) PRU(i,1)

PRU(i+1,0) PRU(i+1,1)

PRU(i+2,0) PRU(i+2,1)

PRU(i+3,0) PRU(i+3,1)

System Model and CQI Definition

System Model :Y = HCx + n

Y (Nr x 1) ==> Received Signal vector at an OFDMA sub carrier

H (Nr x N

t) ==> Channel matrix at an OFDMA sub carrier

C (Nt x 1) ==> Pre-coding vector

X ==> Transmitted symbol vector

n==> zero mean unity variance Gaussian noise vector

CQI at a sub-carrier = ∑ (HC)H HC

CQIi,m

=> the CQI in the i th PRU in the mth sub frame = CQI averaged over all the sub-carriers in the PRU

CQIsubband

=> the CQI averaged over all PRUs in a pair of sub bands spanning in time

CQI Calculation

CQI subband

= ( CQI

i,0 +

CQI

i,0 + CQI

i+1,0 + CQI

i+1,1 +

CQI i+2,0

+CQIi+2,1

+CQI i+3,0

+ CQI

i+3,1 )/8

CQI(i,0) CQI(i,1)

CQI(i+1,0) CQI(i+1,1)

CQI(i+2,0) CQI(i+2,1)

CQI(i+3,0) CQI(i+3,1)

• The channel frequency selectivity within sub-band is quite high in mPED-B channel

• In mPED-B, 2-Tx, 2Tx system the channel hardening effect is similar for N1 and N2 cycling

CQI CDF in 2Tx, 2Rx System (mPED-B Channel, 3 Kmph)

In mPED-B, 2-Tx, 2Tx system the channel hardening effect is similar for N1 and N2 cycling (i.e., not much difference in the upper part of the CDF)

Lower outages for N2 cycling (Diversity gain)

Pr(CQI<0 dB) = 8% for N2 cycling

Pr(CQI< 0 dB) = 16 % for N1 cycling

At the higher end of CDF

Pr(CQI> 5 dB)= 15% for N1 cycling

Pr(CQI> 5 dB)= 10% for N2 cycling

CDF of CQI with N1 Cycling with Best-M

CDF of best-2 sub bands in selected from 12 sub bands

Best-band scheduling gain diminishes at high mobile speeds

CQI Mismatch Caused by Channel Frequency Selectivity

Within the Subband

CQI mismatch due to frequency selective channel

CQI is reported for each sub-band but scheduler may assign only a portion of the sub-band for a given user

Data may be scheduled in a single sub-frame or in multiple sub-frames

Over the sub-band, channel varies due to frequency selectivity

Some amount of CQI mismatch unavoidable in real system

At low speeds, little CQI mismatch when full sub-band is scheduled

Let CQIi,m

be the CQI in the i th PRU in the mth subframe

Case-1: CQI Mismatch when a single PRU pair (two contiguous PRUs in time) is allocated (worst case scenario)

Case-2: CQI Mismatch when a two PRU pairs (two contiguous PRUs in frequency and two contiguous PRUs in time) are allocated

subband

iiCQI CQI

CQICQI

2log10 1,0,

subband

iiiiCQI CQI

CQICQICQICQI

4log10 1,10,11,0,

CQI mismatch due to frequency selective channel (low speed)

• Channel model: mPED-B 3 kmph• At low speeds, feedback delay does not cause significant CQI mismatch• Frequency selective channel causes some CQI mismatch when only a portion of

the sub-band is not allocated to the user• In this scenario N2-cycling (2D-POD) has least CQI mismatch

CQI mismatch for Case-1 CQI mismatch for Case-2

Conclusions

2D-POD (N2 cycling in frequency + precoder cycling over subframes)

Enables a “pure” OL transmit diversity scheme for time only, frequency only and joint time and frequency resource allocations

Least CQI mismatch over all other techniques including N1 cycling

N1 cycling + best-M

Semi-closed loop technique

Gain is prone to mobile speed, feedback delay etc. At 30 Kmph, best-M gains start to vanish

For 5 and 10 MHz BW with 4-frequency partitions, best-M not feasible

Only one or two sub-bands available per frequency partition

At low mobile speeds:

No significant performance difference between 2D-POD+best-M and N1 cycling+best-M

For medium to high mobile speeds

2D-POD outperforms N1 cycling+best-M

2D-POD recommended for OL rank-1 precoding

Proposed Text For Non-Adaptive Precoder Cycling Duration

The OL non-adaptive precoder W changes every PRU and every sub-frame.