Tamed Frequency Modulation (TFM) in the IEEE 802.16.1 uplink

Document Number:IEEE 802.16.1p-00/08

Date Submitted:2000-11-03

Source:Lars LindhNokia Research CenterP.O. Box 407, FIN-00045NOKIA GROUP, Finland

Petri BergholmNokia NetworksP.O.Box 370, FIN-00045NOKIA GROUP, Finland

Venue:IEEE 802.16 Session #10, Tampa, Nov. 6-11, 2000

Base Document:IEEE 802.16.1c-00/08

Purpose:This document is for a presentation about TFM modulation in the 802.16.1 uplink

Notice:This document has been prepared to assist IEEE 802.16. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.

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2 © NOKIA

Tamed Frequency Modulation (TFM) in the IEEE 802.16.1 uplink

properties, performance and implementation

NOKIA

3 © NOKIA

Introduction

• IEEE 802.16.1 is a broadband wireless access (BWA) standard at the moment optimized for SME use in the U.S market

• In order to be accepted as an international standard the residential and SOHO markets should be addressed as well

• Low cost is a driver for these markets

• Tamed Frequency Modulation (TFM) in the uplink makes low cost terminals possible

• IEEE 802.16.1 is an international standard with a potential for world wide adoption. In order to address the residential and SOHO market sector, especially in Europe, TFM is proposed as an optional modulation scheme for the uplink

4 © NOKIA

T FM basic properties

• T FM is a constant envelope modulation scheme working by modulating the phase while keeping the envelope constant

• T FM allows power amplifier to operate at or near saturation level

• No inter modulation distortion arises in the power amplifier -> no spectrum leakage into the adjacent carrier, lower linearity requirements

• Power amplifier can be 6-7 dB smaller than in a corresponding QPSK modulator

• Good system value

• Ideal for low cost terminal solutions

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The TFM signal

• The TFM signal can be written ass(t)=Re{exp(ωct + φ(t))}where φ(t) is given as

an is the data sequence (+1, -1) and g(t) is the shaping filter and h is the modulation index = 0.5g(t) is given as

( ) ( ) ττπφ dnTgahtt

nn∫ ∑

∞−

∞

−∞=

−⋅= 2

)(8

1)(

4

1)(

8

1)( 000 TtgtgTtgtg +++−=

−

−

−

≈

2

3

2

22

0 24

cot2

21

sin)(

Tt

Tt

Tt

Tt

tT

ttg

π

πππ

ππ

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The frequency pulse g(t) and phase response

• Frequency pulse g(t) defines the smooth phase transitions between the constellation points

• Possible phase transitions are 0, +/-45 degr and +/-90 degr-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

-3 -2 -1 0 1 2 3

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

-3 -2 -1 0 1 2 3

I

Q

7 © NOKIA

T FM modulator types

• VCO based modulator

-Low cost-Adjustable

modulation index

• Quadrature based modulator

-Can handle both T FM and QAM

)2sin( 0tfπ−

I-generator

Q-generator

)2cos( 0tfπ

Modulationsymbols

Informationbits

),( αtsiα

)),(cos( αφ t

)),(sin( αφ t

Modulationsymbols

PhaseFilter

VCO

Informationbits iα ),( αts∑ −

nk iTtg )(α

hπ2

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The TFM modulation index

• The modulation index h is part of the gain for the VCO• Nominally 0.5• Calibrated in the manufacturing stage

• May drift with temperature variations

• Must be measured initially and periodically by the BS receiver

• Mechanism for adjusting the index must be made• BS station sends a message indicating the error in the

modulation index with 0.0025 unit resolution (4 bits)

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The TFM spectrum

• Grade A bitrate 36 Mb/s

• Example with 28 Mhz channel

• Conforms to E T SI spectrum mask A-70

-60

-50

-40

-30

-20

-10

0

0 10 20 30 40 50 60

Frequency MHz

Spec

tral

Den

sity

dB

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The TFM spectrum

-70

-60

-50

-40

-30

-20

-10

0

0 10 20 30 40 50 60

Frequency MHz

Spec

tral

Den

sity

dB

• Grade B bitrate 34 Mb/s

• Example with 28 MHz channel

• Conforms to E T SI spectrum mask B

11 © NOKIA

The TFM spectrum

-70

-60

-50

-40

-30

-20

-10

0

0 10 20 30 40 50 60

Frequency MHz

Spec

tral

Den

sity

dB

• Grade C bitrate 32 Mb/s

• Example with 28 MHz channel

• Conforms to E T SI spectrum mask C

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Net Filter Discrimination (NFD)Ratio between the power transmitted by the interfering system and the portion that can be measured after the receiver filter in the adjacent channel

∫

∫∞+

∞−

+∞

∞−=dffHfS

dffS

NFD

I

I

2)()(

)(

Adjacent channelSpectrum SI(f)Receiver

filter H(f)

f

S(f)

Interference

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Net Filter Discrimination• NFD between

T FM and QAM when TFM is the interfererfor different values of alpha (0.15low, 0.25 mid, 0.35 high curve)

• QAM symbol rate = channel spacing / (1 + alpha)

0

510

1520

25

3035

40

1.00 1.04 1.07 1.11 1.14 1.18 1.21 1.25 1.29 1.32 1.36

TFM bitrate / channel spacing ratio

NF

D d

B

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T FM bit rates

• Three different grades of bit rates corresponding to ETSI mask A, B and C

• Gives flexibility, ideal for different cell utilization scenarios

ChannelSize(MHz)

Bit ratesMb/s

12.5 16.1

14 18

20 25.6

25 32.2

28 36.0

36 46.2

40 51.2

50 64.4

ChannelSize(MHz)

Bit ratesMb/s

12.5 15.2

14 17.0

20 24.2

25 30.4

28 34.0

36 43.6

40 48.4

50 60.8

ChannelSize(MHz)

Bit ratesMb/s

12.5 14.4

14 16

20 22.8

25 28.8

28 32

36 41.2

40 45.6

50 57.6

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The TFM Receiver

• The optimal TFM receiver selects the most likely signal from all possible signal sequences

• Squared minimum distance between sequences will determine the performance

• QPSK has squared minimum distance 2, TFM 1.594

• Optimal receiver is complex but a Viterbi based TFM receiver has an virtually equal performance (0.1 dB difference)

• All TFM receivers need an optimal filter• Raised Cosine not suitable for TFM• Asymptotically Optimum Filter (AOF) good choice

• Also a simple MSK-type receiver has moderate performance when used with an optimal filter

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Uncoded BER performance for TFM

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Eb/N0 (dB)

Bit

err

or

pro

bab

ility

12 3

1 QPSK with optimal receiver

2 T FM with optimal receiver

3 T FM with MSK-receiver and AOF-filter

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T FM TX power compared to QPSK

• Minimum PA 1dB compression point 20 dBm

• T FM max Tx power (slightly in saturation) >21 dBm

worst case >21 dBm

• QPSK max power without excessive spectrum spreading:

minimum PA 1 dB compression point 20.0 dBmmin back off -4.0 dBm

16.0 dBmautomatic level control tolerance +-1dB -1.0 dBmnominal Tx power 15.0 dBmworst case 14.0 dBm

• Reduced TX PA size affects the following cost items:• GaAs area• Power consumption and PSU size• Heat sinking mechanics

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VCO modulated 28 GHz transmitter

-5.5…+7.5 dBm (when running +20 dBm out)

xNLO

+13 dBmTotal gain of 2 stages: 14.0 – 22.0 dB

R F out+20 dBm

dBB B in

= expensive 28 GHz GaAscircuitry

• The output from a buffered multiplier is around +13 dBm and to reach +20 dBm level only 13.5 dB of HPA gain is needed

• The gain of 28 GHz amplifier stage with good yield is 9 +-2 dB

• Total of 2 stages are needed, with max gain of 22 dB

• => to compensate tolerances extra 8.0 dB gain adjustment range is needed, with 50 dB output power rang we end up with adjustment range of 58.0 dB + other tolerances

• Note that ALC detector is not necessary with closed loop power control, since HPA can freely saturate. Smaller FETs and less number of stages can be used to produce the output power

• Higher efficiency in power amplifier -> reduced heat sinking mechanics

mean outputpower

TFM QPSK

20 dBm $137 $277

26 dBm $179 $574

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T FM in the IEE E 802.16.1 uplink• Proposed as an additional modulation scheme for the uplink,

optional for both base station and terminal• Co-exists in the same channel with QAM modulation for high

spectrum utilization: dual mode BS and two kinds of terminals• Behaves like a QAM terminal in terms of registration, ranging

and contention. Transmits in uplink mini-slots granted by the base station.

• Different preambles distinguish TFM from QAM in contention slots

• The number of mini-slots needed to transfer N TFM bits is given by

where where QAMsymbolrate and TFMbitrate are the rates defined for the channel and sizeof(minislot) is the number of PSs in each mini-slot.

)(min*4*

*

islotsizeofTFMbitrate

NateQAMsymbolr

20 © NOKIA

Dual mode base station architecture

• The dual mode base station contains separate and independent receivers for QAM and TFM

• Separate sampling and preamble detection ensures independent operation

T FM-ReceiverA

DC

AD

C

QAM-Receiver

FIF

OFI

FO

I

Q

MAC layer

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Conclusions

• T FM needs 6 dB smaller power amplifier

• T FM needed for low cost terminals

• Flexible use of TFM guarantees easy co-existence with QAM

• Seamlessly adopts to 802.16.1 uplink

• Easy implementation of dual mode base station

• Tamed Frequency Modulation (TFM) in the uplink is proposed as an optional modulation scheme for 802.16.1

22 © NOKIA

References

• F. de Jager and C.B Decker. "Tamed frequency modulation, a novel method to achieve spectrum economy in digital transmissions" IEEE Trans on Comm., vol COM-26, no 5, pp.534-542, May 1978

• Carl-Erik Sundberg. "Continuous phase modulation" IEEE Communications Magazine – Vol 24, No. 4, April 1986

• ITU-R F.1101 Characteristics of Digital Radio-Relay Systems below about 17 GHz, 1994

• IEEE 802.16.1pc-00/40, PA Model, Carmelo Decanis and Dan Weinholt, Ericsson

• BRAN HA18d24, Cost comparisons between TFM and 4QAM transmitters for HA applications, Ericsson, Nokia, Telia