generalised multi-carrier (gmc)

TeNeT Group

IIT Madras

K.Giridhar/2006Cellular OFDM – OFDMA Fundamentals

1

Generalised Multi-Carrier (GMC) ++

K. Giridhar

Professor of Electrical EngineeringTelecom & Comp. Networks (TeNeT) Group

Indian Institute of Technology, MadrasChennai 600036

www.tenet.res.in

TeNeT Group

IIT Madras


2

AGC

Sampler DFTError

-- Gross Freq. Offset-- Channel Estimation

and Equalization

OFDM Receiver Algorithms -- Recap

RecoveryP/S and

Detection

Freq.

-- Fine Freq. Offset-- Timing Estimation

TeNeT Group

IIT Madras


3

Block Tx flavours

Multi-Carrier with

Cyclic Prefix (CP)

OFDM

OFDMA

Zero-Padding (ZP)

Unique-Word (UW)

Generalised MC (GMC) FDOSS

IFDMA

DFT-Precoded OFDMA

Filtered MC (5G?)

Single-Carrier with CP, ZP, or UW

TeNeT Group

IIT Madras


4

Conventional OFDM

FDE -- Conventional OFDM

FFTFrequencyDomainEqualiser

RemoveCP

Rx

Algos.

Detection& P/S

IFFTAddCP

Tx

Mod.

SymbolMapping

& S/P

TeNeT Group

IIT Madras


5

TDE + FDE for OFDM

Time & Frequency Domain Equalisation-- for OFDM in large delay spread channels

FFTFrequencyDomainEqualiser

RemoveCP

Rx

Algos.

Detection& P/S

IFFTAddCP

Tx

Mod.

SymbolMapping

& S/P

Time-DomainEqualiser

TeNeT Group

IIT Madras


6

Tx -- low-complexity, TDMARx -- implements SC-FDE;

Linear Equaliser or DFE

Frequency Domain Equalisation-- Single Carrier FDE (SC-FDE)

RxDemod.

AddCP

(of QAMsymbols)

TxMod.

SymbolMapping

FFT

FrequencyDomainEqualiser

Sub.CP

Dete

ctor

to permit FDE DecisionFeedback

Filter

IFFT

-

TeNeT Group

IIT Madras


7

Single Carrier & Generalised Multi-Carrier

Single Carrier with CP offers Low PAPR Freq. Diversity (since each QAM symbol “sees” the entire BW) Ability for multiplexing (of different user streams on down-link) However, not suitable for up-link

(a) poor link margin! (b) multiplexing requires CP between every user burst – inefficient

Generalised Multi-carrier modulation for the Uplink Provides narrow-banding => higher link margin! Provides freq. domain multiplexing – spectrally efficient

F-DOSS – Freq. Domain Orthogonal Spread Spectrum Chang & Chen, IEEE Comm. Letters, Nov.2000

Interleaved OFDMA (I-OFDMA) or DFT spread OFDMA 3GPP LTE has adopted this for UL

TeNeT Group

IIT Madras


8

Let KP=N, where N is FFT size,K – No. of uplink usersP – No. of subcarriers per user

Generalised MC with CP – F-DOSS

RepeatK times

(in time)

TxMod.

Take P

QAMSymbols

user #1 AddCP

nN

j

e0

2

RepeatK times

(in time)

TxMod.

Take P

QAMSymbols

user #2 AddCP

nN

j

e1

2

RepeatK times

(in time)

TxMod.

Take P

QAMSymbols

user #K AddCP

nKN

j

e)1(

2

Assumptions:All K users are freq. synchronised

(within 1% in 802.16e)All K users are ranged

(well within CP length)

TeNeT Group

IIT Madras


9

F-DOSS: FDE at Base Station

RxDemod.

FFT

&CopyMinus

CP

Z.I.– Interleave with K-1

zeroes to suppress noise

FrequencyTranslation

Assuming all K UL

users are ranged

FDE foruser #2

Dete

ctor

Z.I.+

IFFT

user #2Truncate(to P)

FDE foruser #K

Dete

ctor

Z.I.+

IFFT

user #KTruncate(to P)

FDE foruser #1

Dete

ctor

Z.I.+

IFFT

user #1Truncate(to P)

P sub-carriers1,K+1,2K+1,…,(N-K+1)

TeNeT Group

IIT Madras


10

Single Carrier to Generalised Multi-Carrier -- Motivation

F-DOSS offers Ability for multiplexing Uplink users efficiently

Low PAPR

Low Computational complexity

Better link margin (by a factor of N/P=K for each user)

But, flexibility is limited since each user stream “goes thro” uniformly spaced (K-spaced) subcarriers

also, each QAM symbol only present in K (of N) transmit samples

Interleaved OFDMA Some PAPR increase+ increase in computational complexity

But, ensures more flexibility User stream can occupy any P out of N sub-carriers (like OFDMA)

Each QAM symbol is present on all the N transmit sample

Question: Does this “ensure” better CCI averaging in reuse-1 systems?

TeNeT Group

IIT Madras


11

Let KP=N, where N is FFT size,K – No. of uplink usersP – No. of subcarriers per user

Generalised MC – Interleaved OFDMA

PxP

Mixing Matrix

TxMod.

Take P

QAMSymbols

user #1 AddCP

1,,12,1,1

carriers-subon placed Samples

KNKK

Mixing Matrix can simply be a K point DFT matrix!

NxN

IFFT

PxP

Mixing Matrix

TxMod.

Take P

QAMSymbols

user #2 AddCP

2,,22,2,2


KNKK

NxN

IFFT

PxP

Mixing Matrix

TxMod.

Take P

QAMSymbols

user #K AddCP

NKKK ,,3,2,


NxN

IFFT

To mimic FDOSS

TeNeT Group

IIT Madras


12

I-OFDMA: FDE at Base Station

RxDemod.

FFT

(+ De-

Mux)

MinusCP

FrequencyTranslation

FDE foruser #K

Dete

ctor

user #K

FDE foruser #1

Dete

ctor

user #1

P sub-carriers1,K+1,2K+1,…,(N-K+1)

OR, any P P-ptIDFT

P-ptIDFT

TeNeT Group

IIT Madras


13

Summary/Pending issues (GMC)

In GMC techniques, channel estimation requires “lumped” pilots

Use sys-ID in time domain to estimate CIR

Solve Wiener-Hopf (MMSE) equations to define LE/DFE of required order either in TD or FD

Study the pilot-allocation in the UL of LTEs

Understand impact of CCI on UL

generalised multi-carrier (gmc)

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