Ofdma

Orthogonal Frequency Division Multiple-Access

Dr.Ayman ElezabiYasser Monier

900062323

Orthogonal Multiplexing Principle and structure of OFDM symbols in practical standards

An OFDM signal consists of orthogonal subcarriers modulated by parallel data streams. Each baseband subcarrier is of the form

, (1)

where is the frequency of the th subcarrier. One baseband OFDM symbol (without a cyclic prefix) multiplexes modulated subcarriers:

(2)

where is the th complex data symbol (typically taken from a PSK or QAM symbol constellation) and is the length of the OFDM symbol. The subcarrier frequencies are equally spaced

(3)

Orthogonal Multiplexing Principle and structure of OFDM symbols in practical standards

The OFDM symbol (2) could typically be received using a bank of matched filters. However, an alternative demodulation is used in practice. T-spaced sampling of the in-phase and quadrature components of the OFDM symbol yields (ignoring channel impairments such as additive noise or dispersion)

, (4)

Effect of Carrier Frequency Offset and Sampling Time offset

At the front-end of the receiver OFDM signals are subject to synchronization errors due to oscillator impairments and sample clock differences. The demodulation of the received radio signal to baseband, possibly via an intermediate frequency, involves oscillators whose frequencies may not be perfectly aligned with the transmitter frequencies. This results in a carrier frequency offset. Figure 6 illustrates the front end of an OFDM receiver where these errors can occur. Also, demodulation (in particular the radio frequency demodulation) usually introduces phase noise acting as an unwanted phase modulation of the carrier wave. Carrier frequency offset and phase noise degrade the performance of an OFDM system.

Effect of Carrier Frequency Offset and Sampling Time offset

When the baseband signal is sampled at the A/D, the sample clock frequency at the receiver may not be the same as that at the transmitter. Not only may this sample clock offset cause errors, it may also cause the duration of an OFDM symbol at the receiver to be different from that at the transmitter. If the symbol clock is derived from the sample clock this generates variations in the symbol clock. Since the receiver needs to determine when the OFDM symbol begins for proper demodulation with the FFT, a symbol synchronization algorithm at the receiver is usually necessary. Symbol synchronization also compensates for delay changes in the channel.

Channel Estimation Algorithms

System Architecture

System Architecture

System Architecture (cont’d)

1. Input to time domain

2. Guard Interval

3. Channel

4. Guard Removal

5. Output to frequency domain

6. Output

7. Channel Estimation

1,...,2,1,0 NnkXIDFTnx

1,...,1,0,

1,...,1,,

Nnnx

NNnnNxnx

gg

f

nwnhnxy ff

1,...,1,0 Nnnyny f

1,...,2,1,0 NknyDFTkY

1,...,1,0

Nk

kWkIkHkXkY

ICI AWGNChannel

1,...,1,0 NkkH

kYkX

e

e

Estimated

Channel

Pilot for Channel EstimationT

ime

Carriers

Tim

e

Carriers

Comb Type: Part of the sub-carriers are

always reserved as pilot for each symbol

Block Type: All sub-carriers is used as

pilot in a specific period

Block-type Channel Estimation

LS: Least Square Estimation

1

0

110

1

.

.

.

,...,,

N

N

LS

y

y

y

xxxdiagXwhere

yXh

Comb-type Estimation

0,

1,...,1,.inf

lmpx

Lldata

lmLXkXNp pilot signals uniformly inserted in X(k)L=Number of Carriers/Np

xp(m) is the mth pilot carrier value

{Hp(k) k=0,1,…,Np} , channel at pilot sub-carriersXp input at the kth pilot sub-carrierYp output at the kth pilot sub-carrier

LS Estimate

1,...,1,0 p

p

p

p NkkX

kYkH

LMS Estimate

Xp(k)LMS +

e(k)-

Yp(k)

Interpolation for Comb-type

Linear Interpolation

Second Order Interpolation

Ll

mHL

lmHmH

lmLHkH

ppp

ee

0

1

Nl

mpHcmpHcmpHc

c

c

c

where

lmLHkH ee

/

110

11

,2

1

1

,110

,2

1

1

OFDMA, the multi user communication system.

The main motivation for adaptive subcarrier allocation in OFDMA systems is to exploit multiuser diversity. Although OFDMA systems have a number of subcarriers, we will focus temporarily on the allocation for a single subcarrier amongst multiple users for illustrative purposes.


Consider a K-user system, where the subcarrier of interest experiences i.i.d. Rayleigh fading, that is, each user’s channel gain is independent of the others, and is denoted by hk. The probability density function (pdf) of user k’s channel gain p(hk) is given by

Figure 6.4: OFDM with 256 subcarriers, and OFDMA where only 64 of the 256 subcarriers are used. The

total power used is the same, but OFDMA allows much lower peak power.

blast at high power over the entire bandwidth, OFDMA allows the same data rate to be sent over a longer

period of time using the same total power.

6.2 Multiuser Diversity and Adaptive Modulation

In OFDMA, the subcarrier and power allocation should be based upon the channel conditions in order

to maximize the throughput. In this section, we provide necessary background discussion on the key two

principles that enable high performance in OFDMA: multiuser diversity and adaptive modulation. Multiuser

diversity describes the gains available by selecting a user of subset of users that have “good” conditions.

Adaptive modulation is the means by which good channels can be exploited to achieve higher data rates.

6.2.1 Multiuser Diversity

The main motivation for adaptive subcarrier allocation in OFDMA systems is to exploit multiuser diversity.

Although OFDMA systems have a number of subcarriers, we will focus temporarily on the allocation for a

single subcarrier amongst multiple users for illustrative purposes.

Consider a K -user system, where the subcarrier of interest experiences i.i.d. Rayleigh fading, that is,

each user’s channel gain is independent of the others, and is denoted by hk . The probability density function

(pdf) of user k’s channel gain p(hk) is given by

p(hk ) =

(2hke− h2

k if hk ≥ 0

0 if hk < 0.(6.1)

Now suppose the base station only transmit to the user with the highest channel gain, denoted as hmax =

max{ h1, h2, · · · , hK } . It is easy to verify that the pdf of hmax is

p(hmax ) = 2K hmax

⇣1− e− h2

m ax

⌘K − 1

e− h2m ax . (6.2)

8


Now suppose the base station only transmit to the user with the highest channel gain, denoted as hmax = max{h1,h2,··· ,hK}. It is easy to verify that the pdf of hmax is

Figure 6.4: OFDM with 256 subcarriers, and OFDMA where only 64 of the 256 subcarriers are used. The

total power used is the same, but OFDMA allows much lower peak power.

blast at high power over the entire bandwidth, OFDMA allows the same data rate to be sent over a longer

period of time using the same total power.

6.2 Multiuser Diversity and Adaptive Modulation

In OFDMA, the subcarrier and power allocation should be based upon the channel conditions in order

to maximize the throughput. In this section, we provide necessary background discussion on the key two

principles that enable high performance in OFDMA: multiuser diversity and adaptive modulation. Multiuser

diversity describes the gains available by selecting a user of subset of users that have “good” conditions.

Adaptive modulation is the means by which good channels can be exploited to achieve higher data rates.

6.2.1 Multiuser Diversity

The main motivation for adaptive subcarrier allocation in OFDMA systems is to exploit multiuser diversity.

Although OFDMA systems have a number of subcarriers, we will focus temporarily on the allocation for a

single subcarrier amongst multiple users for illustrative purposes.

Consider a K -user system, where the subcarrier of interest experiences i.i.d. Rayleigh fading, that is,

each user’s channel gain is independent of the others, and is denoted by hk . The probability density function

(pdf) of user k’s channel gain p(hk ) is given by

p(hk ) =

(2hke− h2

k if hk ≥ 0

0 if hk < 0.(6.1)

Now suppose the base station only transmit to the user with the highest channel gain, denoted as hmax =

max{ h1, h2, ·· · , hK } . It is easy to verify that the pdf of hmax is

p(hmax ) = 2K hmax

⇣1− e− h2

m ax

⌘K − 1

e− h2m ax . (6.2)

8

THANKS

Technology

Ofdma