Multi-carrier transmission & OFDM

Presented to : Dr. Salwa El RamliPrepared by : Amr Mohamed

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1)Multi carrier transmission:

1.1)Multi carrier transmission.

1.2)drawbacks.

2)OFDM:

2.1)Basic principles.

2.2)OFDM & FDM.

2.3)OFDM modulation

2.4)OFDM demodulation.

2.5)OFDM implementation using IFFT/FFT.

2.6)Cyclic prefix.

2.7)Frequency domain model.

2.8)Frequency diversity.

2.9)Selection of OFDM parameters.

2.10)OFDMA.

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1.1)Multi carrier transmission:

Multi carrier transmission

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1.2)Draw backs :

1- We cant make tight subcarrier

spacing “ decrease BW efficiency”.

2- Larger variations in the instantaneous transmit power similar to higher order modulation.

“high cost power amplifier & high power consumption.

Multi carrier transmission

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2.1)Basic principles:

OFDM

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2.2)OFDM & FDM :OFDM is special case of FDM, the following figures shows what are the difference between Regular FDM-single carrier and OFDM.

OFDM

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2.3)OFDM modulation:

OFDM

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Choosing the no. of subcarriers and the spacing depends on channel conditions.

Given (BW and freq. spacing) we can find the number of subcarriers.

Ex: spacing =15 khz& BW=10 Mhz 600 subcarrier.

The word OFDM subcarriers are mutually orthogonal over the time interval mTu ≤t <(m+1)Tu .

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Basic OFDM transmission can be seen as the modulation of a set of orthogonal functions

OFDM

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The ‘physical resource’ in case of OFDM transmission is often illustrated as a time–frequency grid according to Figure 4.4 where each ‘column’ corresponds to one OFDM symbol and each ‘row’ corresponds to one OFDM subcarrier.

OFDM

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Example of the OFDM signal transmitted versus time.

OFDM

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2.4)OFDM demodulation:

OFDM

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2.5)OFDM implementation using IFFT/FFT:

OFDM

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•Any corruption of the frequency-domain structure of the OFDM subcarriers, e.g. due to a frequency selective radio channel, may lead to a loss of inter-subcarrier orthogonality and

thus to interference between subcarriers

•What is the SOLUTION ? ? ? ?

OFDM

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2.6)Cyclic prefix: In case of time-dispersive channel the orthogonality between the subcarriers will, at least partly be lost, Due to inter-symbol interference within a subcarrier and interference between subcarriers.

Have in mind that time dispersion on the radio channel is equivalent to a frequency-selective channel frequency response.

A relatively modest radio-channel frequency selectivity may cause non-negligible interference between subcarriers, Due to the relatively large side lobes of each OFDM subcarrier.

OFDM

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2.6)Cyclic prefix:

OFDM

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2.6)cyclic prefix: Cyclic-prefix insertion thus increases the length of the

OFDM symbol from Tu to Tu +TCP, with a reduction in the OFDM symbol rate.

Also increasing the samples length from N to N +NCP.

Cyclic prefix is beneficial as long as the span of the time dispersion does not exceed the length of the cyclic prefix.

OFDM

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2.6)Cyclic prefix: The drawback of cyclic-prefix insertion is that only a fraction Tu/(Tu +TCP) of the received signal power is actually utilized by the OFDM demodulator

There is a trade-off between the power loss due to the cyclic prefix and the signal corruption.

Time dispersion typically increases with the cell size, beyond a certain cell size there is no reason to increase the cyclic prefix further as the corresponding power loss due to a further increase of the cyclic prefix would have a larger negative impact.

OFDM

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bk is the transmitted modulation symbol ak scaled and phase rotated by the complex frequency-domain channel tap Hk and impaired by noise nk.

2.7)Frequency domain model:

OFDM

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To properly recover the transmitted symbol for further processing, for example data demodulation and channel decoding, the receiver should multiply bk with the complex conjugate of Hk.

OFDM

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Reference symbols, sometimes also referred to as pilot symbols to estimate HUsing knowledge about the reference symbols,the receiver can estimate the frequency-domain channel around the location of the reference symbol to use it in the receiver.

OFDM

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2.8)Frequency diversity:

OFDM

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2.9)Selection of basic OFDM parameters:

2.9.1)Subcarrier spacing

2.9.2)Number of subcarriers

2.9.3)T =TCP +Tu or equivalently OFDM symbol rate.

OFDM

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2.9) Selection of basic OFDM parameters: The OFDM subcarrier spacing should be as small as possible to minimize the relative cyclic-prefix overhead TCP/(Tu +TCP).

too small subcarrier spacing increases the sensitivity of the OFDM transmission to Doppler spread and different kinds of frequency inaccuracies.

OFDM

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Variations in instantaneous power:

OFDM

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2.10)OFDMA :

OFDM

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SINRtarget at a Throughput = 500 kbps for different channel models:

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SINRtarget(UL) VS ( Nrb.UL & throughput) @ epa5:

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SINRtarget (UL)VS ( Nrb.UL & throughput) @ eva70: