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ABSTRACT
OFDM (orthogonal frequency-division multiplexing) is one of the key digital communication technologies of the current decade. The first part of this paper presents the fundamentals of OFDM and its benefits in the presence of multipath propagation in a tutorial-like fashion. The second part details on some of the most important aspects of OFDM transceiver implementation: concept of receiver channel filtering and A/D conversion, radio impairment compensation (I/Q mismatch), and OFDM demodulator (FFT) design.
OFDM Basics
1. Introduction
The basic principle of OFDM is to split a high-rate DataStream into a number of
lower rate streams that are transmitted simultaneously over a number of
sub carriers. Because the symbol duration increases for lower rate parallel
sub carriers, the relative amount of dispersion in time caused by multipath delay
spread is decreased. Intersymbol interference is eliminated almost completely
by introducing a guard time in every OFDM symbol. In the guard time, the
symbol is cyclically extended to avoid inter carrier interference.
In OFDM design, a number of parameters are up for consideration, such
as the number of sub carriers, guard time, symbol duration, subcarrier spacing,
modulation type per sub carrier. The choice of parameters is influenced by
system requirements such as available bandwidth, required bit rate, tolerable
delay spread, and Doppler values. Some requirement are conflicting. For
instance, to get a good delay spread tolerance, a large number of sub carriers
with small sub carrier spacing is desirable, but the opposite is true for a good
tolerance against Doppler spread and phase noise.
2. Data transmission using multiple carriers
An OFDM signal consists of a sum of sub carriers that are modulated by using
Phase shift keying (PSK) or curvature amplitude modulation (QAM). If i d are
The complex QAM symbol, s N is the number of sub carriers, T the symbol
duration, and
T
f f i i = + 0 the carrier frequency, then one OFDM symbol
starting at s t = t can be written as:
In the literature, often the equivalent complex notation is used, which is
given by (2). In this representation, the real and imaginary parts correspond to
the in-phase and quadrature parts of the OFDM signal, which have to be
multiplied by a cosine and sine of the desired carrier frequency to produce the
final OFDM signal. Figure (1) shows the operation of the OFDM modular in
block diagram.
As an example, figure (2) shows four sub carriers from one OFDM
signal. In this example, all submariners have the phase and amplitude, but in
practice the amplitudes and phases may be modulated differently for each
sub carrier. Note that each sub carrier has exactly an integer number of cycles in
the interval T , and the number of cycles between adjacent sub carries differs by
exactly one. This properly accounts for the orthogoality between sub carriers.

OFDM Basics


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OFDM stands for Orthogonal Frequency Division Multiplexing. OFDM is a technique that allows a base station to split a chunk of radio spectrum into sub-channels. The signal strength of the sub-channels and the number of channels assigned to different devices can be varied as needed. OFDM allows high data rates, even far from a base station, and it copes well with the type of radio interference that is common in urban areas, where signals reflect off walls to produce confusing echoes.
This is a method to transmit data using a large number of carriers that are separated on different frequencies to carry one data stream which has been broken up into many signals.
Speeds that are lower are easier to detect. It was discovered that by using multiple subcarriers the receiver could detect signals easier in environments with interference. Subcarriers transmit a lower speed signal which is converted by to its original high-speed signal at the other end. The subcarriers for OFDM are modulated by several methods including QAM and QPSK.
The basic principle of OFDM is to split a high-rate datastream into a number of lower rate streams that are transmitted simultaneously over a number of subcarriers. Because the symbol duration increases for lower rate parallel subcarriers, the relative amount of dispersion in time caused by multipath delay spread is decreased. Intersymbol interference is eliminated almost completely by introducing a guard time in every OFDM symbol. In the guard time, the symbol is cyclically extended to avoid intercarrier interference.

What is OFDM?

• Orthogonal FDM – it’s multiplexing
• It’s more: – Multi Carrier – Digital modulation (PSK, QAM) – Digital processing
• Demultiplexing
In a conventional serial data system, the symbols are transmitted sequentially, with the frequency spectrum of each data symbol allowed to occupy the entire available bandwidth. A When the data rate is sufficient high, several adjacent symbols may be completely distorted over frequency selective fading or multipath delay spread channel.

STUDY OF OFDM

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What is OFDM?

OFDM is one of the applications of a parallel-data-transmission scheme, which reduces the influence of multipath fading and makes complex equalizers unnecessary.

Why OFDM is used?

To create broadband multimedia mobile communication systems, it is necessary to use high-bit-rate transmission of at least several megabits per second.
If digital data is transmitted at the rate of several megabits per second, the delay time of the delayed waves is greater than 1 symbol time.
Adaptive equalizer at the receiver is one method for equalizing these signals.
There are practical difficulties in operating this equalization at several megabits per second with compact, low-cost hardware.
OFDM reduces the influence of multipath fading and makes complex equalizers unnecessary.

How Equalizers are unnecessary?

OFDM splits high bit rate stream into many low bit rate streams.
Each stream being sent using an independent carrier frequency.

Advantage

Can easily adapt to severe channel conditions without complex time-domain equalization.
Robust against narrow-band co-channel interference.
Robust against intersymbol interference (ISI) and fading caused by multipath propagation.
High spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc.
Efficient implementation using Fast Fourier Transform (FFT).
Low sensitivity to time synchronization errors.
Tuned sub-channel receiver filters are not required (unlike conventional FDM).
Facilitates single frequency networks (SFNs); i.e., transmitter macrodiversity.

Disadvantage

Sensitive to Doppler shift.
Sensitive to frequency synchronization problems.
High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency.
Loss of efficiency caused by cyclic prefix/guard interval.

Interleaving

The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs. A similar design of audio data encoding makes compact disc (CD) playback robust.