OFDM Juan Camilo Sacanamboy

Content

1. Concepts 2.  Basic idea 3.  FDM: The “mother” of OFDM 4. OFDM 5.  Applications

Concepts (1)

Modulation

Carrier signal

Modulation is the process of conveying a message signal (modulating signal) inside another signal (carrier signal) that can be physically transmitted.

Waveform that is modulated with an input signal for the purpose of conveying information. Reference: Link

Concepts (2)

Broadband Wide bandwidth of a transmission medium. Ability to transport multiple signals and multiple traffic types simultaneously.

Concepts (3)

Subcarrier

It is an already-modulated s i g n a l , w h i c h i s t h e n modulated into another signal of higher frequency and bandwidth.

Frequency range in a given bandwidth

Reference: Link

Concepts (4)

Orthogonality

Reference: Link

The peak of one subcarrier coincides with the null of an adjacent subcarrier.

Concepts (5)

Intersymbol Interference

Reference: Link

Concepts (6)

Selective Fading

Reference: Link

Is a radio propagation anomaly caused by partial cancellation of a radio signal by itself – the signal arrives at the receiver by two different paths, and at least one of the path is changing (lengthening or shortening).

Basic idea

•  Orthogonal Frequency Division Multiplexing •  Multicarrier broadband modulation technique for

transmitting large amounts of digital data.

FDM: The “mother” of OFDM •  Frequency Division Multiplexing •  Signals from multiple transmitters are transmitted

simultaneously over multiple frequencies. •  Each subcarrier is modulated separately by different data

stream and a guard band is placed between subcarriers to avoid signal overlap.

Reference: Link

OFDM (1) •  Like FDM, OFDM uses multiple subcarriers BUT:

o  There are closely spaces to each other without causing interference, removing guard bands.

o  Its possible because subcarriers are orthogonal.

Reference: Link

OFDM (2)

Basic OFDM System •  A very high rate data stream is divided into multiple parallel

low rate data streams. •  Each smaller data stream is then mapped to individual data

subcarrier and modulated using some sorts of PSK or QAM. i.e. BPSK, QPSK, 16-QAM, 64-QAM.

OFDM (3) Basic OFDM System

Basic OFDM System (Hanzo, Webb, & Keller, 2000)

OFDM (4) Characteristics •  High Spectral Efficiency: OFDM needs less bandwidth than FDM

to carry the same amount of information. •  Resilience: More resilient in NLOS environment than FDM. •  Fault Tolerance: It can efficiently overcome interference and

frequency-selective fading caused by multipath because ecualizing is done on a subset of subcarriers instead of a single broader carrier.

•  Supress effect of ISI (Inter Symbol Interference): Longer symbol period of the parallel OFDM subcarriers than a single carrier system.

OFDM (5)

Disadvantage The main disadvantage of the OFDM system is the complexity of implement N modulators at the transmitter and N demodulators at the receiver. Solution: This problem can be reduced using the discrete Fourier transform (DFT), implemented as a fast Fourier transform (FFT).

OFDM (6)

QAM-OFDM The basic system has N sub-bands, each separated from its neighbour by a guard band. The available spectrum can be used much more efficiently if the spectra of the individual sub-bands are allowed to overlap.

Detailed OFDM System (Hanzo, Webb, & Keller, 2000)

Serial to parallel convertor

OFDM (7) Detailed OFDM System •  The input serial data stream is rearranged into a sequence { 𝑑↓𝑛 } of N QAM symbol at baseband.

•  At the 𝑛th symbol instant, the QAM symbol is represented by th symbol instant, the QAM symbol is represented by an in-phase component 𝑎(𝑛) and a quadrature component 𝑏(𝑛). ▫  𝑑(𝑛)=𝑎(𝑛)+𝑗𝑏(𝑛)

•  A block of N QAM symbols are applied to a serial-to-parallel convertor and the resulting in-phase symbols are applied to N pairs of balanced modulators.

OFDM (8)

Detailed OFD System •  The quadrature components 𝑎(𝑛) and 𝑏(𝑛)

modulate the carriers cos(𝑤↓𝑛 𝑡)and sin( 𝑤↓𝑛 𝑡) respectively.

•  The modulated carriers 𝑎(𝑛) cos( 𝑤↓𝑛  𝑡)  and 𝑏(𝑛)sin( 𝑤↓𝑛 𝑡) when added together constitute a QAM signal. The 𝑛th QAM signal is given by: th QAM signal is given by: ▫  𝑋↓𝑛 =𝑎(𝑛)cos(𝑤↓𝑛 𝑡) +𝑏(𝑛)sin( 𝑤↓𝑛 𝑡)

OFDM (9)

FDM/QAM signal 𝐷(𝑡)=  ∑𝑛=0↑𝑁−1▒𝑋↓𝑛 (𝑡) 

OFDM (10)

Modulation by Discrete Fourier Transform •  Large number of sub-channels modems •  Taking the DFT of the original block of N QAM symbols and

then transmitting the DFT coefficients serially is exactly equivalent to the operations required by the OFDM transmitter of a Detailed OFDM System.

•  Simplifications can be achieved if the bank of sub-cannel modulators/demodulators is implemented using (IFFT/FFT).

OFDM (11)

Reference: Link

FFT and IFFT are linear transformations on signals. One is the reverse of the other one. It doesn’t matter the order if apply IFFT in the transmitter or in the receiver because IFFT and FFT are inverse. The question becomes why use IFFT in the transmitter. That’s because signals need to be modulated by N-QAM of the ortogonal subcarriers. Mathematically, the process can be represented by IFFT.

Applications

• ADSL • HomePlug AV • WiMedia UWB • Wifi (801.11 a/g/ac) • WiMax

References

• Conniq. (s.f.). Introduction to FDM, OFDM, OFDMA, SOFDMA.

• Hanzo, L., & Keller, T. OFDM and MC-CDMA. • Hanzo, L., Webb, W., & Keller, T. (2000).

Single- and Multi-carrier Quadrature Amplitude Modulation : principles and applications for personal communications, WLANs and broadcasting.