Wireless Communication

Assignment # 3

Submitted To: Dr. Khawar

Submitted By: M. MohsinSardar

OFDM System Design

Introduction:

Orthogonal Frequency Division Multiplexing(OFDM) is a method that allows transmission of high data rates over extremely hostile channels at a comparable low complexity. OFDM is a multicarrier system that uses discrete Fourier Transform/Fast Fourier Transform (DFT/FFT). Orthogonal FDM’s (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. OFDM provides high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. OFDM is sometimes called multi-carrier or discrete multi-tone modulation.

Single Carrier System Multicarrier System

Designing of OFDM System:

Applications of OFDM:

Digital Television

  • European and Australian standard

Wireless Local Area Networks (LANs)

  • Hiperlan 2

ADSL (asymmetric digital subscriber loop)

  • High speed data transmitted along existing telephone lines

Mobile Telephony

  • 4G mobile networks

Advantages of OFDM:

  • OFDM makes efficient use of the spectrum by allowing overlap.
  • By dividing the channel into narrowband flat fading subchannels, OFDM is more resilient to frequency selective fading as compared to single carrier systems.
  • Eliminates ISI and IFI through use of a cyclic prefix.
  • Using adequate channel coding and interleaving one can recover symbols lost due to thefrequency selectivity of the channel.
  • Channel equalization becomes simpler than by using adaptive equalization techniques withsingle carrier systems.
  • It is possible to use maximum likelihood decoding with reasonable complexity.
  • OFDM is computationally efficient by using FFT techniques to implement the modulationand demodulation functions.
  • Is less sensitive to sample timing offsets than single carrier systems are.
  • Provides good protection against co-channel interference and impulsive parasitic noise.

Disadvantages of OFDM:

  • The OFDM signal has a noise like amplitude with a very large dynamic range, therefore itrequires RF power amplifiers with a high peak to average power ratio.
  • It is more sensitive to carrier frequency offset and drift than single carrier systems are dueto leakage of the DFT.

References:

Wirelesscafe, OFDM and its wireless applications. [online] Available at:

[accessed on 10th May, 2011]

Scribd, OFDM Introduction and System Model. [online] Available at:

on 13th May, 2011]

Ieeevic, orthogonal Frequency Division Multiplexing. [online] Available at:

on 11th May, 2011]