Wireless Communication

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The goal of third and fourth generation mobile networks is to provide users with high data rates, and to provide a wider range of services, such as voice communications, videophones, and high speed Internet access. A common challenge in designing a wireless system is to overcome the effects of the wireless channel, which is characterized as having multiple transmission paths and as being time varying. OFDM has a promising future as a new technology in several next generation wireless communication systems. The ability of OFDM systems to combat the effects of multipath propagation with a comparatively simple receiver structure made it the modulation of choice for some of the most prominent wireless technologies such as the IEEE 802.11 …show more content…

1.1.1 OFDM Advantages
In summary, OFDM has the following advantages:
1. OFDM is an efficient way to deal with multipath effects. For a given channel delay spread, the implementation complexity is much lower than that of a conventional single carrier system with time domain equalizer.
2. Bandwidth efficiency is high since it uses overlapping orthogonal subcarriers in the frequency domain.
3. Modulation and demodulation are implemented using inverse discrete
Fourier transform (IDFT) and discrete Fourier transform (DFT), respectively. The fast Fourier transform (FFT) algorithm can be applied to make the overall system efficient.
4. Capacity can be significantly increased by adapting the data rate per subcarrier according to the signal-to-noise ratio (SNR) of that particular subcarrier. 5. OFDM is robust against narrowband interference because such interference affects only a small percentage of the subcarriers.
1.1.2 Problems Associated with OFDM
OFDM communication systems have two primary drawbacks. The first is the high sensitivity to carrier frequency offsets and phase noise. When there are frequency offsets in the subcarriers, the orthogonality among the subcarriers breaks and this causes intercarrier interference (ICI). The second drawback, is that the transmitted OFDM signal has large amplitude fluctuations and so a high
PAPR. This high PAPR requires system components with a wide linear range to accommodate for the signal variations.

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