Page 58 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes 3.4.5 Orthogonal Frequency Division Multiplexing (OFDM)

Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on
multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital
communication, whether wireless or over copper wires, used in applications such as digital
television and audio broadcasting, DSL broadband internet access, wireless networks, and 4G
mobile communications.
OFDM is essentially identical to coded OFDM (COFDM) and discrete multi-tone modulation
(DMT), and is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier
modulation method. The word “coded” comes from the use of forward error correction (FEC). A
large number of closely spaced orthogonal sub-carrier signals are used to carry data on several
parallel data streams or channels. Each sub-carrier is modulated with a conventional modulation
scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate,
maintaining total data rates similar to conventional single-carrier modulation schemes in the
same bandwidth.

The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe
channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband
interference and frequency-selective fading due to multipath) without complex equalization
filters. Channel equalization is simplified because OFDM may be viewed as using many slowly
modulated narrowband signals rather than one rapidly modulated wideband signal. The low
symbol rate makes the use of a guard interval between symbols affordable, making it possible to
eliminate intersymbol interference (ISI) and utilize echoes and time-spreading (on analogue TV
these are visible as ghosting and blurring, respectively) to achieve a diversity gain, i.e. a signal-to-
noise ratio improvement. This mechanism also facilitates the design of single frequency networks
(SFNs), where several adjacent transmitters send the same signal simultaneously at the same
frequency, as the signals from multiple distant transmitters may be combined constructively,
rather than interfering as would typically occur in a traditional single-carrier system.

3.4.6 Ultra Wideband Modulation (UWB)

Ultra-wideband (UWB) signals are generally defined as signals with fractional bandwidth greater
then twenty percent of their central frequency or as signals with bandwidth more than 500MHz,
whichever is less. The bandwith is specified by 10dB decrease of the signal power spectral density.
Many different generation techniques may be used to satisfy these requirements. The principal
group of generation techniques is based on spectral characteristics of very short pulses and we
can denominate these techniques as Impulse Radio concepts. These very short duration pulses
(hundreds of picoseconds) have very wide spectrum, which must adhere to the spectral mask
requirements. Very low power levels are permitted for typical UWB transmission because of the
compatibility with other radiocommunication services. Many pulses are typically combined to
carry the information for one bit and to separate individual transmissions of several users. For
the same purposes, a variety of pulse shapes, wavelets and waveforms can be used. Pulses can
be sent individually, in bursts, or in near-continuous streams, and they can encode information
in pulse amplitude, polarity, shape, and position.

3.5 Sending Data Packets in the Air

A central problem for business individuals on the move, concerns the ability to communicate
data between the work base and remote locations. A new technology has evolved over the past
few years which allows the transmission of digital data across existing air link analogue cellular
voice channels as well as across existing Circuit Switched telephone networks. This technology is
known as Cellular Digital Packet Data (CDPD).

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