User's Manual Chapter 13

Base FCC Regulatory Test Report
7-94
AT&T Wireless Services
FCC Type Acceptance Application
10991 Rev. 1.0 7/6/99
Figure 7.8— Illustration of OFDM Burst Parameters
The amount of time allocated to the guard interval is sufficient to allow
the channel’s time-delayed multipath components to dissipate prior to
the transmission of the following burst. The guard interval essentially
eliminates the inter-symbol interference (ISI) common to multipath
communications channels.
Although OFDM requires a large number of tones to carry data across
the channel, its frequency spectrum requirements are minimized
through the use of very narrow individual tone spacing. Normally, such
narrow spacing would be problematic due to the nature of pulsed RF
transmissions, which exhibit spectrum spreading. This spreading takes
the form of a sinc function, the characteristics of which are dependent
upon the burst risetime and burst duration. The inter-tone (sin x)/x
interference that would normally occur with narrow tone spacing is
minimized in an OFDM system. This is accomplished by taking
advantage of the orthogonality that exists between tones when their
frequency spacing is equal to the reciprocal of the burst period. Such
tone spacing greatly minimizes the effects of inter-tone (sin x)/x
interference if the receiver window is properly synchronized with the
transmitted OFDM bursts.
Once all the active frequency-domain tones have been generated for a
burst, these tones are transferred to the time domain using an Inverse
Fast Fourier Transform (IFFT). The resulting OFDM time-domain
waveform is simply a summation of the phase and amplitude
components associated with each active tone during the burst period
(refer to Figure 7.9—). Following the execution of an IFFT, a cyclic
prefix is added to the time domain signal samples, and the samples are
filtered. The filtered digital samples are converted to analog signals,
which are then converted to the operating frequency for transmission
over the airlink.
Burst Period
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Guard Time
T
burst
T
guard
T
slot
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