1.1
Radio Channel Behavior
5
Alternatively, OFDM can efficiently deal with all these ISI effects, which occur in
multi-path propagation situations and in broadband radio channels. Simultaneously,
the OFDM transmission technique needs much less computational complexity in the
equalization process inside each receiver. The performance figures for an OFDM
based new air interface and for next generation of mobile communications are very
promising even in frequency-selective and time-variant radio channels.
1.2 Basics of the OFDM Transmission Technique
If a high data rate is transmitted over a frequency-selective radio channel with a
large maximum multi-path propagation delay τ
max
compared to the symbol dura-
tion, an alternative to the classical SC approach is given by the OFDM transmission
technique. The general idea of the OFDM transmission technique is to split the
total available bandwidth B into many narrowband sub-channels at equidistant fre-
quencies. The sub-channel spectra overlap each other but the subcarrier signals are
still orthogonal. The single high-rate data stream is subdivided into many low-rate
data streams for the sub-channels. Each sub-channel is modulated individually and
will be transmitted simultaneously in a superimposed and parallel form. An OFDM
transmit signal therefore consists of N adjacent and orthogonal subcarriers spaced
by the frequency distance ∆f on the frequency axis. All subcarrier signals are mu-
tually orthogonal within the symbol duration of length T
S
, if the subcarrier distance
and the symbol duration are chosen such that T
S
=
1
∆f
. For OFDM-based systems,
the symbol duration T
S
is much larger compared to the maximum multipath delay
τ
max
. The k-th unmodulated subcarrier signal is described analytically by a complex
valued exponential function with carrier frequency k∆f, ˜
g
k
(t), k = 0, . . . , N − 1.
˜
g
k
(t) =
⎧
⎨
⎩
e
j2πk∆f t
∀t ∈ [0, T
S
]
0
∀t ∈ [0, T
S
]
(1.3)
Since the system bandwidth B is subdivided into N narrowband sub-channels, the
OFDM symbol duration T
S
is N times larger than in the case of an alternative SC
transmission system covering the same bandwidth B. Typically, for a given system
bandwidth, the number of subcarriers is chosen in such a way that the symbol
duration T
S
is sufficiently large compared to the maximum multi-path delay τ
max
of
the radio channel. Additionally, in a time-variant radio channel, the Doppler spread
imposes restrictions on the subcarrier spacing ∆f . In order to keep the resulting
Inter-Carrier Interference (ICI) at a tolerable level, the system parameter of the
subcarrier spacing ∆f must be large enough compared to the maximum Doppler
frequency f
Dmax
. In [18] the appropriate range for choosing the symbol duration T
S
as a rule of thumb in practical systems is given as (cf. Eq. (1.2)):
4τ
max
≤ T
S
≤ 0.03
1
f
Dmax
(1.4)
The duration T
S
of the subcarrier signal ˜
g
k
(t) is additionally extended by a cyclic
prefix (so-called guard interval) of length T
G
, which is larger than the maximum
1.2 Basics of the OFDM Transmission Technique
7
s
(t) =
∞
n=0
N −1
k=0
S
n,k
e
j2πk∆f (n−nT )
rect
2(t − nT ) + T
G
− T
S
2T
(1.7)
The analytical signal description shows that a rectangular pulse shaping is applied
for each subcarrier signal and each OFDM symbol. Due to the rectangular pulse
shaping, the spectra of all the considered subcarrier signals are sinc-functions which
are equidistantly located on the frequency axis, e.g., for the k-th subcarrier signal,
the spectrum is described in Eq. (1.8):
G
k
(f ) = T · sinc [πT (f − k∆f)]
(1.8)
The typical OFDM spectrum shown in Fig. 1.6 consists of N adjacent sinc-
functions, which are shifted by ∆f in the frequency direction.
( )
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