Agilent Technologies N1911A Manuel d'utilisateur télécharger pdf (Page 4)

4

OFDM/OFDMA Subcarriers

The WiMAX’s speciﬁ cations for the

256-carrier OFDM PHY deﬁ ne three

types of subcarriers: data, pilot, and

null. 200 carriers are used for data and

pilot subcarriers. Eight pilot subcarri-

ers are permanently spaced through-

out the OFDM spectrum. Therefore the

data subcarriers take up the remaining

192 active carriers. The remaining 56

potential carriers are nulled and set

aside for guard bands and removal of

the center frequency subcarriers [2].

Figure 2 show the comparison be-

tween 802.11a (Wireless LAN) and

802.16 (WiMAX). In WLAN, 52 carriers

are used while 48 provide data and

4 provide pilots. In WiMAX, only 200

of the possible 256 carriers are used.

These are numbered between –100

to +100. The center frequency carrier

(#0) is not used. From the 200 carriers,

Figure 2 802.11a vs 802.16 WiMAX

Figure 3 802.16e OFDMA Mobile WiMAX

a longer symbol period is because a

longer symbol period is more resis-

tant to channel impairments such as

multipath fading. This longer symbol

period allows WiMAX to be used

more effectively than WLAN over

longer distances and in NLOS applica-

tions. However, for spectral efﬁ ciency,

WLAN and WiMAX are approximately

equal.

In OFDMA mode, various subscrib-

ers can be served simultaneously by

assigning each subscriber a speciﬁ c

carrier group (subchannelization)

that carries the data intended for that

subscriber (see Figure 3).

192 of these are used for data and 8

are used for pilots. WiMAX can be

conﬁ gured with the bandwidth rang-

ing between 1.25 MHz to 28 MHz.

Figure 2 shows an example of

1.5 MHz bandwidth, whereby carri-

ers are only spaced 6.7 kHz apart. As

compared to WLAN, the individual

subcarriers are spaced over 300 kHz

apart.

While WiMAX has signiﬁ cantly more

subcarriers and therefore more data

in each symbol, it does not have a

signiﬁ cantly higher data rate than

WLAN. This is because in OFDM,

when the carriers become more

closely spaced, the symbol period

must be increased proportionately to

maintain the orthogonality of the in-

dividual carriers. The beneﬁ t of using

more closely spaced carriers and thus

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