Agilent Technologies N1911A Bedienungsanleitung PDF herunterladen (Seite 5)

5

Modulation Type

The IEEE 802.16-2004 standard

deﬁ nes a set of adaptive modulation

and coding (AMC) rate conﬁ guration

that can be used to trade off data rate

for system robustness under various

wireless propagation and interference

conditions. When the radio link qual-

ity is good, the WiMAX system can

use higher-order modulation schemes

(more bits/symbol) that will result in

more system capacity. When link con

802.16 Concepts: Subchannel-

ization

Subchannelization is an optional

capability that can be supported by

subscriber and Base Stations (BS).

Figure 4 shows the Uplink subchan-

nelization. One subchannel contains

12 carriers. OFDM WiMAX has 192

active data carriers, so there are a to-

tal of 16 subchannels . These are allo-

cated in various combinations for the

creation of 31 different conﬁ gurations

to be assigned to a user. (Details on

subchannelization can be found in [1],

Table 213 for OFDM.)

In the most basic operation, all 16

subchannels (all 192 carriers) are

assigned to a single user. Whereas in

the most complex conﬁ guration, 16

users are assigned to one subchan-

nel, the downlink symbol would still

contain all 192 carriers, but user’s

data for the individual user would

only have 12 carriers. For uplink,

all 16 users would transmit signal

simultaneously, therefore their signals

would arrive at BS at the same time

with approximately the same power.

It is important to get the subscriber’s

station time, frequency, and amplitude

aligned so that the BS can receive,

demodulate and decode informati

from 16 different transmitters simul-

taneously.

ditions are poor due to problems such

as signal fading or interference, the

WiMAX system can change to lower

modulation schemes to maintain an

acceptable radio link margin. The

allowed modulation types are Binary

Phase Shift Keying (BPSK), Quadra-

ture Phase Shift Keying (QPSK), 16

Quadrature Amplitude Modulation

(16QAM) and 64QAM [2].

Figure 4 Uplink Subchannelization

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