Agilent Technologies N1911A User Manual download pdf (Page 6)

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Figure 5 Various duplexing techniques (TDD, FDD and H-FDD)

Duplexing Techniques

WiMAX systems can be deployed

as Time Division Duplexing (TDD),

Frequency Division Duplexing (FDD),

or Half-Duplex FDD. (See Figure 5)

In the TDD conﬁ guration, the BS and

subscriber equipment transmit on the

same RF frequency, but at a different

time. The BS transmits a Downlink

subframe, followed by a short gap

called Transmit/Receive Transi-

tion Gap (TTG), whereas the Uplink

subframe is transmitted by individual

subscriber. The subscribers are ac-

curately synchronized so that their

transmissions do not overlap each

other as they arrive at the BS. There

is another short gap called Receive/

Transmit Transition Gap (RTG) after

the Uplink subframe allocated before

the BS can start transmitting again.

In the FDD conﬁ guration, the down-

link and uplink frames are identical to

TDD; however they are transmitted on

separate RF frequencies and overlap

each other on a speciﬁ c time. No TTG

or RTG gaps are required in this FDD

mode.

The Half-Duplex FDD conﬁ guration

combines the characteristics of FDD

and TDD. Notice the BS and subscrib-

er equipment transmit on different

frequencies (like FDD) but they do not

transmit at the same time (like TDD).

A Downlink subframe always begins

with a Preamble (P), followed by a

Header (H), and one or more downlink

bursts of data. These downlink bursts

are generally made up of multiple

symbols within the burst. Within

each burst, the modulation type is

constant, however, from burst to burst

the modulation type can change.

“Robust” bursts such as BPSK and

QPSK are required to be transmitted

ﬁ rst, followed by less robust modula-

tion types (QAM). The order is BPSK

followed by QPSK followed by 16QAM

followed by 64QAM. Midambles

are sometimes inserted between

Data Bursts. Note that each Uplink

subframe is preceded by a Preamble.

This is called “short Preamble” and it

allows the BS to synchronize on each

individual subscriber itself.

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