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Wireless Fundamentals

Understanding Spread Spectrum Technologies

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Spread Spectrum

Narrowband and spread spectrum are the two main ways of sending a signal.

Spread spectrum uses less energy at peak.

The bandwidth required depends on the amount of information to be sent.

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FHSS is a time-based narrowband hopping of frequencies.

DSSS is a broadband use of frequencies.

FHSS Versus DSSS

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DSSS: Encoding

Each bit is transformed into a sequence, called “chip” or “symbol.”

In this example, the chipping code is called Barker 11.

Up to 9 bits can be lost.

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When using DBPSK, the phase shifts with 180° angles; each shift represents 1 bit.

When using DQPSK, shifts are 90°; each shift represents 2 bits.

DBPSK allows 1 Mb/s.

DQPSK allows 2 Mb/s.

DSSS Modulations: DBPSK and DQPSK

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With CCK, each symbol of 6 bits is associated to a unique code sequence as shown on the example here.

Coding 4 bits per symbol allows 5.5 Mb/s, coding 8 bits per symbol allows 11 Mb/s.

DSSS Modulation: CCK

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Orthogonal Frequency-Division Multiplexing

Of 64 subcarriers:

12 zero subcarriers (in black) on sides and in center

Sides function as frequency guard band, leaving 16.5-MHz occupied bandwidth

Center subcarrier zero for DC offset/carrier leak rejection

48 data subcarriers (in green) per symbol

4 pilot subcarriers (in red) per symbol for synchronization and tracking

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Uses the same principles as DBPSK and DQPSK: BPSK shifts 180º; QPSK shifts 90º.

Speed depends on density of signal per tone.

Modulation Data Rate per

Subchannel (kb/s)

Total Data Rate

(Mb/s)

BPSK 125 6

BPSK 187.5 9

QPSK 250 12

QPSK 375 18

OFDM Modulations: BPSK and QPSK

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With QAM, 90º shifts are associated with amplitude modulation.

With four amplitude positions, 16 values are possible.

OFDM for wireless uses 16-QAM and 64-QAM, with speeds up to 54 Mbps.

OFDM Modulation: QAM

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Channels and Overlap Issues

With channels built for 5-MHz interchannel space, each DSSS channel uses more than one channel.

Only three or four nonoverlapping channels are available in the 2.4-GHz ISM band.

Channel overlap can be co-channel interference or adjacent channel interference.

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Summary

Spread spectrum technologies offer better resistance to narrowband interferences.

Wireless networks use DSSS.

DBPSK allows 1 Mb/s, DQPSK 2 Mb/s.

Using CCK increases the speed to 11 Mb/s.

OFDM uses subcarriers to carry the signal.

BPSK allows 9 Mb/s, QPSK 18 Mb/s.

Using QAM increases the speed to 54 Mb/s.

Larger channels imply interference and channel collocation planning.

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