The quadrature phase-shift keying (QPSK) digital transmitter of Figure 1 is one of many DSP systems used in the communications industry. The following sections describe the transmitter in detail.

**Quadrature phase shift keying (QPSK)**

QPSK is a method for transmitting digital information across
an analog channel. Data bits are grouped into pairs, and
each pair is represented by a particular waveform, called a
symbol, to be sent across the channel after modulating the
carrier. (The receiver will demodulate the signal and look
at the recovered symbol to determine which pair of bits was
sent.) This requires having a unique symbol for each
possible combination of data bits in a pair. Because there
are four possible combinations of data bits in a pair, QPSK
creates four different symbols, one for each pair, by
changing the I gain and Q gain for the cosine and sine
modulators in Figure 1. To transmit each pair
of bits in the source data, the gains are kept constant over
a fixed number of output samples known as the symbol
period,

The QPSK transmitter system uses both the sine and cosine at
the carrier frequency to transmit two separate message
signals,

QPSK Transmitter |
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**Pseudo-noise generation**

The input bits to the transmitter are provided by a special
shift-register, called a pseudo-noise generator
(PN generator), in Figure 2. A PN
generator produces a sequence of bits that appears random.
The PN sequence will repeat with period

As shown in Figure 2, the PN generator is simply a shift-register and XOR gate. Bits 1, 5, 6, and 7 of the shift-register are XORed together and the result is shifted into the highest bit of the register. The lowest bit, which is shifted out, is the output of the PN generator.

The PN generator is a useful source of random data bits for system testing. We can use the output of a PN generator as a "typical" sequence that could be transmitted by a user. The sequence is a good data model because communications systems tend to randomize the bits transmitted for efficient use of bandwidth. PN generators have other applications in communications, notably in the Code Division Multiple Access schemes used by cellular telephones.

Pseudo-Noise Generator |
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**Series-to-parallel conversion**

The PN generator produces one output bit at a time, but each symbol the system transmits will encode two bits. Therefore, we require the series-to-parallel conversion to group the output bits from the PN generator into pairs of bits so that they can be mapped to a symbol.

**I/Q look-up table**

This block is responsible for mapping pairs of bits to
in-phase and quadrature gains. Such a mapping is often
described by a signal constellation. Figure 3
shows the data mapping constellation for the QPSK system.
In this case the data are grouped into pairs and each pair
maps to a separate in-phase
(

QPSK Constellation |
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One way to implement this mapping is by using a look-up
table. A pair of data bits can be interpreted as an
offset into an

The constellation bit-assignments are such that any two adjacent constellation points differ by only one bit. This assignment is called Gray coding and helps reduce the number of bit errors made in the event of a received symbol error.