Adaptive Modulation and Cell Planning

Adaptive modulation is a term denoting the ability of a radio to change its modulation scheme
on the fly to adapt to varying signal conditions. In existing commercial products the radio is
not going to shift from CDMA to OFDM or anything that fundamental. Instead, the choice will
probably be between quadrature phase shift keying (QPSK) and 64 quadrature amplitude
modulation (QUAM) or perhaps 16 QUAM.
Here I will provide only the briefest definition of the two basic techniques.
Phase shift keying is somewhat akin to frequency modulation. The phase of the carrier
wave is retarded or advanced by so many degrees, and the phase shifts are used to represent
bits. By providing, for example, four different degrees of phase shift, the signal can carry 4 bits
of information per hertz or wave cycle, though with normal losses this rate is unlikely to be
consistently realized in practice.
QUAM adds amplitude gradations to those of phase—in other words, the intensity of the
carrier wave as well as its phase position is made to vary by so many discrete steps, and the
more steps, the more bits can be encoded in a single waveform. In theory hundreds of bits per
hertz could be encoded by this means, but with normal losses and overhead involved in radio
transmissions, 5 bits per hertz is about the maximum throughput that can be achieved today.
The greater the number of states that can be represented by a single waveform, the smaller
the differences between states and the greater likelihood that the encoded information will be
lost to interference or noise. Thus, QUAM modulation systems can only be used when signal
strength is very high.
A radio capable of adaptive modulation will constantly evaluate signal quality and will
shift from high-order QUAM to lower-order PSK when a more robust signal is needed. The
throughput rate will drop accordingly. In some cases adaptive modulation can be overridden
in a radio, but the network operator can generally expect higher bit error rates when that
is done.
The effects of adaptive modulation on throughput rates, especially over distance and in
the presence of multipath environments, must be taken into account when the network is
being planned. Generally, adaptive modulation will degrade capacity while improving signal
integrity, but the operator needs to know exactly to what degree for each within each individual
cell and sector.