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Chapter: Analog and Digital Transmission

Digital Modulation Methods

Modern cellular systems use digital modulation methods, as this has several advantages over analogue transmission, including the possibility to apply advanced signal processing, such as error correction coding, security and diversity.

 

 

Some basics about modulation: ppt

Example of popular narrowband modulation methods are

Other methods are

What Modulation Suits the Channel?

Multipath reception and user mobility lead to channel behavior that is time and frequency dependent. In the following frequency-time diagrams, we depict where in frequency and when in time most of the bit energy is located. It is important to select a modulation scheme that is appropriate for the Doppler spread and the delay spread of the channel. It is seen from the channel scatter function or from samples of the channel that:

In narrowband communication (NB), the occupied bandwidth is small and the bit duration is long. The bit energy footprint is stretched in vertical direction. Intersymbol interference is neglectable is the bit duration is, say, ten times or more longer than the delay spread. However, due to multipath the entire signal (bandwidth) may vanish in a deep fade. Is fading is very fast, the channel may change during a bit transmission. This can occur if the Doppler spread is large compared to the transmit bandwidth.

In wideband transmission (WB), i.e., transmission at high bit rate, the signal is unlikely to vanish completely in a deep fade, because its bandwidth is so wide that some components will be present at frequencies where the channel is good. Such frequency selective fading, however, leads to intersymbol interference, which must be handled, for instance by an adaptive equalizer.

In OFDM or multicarrier systems, multiple bits are transmitted in parallel over multiple subcarriers. If one subcarrier is in a fade, the other may not. Error correction coding can be used to correct bit errors on faded subcarriers. Rapid fading (Doppler) may erode the orthogonality of closely spaced subcarriers.

In the above frequency-time plot, every tile represents one bit. In spread-spectrum transmission one bit is often transmitted in multiple 'chips'. Below, one such chip is plotted as a tile. One bit is spread over multiple tiles.

Direct sequence intentionally broadens the transmit spectrum by multiplying user bits with a fast random sequence. The wideband signal is unlikely to fade completely. If frequency selective fading occurs, the receiver sees a series of time-shifted versions of the chipped bit. A rake receiver can separate the individual resolvable paths.

Frequency hopping. The carrier frequency is shifted frequently, to avoid fades or narrowband interference. While the signal may vanish during one hop to a particular frequency, mostly likely other hops are to frequencies at which the channel is good. Error correction is applied to correct bits lost at faded frequencies. Slow and fast hopping methods differ in the relative duration spent at one frequency, compared to the user bit duration.

Orthogonal multicarrier spread spectrum is a spectrum-spreading method to exploits advantages of OFDM and DS-CDMA. each user bit is transmitted simultaneously and in parallel over multiple subcarriers. This corresponds to a 90 degree rotation of a bit energy map in the frequency-time plane.
While the signal may be lost some subcarriers, reliable communication is ensured through appropriate combining of energy from various subcarriers, taking into account the signal-to-interference ratios. In an interference-free environment, maximum ratio diversity combining is the preferred receive strategy.


Pulse Shapes

Signal pulse shapes usually are chosen to satisfy the following requirements

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