Semana 13

Adaptive Modulation and Coding

The process of equaliza-
tion involves some method of gathering the dispersed symbol energy

Diversity Techniques and MIMO

Diversity is based on the fact that individual channels experience independent fading
events.

(MIMO) Antennas

Diversity: Diversity can be accomplished to have multiple received signals
through multiple transmit and/or receive antennas.

Beam-forming: Multiple antennas can be configured to create directional
antenna patterns to focus and increase energy to intended recipients.

MIMO Principles

Multi-user MIMO (MU-MIMO): With enough MIMO antennas, directional
antenna beams can be established to multiple users simultaneously.

Multilayer transmission: Multiple, parallel data streams can flow between a
pair of transmit and receive antennas.

Multi-User MIMO

The MIMO antenna architecture has become a key technology in evolving
high-speed wireless networks,

OFDM and Spread Spectrum

MIMO and OFDM technol-
ogies are the cornerstone of emerging broadband wireless networks.

Bandwidth Expansion

Carrier aggregation combines multiple channels. For example, 802.11n and
802.11ac combine the 20 MHz channels from earlier 802.11 standards into 40,
80, or 160 MHz channels.

Multipath Propagation

One of the key effects causing fading is multipath propagation.

This is generally the case for many
satellite facilities and for point-to-point microwave.

The Effects of Multipath Propagation

As multipath signals add together,
the resulting signal power can be stronger, but can also be lower by a factor of 100 or

1000 (20 or 30 dB). The signal level relative to noise declines, making signal detec-
tion at the receiver more difficult.

Types of Fading

Fading effects in a mobile environment can be classified as
either small-scale or large-scale.

This change is mainly caused

by shadowing and differences in distance from the transmitter.

The Fading Channel

The other small-scale effect, multipath fading, can cause distortion and inter-
symbol interference.

Forward Error Correction

those in
which the transmitted signal carries digital data or digitized voice or video data.

in satellite communications, the amount of delay involved

makes retransmission undesirable.

Adaptive Equalization

Adaptive equalization can be applied to transmissions that carry analog information
(e.g., analog voice or video) or digital information (e.g., digital data, digitized voice

or video) and is used to combat intersymbol interference.