Antenna Diversity Techniques: Types and Basics

This page describes antenna diversity types and basics. It provides a comparison between space diversity, frequency diversity, and time diversity.

There are two main types of fading: small-scale fading and large-scale fading. To mitigate these fading types and utilize them effectively to recover transmitted information at the receiver end, various diversity techniques are employed at mobile phones and base stations.

There are two main types of diversity techniques: microscopic diversity and macroscopic diversity. Microscopic diversity is used when fading is of the small-scale type, while macroscopic diversity is used in large-scale fading conditions. We will discuss below microscopic antenna diversity types used at mobile stations/handsets.

Antenna Diversity Types

There are various antenna diversity types, such as space diversity, frequency diversity, time diversity, polarization diversity, pattern diversity, directional diversity, and transmit-receive diversity, as described below.

Space Diversity

When two antennas are separated by a distance greater than lambda/2 (λ/2, where lambda is the wavelength), the signals received at both antennas are uncorrelated. This helps recover the transmitted information. More than two antennas can also be used depending on the high data rate system requirements. This type of diversity is used in MIMO-based systems such as LTE, Mobile WiMAX, and WLAN-11n/11ac, where multiple antennas are employed at the receiver end and also at the transmitter end, depending on MIMO configurations.

Frequency Diversity

Here, information is transmitted over multiple carriers or spread across a larger bandwidth. The concept is that each frequency carrier experiences a different channel and, therefore, will experience different uncorrelated fading for information transmitted from the same transmitter. This frequency diversity type is very efficient and useful in frequency-selective channel environments. For example, in CDMA, information is spread across a larger bandwidth of about 1.25 MHz, and in OFDM or OFDMA, information is sent over multiple carriers based on FFT size (2048/1024/256). Frequency hopping used in GSM and other technologies is also based on this diversity concept.

Time Diversity

Information is divided and sent across the channel at different time instants. This type of diversity is useful in time-varying channel environments where data are more corrupted at one time instant but less corrupted at another time instant. As different time instants “see” the channel differently, it helps recover the information in totality at the receiver end. For example, in TDMA-based systems such as GSM, the Interleaver module in the physical layer chain spreads data across different time instants and, therefore, introduces time diversity. Time diversity can also be introduced without an interleaver by dividing and sending transmissions of data units using some permuted order. This technique performs well when a mobile device moves at higher speeds, producing a fast time-varying channel environment.

Polarization Diversity

This technique requires information to be transmitted using two different polarizations of the E-M wave (Electro-magnetic wave). Unlike space diversity, the distance between antennas is not a constraint for the optimum functioning of polarization diversity in the wireless system to help recover transmitted data at the receiver end. For good performance, antennas should be orthogonally polarized, as this exhibits about a 30dB difference in signal strength at the receiver between these two orthogonally polarized conditions.

Pattern Diversity

Antennas with different antenna radiation patterns are used in this type of diversity technique. These two different types of antennas will experience uncorrelated fading, and this diversity concept is exploited to recover the transmitted information at the receiver end. To achieve good performance under this scheme, antennas should be placed closer. For example, a mobile device having dual antennas, viz. dipole and microstrip patch antenna.

Directional Diversity

In this type of diversity, directional antennas are employed rather than Omni-directional antennas at the mobile device. This provides better RSSI at the mobile device for the same transmit power compared to the non-directional antenna case. Alternatively, you can reduce the transmit power to achieve the same performance with a directional antenna. This eliminates the need for digital beamforming circuits at the transmit end. The challenge here is in the placement of the antenna on the device, as the device will have different orientations and, therefore, will experience different RSSI under different mobility and orientations. The best performance can be achieved when the antenna is mounted on the boundary of the device. Microstrip antennas are often used for this.

Transmit-Receive Diversity

In this diversity, two antennas are dedicated: one for the transmit direction and the other for the receive direction. This avoids the need for a duplexer in the receiver.