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Antenna diversity basics

This page describes antenna diversity type and basics. It provides difference between space diversity vs frequency diversity vs 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 receiver end various diversity techniques are employed at mobile phone and at base station.

There are two main types of diversity techniques viz. microscopic diversity and macroscopic diversity. Microscopic type is used when fading is of small scale type and macroscopic is used in large scale fading conditions.

We will discuss below microscopic antenna diversity types used at mobile station/handset.

antenna diversity type

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

Space Diversity

When two antennas are separated by distance more than lambda/2, signals received at both antennas are uncorrelated and hence it helps recover transmitted information. More than two antennas are also used depending upon high data rate system requirement.

This type of diversity is used in MIMO based systems such as LTE, Mobile WiMAX and WLAN-11n, 11ac etc where multiple antennas are employed at receiver end and also at transmitter end depending on MIMO configurations.

Frequency diversity

Here information is transmitted over multiple carriers or spread across larger bandwidth. The concept is that each frequency carrier sees different channel and hence 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 environment.

For example in CDMA information is spread across 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 also is 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 the time varying channel environment where the data are more corrupted at one time instant but will be less corrupted at other time instant. As different time instants sees channel differently it helps to recover the information in totality at the receiver end.

For example in TDMA based systems such as GSM, Interleaver module in physical layer chain spreads data across different time instants and hence introduces time diversity. The time diversity can also be introduced without interleaver by dividing and sending transmission of data units using some permuted order.

This technique performs well when mobile device moves at higher speed producing fast time varying channel environment.

Polarization diversity

This technique requires information to be transmitted using two different polarizations of E-M wave (Electro-magnetic wave).Unlike space diversity distance between antennas is not a constraint for optimum functioning of the polarization diversity in the wireless system to help recover transmitted data at receiver end. For good performance antennas should be orthogonally polarized, as in this way about 30dB difference of signal strength is exhibited at 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 hence this diversity concept is exploited to recover the transmitted information at receiver end. To achieve good performance under this scheme, antennas should be placed closer. For example, 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 mobile device. This provides better RSSI at mobile device for the same transmit power compare to non-directional antenna case or you can reduce the transmit power to achieve same performance with directional antenna. This eliminates need of digital beam forming circuits at transmit end.

The challenge here is in placement of the antenna on the device; as device will have different orientations and hence will experience different RSSI under different mobility and orientations. The best performance can be achieved when 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 transmit direction and the other for receive direction. This avoids need of duplexer in the receiver.


RELATED LINKS

MIMO description and MIMO types, READ MORE.
Antenna Tutorial, READ MORE.
Channel model, READ MORE.
Diplexer versus Duplexer, READ MORE.
CDMA tutorial, Read more
OFDM versus OFDMA, Read more
GSM Tutorial, Read more
WLAN 802.11ac, Read more

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