LaNWiFi.NeTWorld's Wireless Technology Center // Wireless For Freedom.

To improve physical layer performance in a wireless environment, a common technology is to consider directional communications or use multiple antennas on the same communication node. It should be noted that a multi-antenna communication system consists of both RF components and baseband processing.

Directional Antenna
Directional antennas enable directional transmission and reception in a wireless network, and thus have several advantages.
• Better spatial reuse efficiency. Since transmission and reception are directional, channel reuse does not need to rely on spacial separation, which significantly improves channel spatial reuse efficiency. This feature helps to increase network capacity.
• Lower interference. Directional transmission and reception reduces the collisions and interference among different nodes. This feature improves the QoS and throughput of a network.
• Less energy consumption for the same network capacity. For the same transmission range, less transmit power is needed by a directional antenna than an omni-directional antenna. Thus, for the same transmission rate, less interference will produce by a node to other nodes. In other words, this feature not only improves the energy efficiency , but also increases the network capacity.
• Better security. Due to directional transmission, eavesdropping becomes much more difficult, and thus enhances security of the network at the physical layer. Directional antennas can be realized in the following methods.
• Steerable antenna. In this case, one antenna is used on each node, pointing in a specific direction. For networking with other nodes, the antenna needs to be mechanically or electronically steerable so that the antenna points to the right direction at the right time . Since the process of changing the direction of a steerable antenna may be slower than ad hoc networking needs, it is not always a good choice for WMNs.

Antenna Diversity and Smart Antenna

Considering communications between nodes A and B in Figure 2.2, node A is assumed to have M antennas for transmission and N antennas for reception, while in node B there are K antennas for transmission and L antennas for reception. Different values of M, N, K, L result in various multiple-antenna systems.

Single Transmitting Antenna Multiple Receiving Antennas

If multiple antennas are in the receiver but single antenna in the transmitter (i.e., K = 1, M = 1 and either L>1 or N >1), techniques such as antenna diversity and adaptive/smart antennas can be used for a multi-antenna system. They have been proposed for point-tomultipoint one-hop cellular networks.
Antenna diversity is based on the fact that signals received from uncorrelated antennas have independent fading. Thus, it has high probability that at least one good signal can be received at the receiver. Antenna uncorrelation is usually achieved through different types of diversity.

• Space diversity. This is the simplest version of antenna diversity, which is achieved via antenna separation by a certain number of wavelengths.When antennas are at the same location, space diversity will disappear.
• Polarization diversity. Since antennas can be at the same location with polarization diversity, it has become a more favorable approach to achieving antenna diversity. However, it is a more complicated technology than space diversity.
• Pattern diversity. By adjusting the radiation patterns at different antennas, diversity can be achieved, even if antennas are at the same location. However, pattern diversity also has higher complexity than space diversity.

To utilize diversity, signal processing is needed. The most common techniques are explained as follows .
• Switch diversity. The antenna with the best signal is selected. The signal qualitymetrics can be signal strength, BER, etc.
• Equal gain combining. To enhance the performance of switch diversity, an equal gain combining technique can be used to co-phase signals and add them together.
• Maximum ratio combining (MRC). MRC weights signals by SNRs before combining them. It is the optimum method in the presence of noise.

Multiple Transmitting Antennas Single Receiving Antenna

If multiple antennas are in the transmitter and single antenna in the receiver, i.e. N = 1, L = 1 and eitherK >1 orM >1, either antenna diversity or smart antenna techniques are difficult to implement. Since the receiver has only one antenna, the transmitter antennas must be designed appropriately so that the arrival signals at the receiver can still keep the performance gain of antenna diversity or smart antenna. To reach this goal, one important requirement is that the channel state information (CSI) must be available at the transmitter. For example, schemes such as [61] assume that CSI is perfectly known. However, usually only partial information of channel state is available. For a time division duplex (TDD) system, such information can be derived from a reverse link, but is still not accurate to reflect forward link CSI, due to channel variations in time. For a frequency division duplex (FDD) system, CSI of forward and reverse links is independent.

MIMO

If multiple antennas are used at both the transmitter and the receiver, i.e.,M >1, L>1 or K >1, N >1, the multiple-antenna system is a MIMO system. Since a MIMO can utilize both diversity and multiplexing of simultaneous data streams, it can potentially increase the system capacity by three times or even more . Currently MIMO has been adopted into IEEE 802.11n . MIMO systems can be built based on spatially separated antennas. For some applications, compact antennas are needed, and thus MIMO systems must be designed based on vector antennas . These vector antennas are built based on co-located elements, e.g., one loop and two dipoles. In fact, vector antennas are examples of pattern diversity. MIMO based on co-located antennas can also increase the capacity by several times. However, its capacity and BER performance is still lower than MIMO systems with spatially separated antennas. Depending on where MIMO signal processing is performed, a MIMO system can be classified into three types: receiver processing only, transmitter processing only, and both transmitter and receiver processing MIMO systems.

MIMO has been researched for many years, and there are some chipsets available on the market. To further improve transmission rate and reliability, developing new MIMO schemes is an ever-increasing demand. Moreover, new algorithms are also needed to improve performance of existing MIMO technologies, e.g., lower signal processing complexity and less dependence on channel state information.