Antenna Array

An antenna array is a configuration of multiple antennas (elements) arranged to achieve a given radiation pattern. A single-element antenna is usually not enough to achieve technical needs. That happens because its performance is limited. The set of discrete elements, which constitute an antenna array, offers the solution to the transmission and/or reception of electromagnetic energy. The geometry and the type of elements characterize an antenna array. Antenna array factor quantifies the effect of combining radiating elements in an array without the element specific radiation pattern taken into account. The overall radiation pattern of an array is determined by this array factor combined with the radiation pattern of the antenna element. The overall radiation pattern results in a certain directivity and thus gain linked through the efficiency with the directivity. Directivity and gain are equal if the efficiency is 100%.

Classification
The phasing of the uniform linear array elements may be chosen such that the main lobe of the array pattern lies along the array axis (end-fire array) or normal to the array axis (broadside array).

Broad side array
Broad side array is the arrangement of identical antennas, which are placed along the axis perpendicular to the direction of maximum radiation. The identical antennas are equally spaced along the line of axis and all the elements are fed with equal magnitude of current with the same phase. This results in array pattern known as broad side array. It is evident that broad side array is bidirectional where maximum radiation is obtained in the direction of axis perpendicular to the array axis. By placing an identical array at a distance of λ/4 behind the array, bidirectional array can be converted to unidirectional array and by lead current in phase by λ/2.

End Fire Array
An End fire array looks similar to broad side array except that the individual elements are fed with the current that is equal in magnitude but opposite in phase. In other words, the individual elements are excited in such a way that a progressive phase difference between adjacent elements becomes equal to the spacing between the antennas (elements). “The arranging of identical antennas along a line drawn perpendicular to their respective axis so that the principle direction of radiation coincides with the direction of the axis of array” is known as End fire array. The radiation is maximum in the direction along the axis of the array i.e., 0 degree (or) 180 degree. If two equal radiators are operated in phase quadrature at a distance of λ/4 apart, an end fire couplet is said to be formed.

What is MIMO

Multiple-Input Multiple-Output (MIMO) technology is a wireless technology that uses multiple transmitters and receivers to transfer more data at the same time. Wireless products with 802.11n support MIMO. This is part of the technology that allows 802.11n to reach much higher speeds than products without 802.11n.

MIMO technology takes advantage of a radio-wave phenomenon called multipath where transmitted information bounces of walls, ceilings, and other objects, reaching the receiving antenna multiple times via different angles and at slightly different times. Multipath is a natural occurrence for all radio sources. Radio signals bounce of objects and move at different speeds towards the receiver. In the past multipath caused interference and slowed down wireless signals. MIMO takes advantage of multipath to combine the information from multiple signals improving both speed and data integrity.

MIMO technology leverages multi path behavior by the use of multiple, smart transmitters and receivers with an added spatial dimension to dramatically increase performance and range. MIMO allows multiple antennas to send and receive multiple spatial streams at the same time. Smart transmitters and receivers are used with all 802.11n devices. Using multiple antennas the data can be sent and received through multiple signals. More antennas usually equates to higher speeds. A wireless adapter with 3 antennas may have a speed of 600 Mbps while an adapter with 2 antennas has a speed of 300mbps. The router also needs to have multiple antennas and fully support all of the features of 802.11n to gain the highest speed possible With this extra bandwidth cannot be needed for achieving high data rate.

MIMO makes antennas work smarter by enabling them to combine data streams arriving from different paths and at different times to effectively increase receiver signal-capturing power. Smart antennas use spatial diversity technology, which puts surplus antennas to good use. If there are more antennas than spatial streams, the additional antennas can add receiver diversity and increase range. In order to implement MIMO, either the station (mobile device) or the access point (AP) need to support MIMO. Optimal performance and range can only be obtained when both the station and the AP support MIMO. Legacy wireless devices can't take advantage of multipath because they use a Single Input, Single Output (SISO) technology. Systems that use SISO can only send or receive a single spatial stream at one time.