3.2. Beam forming
A single output of the array is formed when signals induced on
different elements of the array are combined. A plot of the array response as a
function of angle is usually specified as the array pattern or beam pattern. It
can also be known as power pattern when the power response is plotted.
This method of combining the signals from several elements is
understood as beam forming. The direction in which the array has maximum
response is said to be the beam pointing direction, and thus this is the
bearing where the array has the utmost gain.
Conventional beam pointing or beam forming can be achieved by
adjusting only the phase of the signals from different elements. In other
words, pointing a beam in the desired direction. However, the shape of the
antenna pattern in this case is fixed, that is, the side lobes with respect to
the main do not change when the main beam is pointed in different directions by
adjusting various phases. Nevertheless, this can be overcome by adjusting the
gain and phase of each signal to shape the pattern as required and the degree
of change will depend upon the number of elements in the array [26].
For example, signals can also be coupled together without any
gain or phase shift in a linear array, and it is known as broadside to the
array, which is, perpendicular to the row joining all the elements of the
array. The array pattern formed thus falls to a low value on either side of the
beam pointing direction and the region of the low value is known as a null. In
this case, it must be noted that the null is actually a position where the
array response is zero and the term should not be misused to denote the low
value of the pattern.
Lastly, it is very convenient to make use of vector notation
while working with array antennas. Thus the term weight vector (W) is
introduced. It is important because the weight vector will have significant
impact on the array output.
3.2.1. Nulling Beam Forming
The flexibility of array weighting to being adjusted to
specify the array pattern is an important property. This may be exploited to
cancel directional sources operating at the same frequency as that of the
desired source, provided these are not in the direction of the desired source
[26].
In circumstances where the directions of these interferences
are identified, cancellation is feasible by positioning the nulls in the
pattern corresponding to these directions and concurrently steering the main
beam in the direction of the desired signal. This approach of beam forming by
placing nulls in the directions of interferences is commonly referred to as
null beam forming or null steering [27].
3.2.2. Steering Vector
The steering vector contains the response of all elements of
the array to a narrow-band source of unit power. As the response of the array
is different in different directions, a steering vector is associated with each
directional source. The uniqueness of this
Association depends upon the array geometry [26]. Every
component of this vector has unit magnitude for an array of identical elements.
The phase of its i th component is similar to the
phase difference between signals induced on the ith
element and the reference element due to the source associated with the
steering vector. This vector is also known as the space vector because each
component of the vector represents the phase delay that is resulted from the
spatial position of the corresponding element of the array. In addition, it can
also be referred to as the array response vector for it measures the response
of the array due to the source under consideration.
Beam forming is used by the smart antennas, in order to obtain
a radiation pattern which only receives from and transmits to the desired
directions, while attenuating undesired directions. The only available
information for downlink transmission is the directional to the mobile station.
It is furthermore reasoned that in that situation, it is desired to send as
much power in the direction of the mobile station as possible, attenuating all
other directions. Most of basic beam forming methods, point the antenna beam in
a certain direction, by applying phase shifts to the signals to the individuals
antenna in the array. [28]. The phase shifts can be applied in same digital
baseband.
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