DESIGN PARAMETRES AT BASE STATION Prittu Ann Thomas Roll no :14

BASE STATION It is a structure which connect mobile phones within its range to the regular landline phone system. A cellular base station acts as low power, multi- channel two-way radio

Base stations consist of two main portions  Antenna  the tower or mast which raises the antenna above the ground The use and construction of a cellular base station is regulated by FCC The antennas must be kept inaccessible to the public and at a safe range.

Gain, radiation pattern, efficiency, and many other characteristics have the same value whether an antenna is transmitting or receiving a signal – Reciprocity theorem The development of the network and the antenna systems will be determined by current usage patterns and also by those foreseen in the future

DESIGN PARAMETERS OF ANTENNA Gain Gain is a function of both the azimuth and elevation beamwidth of antenna G = 41,253 θ φ θ, φ – Azimuthal and elevation angle or, G = D ξ where D – directivity, ξ - efficiency

Broadside array delivers the maximum possible directivity if all its elements are excited with equal co-phased currents Beam shaping require non uniform element currents for side lobe suppression and null fill

Power Base station power depend on type of antenna, the number of analog and digital channels that operate at a given time and at their maximum strength, and antenna gain Power received by an antenna is :

Beamwidth Azimuthal Beamwidth Depend on vertical radiating element and reflecting surface behind it It is a function of frequency Typically 65° ±3° or 90° ±5° over the whole band Base stations support three cells spaced 120° apart in azimuth

Elevation Beamwidth Elevation beamwidth of a BS array is a function of its electrical length in the vertical plane Smaller the elevation beamwidth larger the physical dimension of antenna Control of elevation beamwidth improves C/I ratio in neighboring cells as well as increase field strength of user in intended cell

Re-use distance.

Beam Tilt Angle by which elevation pattern maximum is placed below the horizontal is known as the beamtilt of the antenna Provided by 2 means  Mechanical Tilt  Electrical Tilt

Passive Intermodulation Product When multiple-frequency transmissions occur, if any conductor or joint has a nonlinear voltage/current relationship passive intermodulation(PIM) products will be generated PIM are spurious signals with frequencies related to those that give rise to them.

For two unmodulated carriers, intermodulation products are generated at frequencies given by: Second order: f 2 − f 1, f 2 + f 1 Third order: 2f 2 − f 1, 2f 1 − f 2, 2f 1 + f 2, 2f 2 + f 1 Fourth order: 3f 1 − f 2, 2f 1 − 2f 2, … Fifth order: 3f 1 − 2f 2, 2f 2 − 2f 1, 4f 1 − f 2, 4f 2 − f 1, …

Grounding It is meant for lightning protection Careful attention must be paid to bonding of conducting parts of the antenna lightning spike mounted to the supporting structure provide protection

Mechanical Design Careful attention to the selection of materials, the design of joints, and arrangements for exclusion of water from critical areas stable operating characteristics despite the effects of wind, rain, and pollution cost and weight of antennas is a major constraint

Diversity Systems Diversity system uses two antennas and combines their outputs without any correlation. Average received signal is improved TYPES Spatial Diversity  Uses vertically polarized antennas Polarization Diversity  Used in Urban areas

(a) Space diversity and (b) polarization diversity high-band antenna systems mounted on monopoles typically 15-m high

Front to Backlobe Ratio It is the ratio between the peak amplitudes of the main and back lobes expressed in dB Defined for directional antenna Dipole has same radiation for front and back lobe

DESIGN OF BASE STATION Base station design depend on  Radiating Element  Feed System Feed system is created using printed circuit techniques or using coaxial cables

Antennas Used at Base stations Stacked patch antenna Crossed Dipoles Square dipoles Log periodic dipole

Figure 2.6. Dual-polar radiating elements: (a) patch, (b) crossed dipole, and (c) square dipole array

Typical designs for a vertically polarized radiating element. The reflecting plane behind the elements may be flat, curved, bent, or have up-standing flanges along its longitudinal edges.

Input Impedance VSWR at the input of a base station antenna should be 1.4:1 or 1.5:1. Base station is a complex assembly of radiating elements, power dividers, and interconnecting transmission lines Control of impedance matching at each stage between the radiating elements and the array input.

Array Dimension Directivity increases to a maximum when the spacing is around one wavelength If spacing exceeds one wavelength grating lobes will arise

Feed Array Compute required power division ratio needed at each junction mismatch at junctions can be mitigated by using hybrid power dividers Microstrip feed are relatively easy because both the Z 0 and the length of any line section can be adjusted at will. Coaxial line dividers must make use of the characteristic impedances of available cables

Feed Network

Base station design depends on: Gain Power Azimuthal Beamwidth Elevation Beamwidth Beam Tilt Passive Intermodulation Products Grounding

Diversity System Mechanical Design Front lobe to Back lobe ratio Radiating Elements Array Dimension Input Impedance Feed Array