Equipment related parameters include the base station, antenna, and terminal. The link budget parameters vary with the base stations, antennas, and terminals of different vendors. These parameters affect the link budget result. As a result, the downlink is unaffected in most scenarios.
Transmit power includes that of the base station and terminal sides. The transmit power at the base station side affects the downlink budget. The transmit power at the terminal side affects the uplink budget. With the adoption of MIMO technology, two or more antennas are used at the same time at the base station for transmission. Therefore, the power combining gain must be considered.
The formula for calculating the power combining gain is as follows:
Power Combining Gain=10*Log(N)
Where, N indicates the number of transmit channels of the base station.For example, when a base station contains two transmitters and two receivers, the power combining gain is 3 dB.
Therefore, the transmit power in each sector (2T2R) is as follows:
46 dBm (40 Watt) in total for a 2×2 system with 20W from each transmit path
The receiver sensitivity indicates the minimum signal strength required to enable decoding by the eNodeB or UE receiver if there is no interference. In link budget tool, each of the subcarrier receiver sensitivity can be calculated by the following formula:
Sensitivity = SINR + N floor + 10.log + NF
SINR indicates the demodulation threshold of the receiver. The demodulation threshold is related to the specific code modulation mode involved, the BLER chosen and whether other quality affecting features are implemented e.g. MIMO and Coding repetition.
The SINR used in the link budget is obtained from the system simulation result. Nfloor indicates the multiplication result of K and T and is the density of the thermal white noise power. The value is -174 dBm/Hz.
Noise figure is the ratio of the SINR at the input end to the SINR at the output end of the receiver. The unit is dB. NF is an important index used to measure the performance of a receiver. Noise figure is highly dependent on both operating bandwidth and eNodeB type. The NF of a common LTE terminal is generally 6 dB to 8 dB and the typical value used is 7 dB.
The antenna gain indicates the power density ratio of the signals generated from the same point by the actual antenna and ideal radiation unit when the input power is identical. The antenna gain quantifies the degree to which an antenna transmits input power in concentration. To increase the gain, reduce the lobe width of the radiation at the vertical plane and maintain the omni-directional radiation performance at the horizontal plane.
Two units are used to indicate antenna gain: dBi and dBd.
The dBi indicates the gain of the antenna compared with the isotropic radiator to all directions. The dBd indicates the gain of the antenna compared with the symmetric oscillator.
The formula for the conversion between these two units is as follows:
dBi = dBd + 2.15.
The relationship between antenna gain, horizontal beamwidth, and vertical beamwidth is as follows:
In this formula, A and B indicate the horizontal beamwidth and vertical beamwidth. G Indicates antenna gain.
In LTE system, we often use 65° 18-dBi directional antennas and 11-dBi omni-directional antennas as the antennas in base stations. Figure show the antenna lobes of the 65° 18-dBi directional antenna and 11-dBi omni-directional antenna.
We recommend the 65° dual-polarized 18-dBi directional antennas for the base stations that are distributed in densely populated urban areas and common urban areas. The 90° or 65° directional antennas can be used for base stations in suburban areas.
Which Antenna for LTE to USE ?
We recommend the 11-dBi omnidirectional antennas for coverage in rural areas, especially in isolated towns. The 33° horizontal beamwidth antennas can be used for highway coverage. The gain of such antennas can reach 21 dBi, which helps increase coverage radius.
The antenna gains of the terminals in the LTE system vary. This results in a large difference in the coverage scopes of different terminals. However, the LTE terminal market is dominated by USB dongle at this stage and CPE antenna gain values will be based on final product availability. However, since CPE antenna is external, gain similar to those currently available in 3G/WiMAX product is expected.