Our modern digital communication system has been continuously evolving and it demands higher data rate and system efficiency to support millions of individuals around the clock. Since the complexity of the system increases the challenges also increase in designing, testing, and implementation of transmission network and user equipment.
Adjacent Channel Leakage Ratio in any RF communication device is generally described as the unwanted frequency generated by second or third-order intermodulation distortion of the devices that fall near the transmitting frequency band. In modern digital communication, any power leakage from the transmitting channel to the adjacent channel causes distortion in other neighboring channel’s transmission which affects the system’s efficiency. In order to make sure the system works perfectly within the allowed limits, an ACLR measurement has to be performed with a reference transmit signal.
In LTE, ACLR is the ratio of the filtered mean power centered on the assigned channel frequency to the filtered mean power centered on an adjacent channel frequency as defined by the 3GPP specification. It is one of the significant characteristics of LTE transmission defined by 3GPP’s specification.
Specification limits are different for E-UTRA and UTRA adjacent channels and both frequency division multiplexing (FDD) and time division multiplexing (TDD) schemes. In E-UTRA, ACLR measurements are calculated using a square measurement filter and an RRC filter used for UTRA.
ACLR Measurement with spectrum analyzer:
The above picture illustrates the ACLR mode of a modern spectrum analyzer with one transmit (blue) channel and two adjacent channels (green).
Equipment Settings:
Signal Source: LTE signal (E-UTRA)
Operating band: LTE band 13
RBW: 100 KHz
VBW: 1 MHz
LTE Signal frame configuration:
Signal source frame configuration settings:
Modulation: QPSK
Number of resource blocks: 12
Resource block offset: 18
Spectrum analyzer optimization
Optimization of spectrum analyzer is very important in ACLR measurement. The mean power of the adjacent channels varies with the difference in the reference level of the spectrum analyzer. Modern signal analyzers are capable of performing complex measurements with greater accuracy.
