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 increases in designing, testing and implementation of transmission network and user equipment.
Adjacent Channel Leakage Ratio in any RF communication devices are generally described as the unwanted frequency generated by second or third order intermodulation distortion of the devices which fall near to the transmitting frequency band. In modern digital communication, any power leakage from transmitting channel to the adjacent channel cause distortion in other neighboring channel’s transmission and which affects the system 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 3GPP specification. It is one of the significant characteristic in 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 square measurement filter and 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 block: 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 difference in the reference level of the spectrum analyzer. Modern signal analyzers are capable of performing complex measurements with greater accuracy.
zhu says
February 20, 2018 at 9:56 amHI, thanks for you illustration.
When using Rohde & Schwarz spectrum analyzer, which trace mode should i select to get the correct ACLR? Max Hold or Average?
Looking forward to your reply…
Rajiv says
February 22, 2018 at 3:33 amHi Zhu, You may use LTE square option for ACLR measurements. I would suggest trace mode: average since the measurement is across a bandwidth (not a single peak).
Try both options (average and max hold) and see the differences too. Every test case is different depends on power level, channel BW etc…