Adjacent channel power ratio ( ACPR)

In multicarrier systems, the carriers can be spaced quite close to each other. When this is the case a quantity referred to as the adjacent channel power ratio or ACPR becomes important. As mentioned above, multicarrier systems have a number of carriers which may generate signals whose power may add in phase. As more tones or signals start interacting, the peak additive power will increase. The average power of these signals may well be within the dynamic range of the system. However, the peaks of power may exceed the dynamic range. This will cause non linear odd - order distortion in the system. When this happens it results in adjacent channel power output or ACP. The ACPR is the ratio of the system output power at an offset frequency with respect to the power of the channel of interest. This can be considered one measure of linearity of a transmitter ( or RFPA). If the transmitter or the PA generates unwanted sidebands at an offset frequency that lies within the passband of an adjacent channel. For a given modulation scheme, the relationship between third order intermodulation products and the ACPR at a given power level is: ACPR = IMR2-tone + 10*log[ n**3/(16X + 4Y)].For a given modulation scheme, the relationship between third order intermodulation products and the ACPR at a given power level is: ACPR = IMR(2-tone) + 10*log[ n**3/(16X + 4Y)]. Here X and Y are given by:

X = (2n**3 – 3n**2 – 2n)/24 + [mod(n/2)]/8.0

And

Y = n**3 – {[mod(n/2)]/4.0}

All ratios here are in dBc. i.e. the ratio of the two tone intermodulation to signal carrier IMR and ACPR. Check out our website and engineer's corner. Go to http://www.signalpo.biz.