How to define the IIP3 of receiver???

If defining a receiver's spec, how much is enough as IIP3???

How to calculate the needed appropriate IIP3 for a receiver???

Hi
how much is enough as IIP3,is depend from the maximum input signal that you expected to receive without distortion,you can try to calculate IIP3 with AppCAD (it's free).

And be aware that you may need some margin due to modern modulation schemes (CDMA, OFDM).
Mazz

Here is an example for a CDMA receiver from a M/A/X/I/M app note.

The intermodulation response attenuation is a measure of a receiver's ability to receive a cellular band CDMA signal on its assigned channel frequency in the presence of two interfering continuous wave (CW) tones. Theses tones are separated from the assigned channel frequency and from each other such that the third order intermodulation product of two tones produces an in-band interfering signal to the desired signal due to non-linearity of LNA and mixer. IS-98A standard calls for three different cases of different two-tone levels and desired signal levels, under which the FER should not exceed 1% and should be 4.3dB. It is supposed that 1.5dB margin is added for traffic channel system consideration, and then is required as above 5.8dB. In the small two tone level case, the equivalent white noise interference source Ioc consists of the receiver's thermal noise No and the transmitter's thermal noise in the receiver band Nt, and an equivalent in-band interference component due to the third order intermodulation product IIM3. In a 1.233MHz bandwidth, Ioc should not exceed -101.3dB. IIM3 can be expressed in linear form as:

IIM3 = IOC -(No + Nt) · BWRF (3)

Where No is set by -166.2dBm/Hz and Nt is set by -178dBm/Hz. From equation (5), IIM3 should be less than -103.7dBm/1.23MHz. As a result, IP3 in the small two-tone level's case should be greater than -12.7dBm by using formula (4).

IIP3 = Pin + IM3 / 2 (4)

Where Pin is set by -43dBm (IS-98A) for two tones and IM3 is set by 60.7dBm (-103.7dBm+43dBm). In the large two tone case, the desired CDMA signal level is 25dB above the minimum receiver sensitivity level. The equivalent in-band white noise interference source is totally dominated by the third order intermodulation product IIM3 when the power level of two tones is -21dBm as IS-98A standard requires. This means that IIM3 is equal to Ioc and should not exceed -79.3dBm/1.23MHz. Similarly, 1.5dB margin is added for =5.8 dB. The third order intermodulation spurious response attenuation IM3> 58.3dB. This case requires the IIP3 of the LNA should exceed +8.5dBm for large two tones case. Following the IS-98A standard requirement, there are three two-tone test cases, the middle power level of two tones is -30dBm. This means that the LNA of the cellular band needs variable gain control or three linearity modes to handle the third order intermodulation issues.