As discussed in our previous blog post, the quality of a transmitted waveform is usually measured using error vector magnitude, frequency error, output power, and dynamics, as well as time alignment between transmit branches and downlink RS power requirements

[1]. Another key consideration is unwanted emissions, which consist of out-of-band emissions and spurious emissions.

Out of band emissions are unwanted emissions immediately outside the channel bandwidth that result from the modulation process and non-linearity in the transmitter. Spurious emissions are emissions that are caused by unwanted transmitter effects such as harmonic emissions, parasitic emissions, intermodulation products, and frequency conversion products.

The out-of-band emission requirements for the Evolved Node B (eNB) and user equipment (UE) transmitters, for instance, are specified in terms of both adjacent channel leakage power ratio (ACLR) and unwanted emissions on the operating band.

 

Relating ACLR to IMD

 

The increase in ACLR is mainly due to increased adjacent channel occupancy by 3rd and 5th order inter-modulation components [3]. In their application note [4], Maxim Integrated related ACLR to IMD3, calculating an ACLR for n subcarriers using two-tone IMD3 and a correction factor, with the following formula:

ACLRn = IMD3 + Cn

which uses the correction factors listed in the following table:

correction factors table

For a larger number of subcarriers, as in LTE and WiMAX, a set of closed form formulas are presented by Carvalho and Pedro [5]. Their article relates ACPR to the two-tone intermodulation ratio, IMR2, which in turn is linked to the third order intercept point, IP3, and the total output power, POT , as follows:

IMR2 = 2(IP3 – POT) + 6

Following their established theory, Carvalho and Pedro provided the formula for n-tone ACPR related to IMR2 as follows:

ACPR = IMR2 + 10log (n3/(16N + 4M))

with N = (2n3 – 3n2 – 2n)/24 and M = n2/4, where n is an integer multiple of 2.

ACPR will get asymptotically close to IMR2 when the number of subcarriers is high. This approximation can be applied for a 20 MHz LTE signal, for instance, with 2048 subcarriers.

For illustration, the measured 0 dBm two-tone IMR2 for Nutaq’s Radio420X is about -60 dBc. This suggests that the expected ACPR is IMR2, with -24.09 dBm output power per tone, in the case of 5 MHz LTE signal bandwidth. You would also expect such an ACLR level given that Radio420X exhibits a typical P1dB of +15 dBm.

Overall, Radio420X is suitable for LTE radio testing and development at +5 dBm total average output power. When it’s coupled with off-the-shelf pre-driver evaluation boards, such as the SKY77xxx family from Skyworks®, you can expect a clear LTE signal at +15 dBm (leaving typically more than 12 dB backoff) for femtocell applications.

References

  1. European Telecommunications Standards Institute. 2012. LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); Base Station (BS) Conformance Testing (3GPP TS 36.141 version 9.11.0 Release 9). ETSI 3rd Generation Partnership Project (3GPP). http://www.etsi.org/deliver/etsi_ts/136100_136199/136141/09.11.00_60/ts_136141v091100p.pdf
  2. European Telecommunications Standards Institute. 2008. Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) Radio Transmission and Reception, (3GPP TS 36.803 version 1.1.0 Release 4).  ETSI 3rd Generation Partnership Project (3GPP). http://www.3gpp.org/ftp/Specs/html-info/36803.htm
  3. Sesia, Stefania, Matthew Baker, and Issam Toufik, eds. 2009. LTE, the UMTS Long Term Evolution: From Theory to Practice. West Sussex: John Wiley & Sons. ISBN: 0470697164.
  4. Maxim Integrated, Inc. 2006. “Adjacent Channel Leakage Ratio (ACLR) Derivation for General RF Devices.” Application note. http://www.maximintegrated.com/app-notes/index.mvp/id/3902
  5. Carvalho, Nuno Borges de, and José Carlos Pedro. 1999. “Compact Formulas to Relate ACPR and NPR to Two-Tone IMR and IP3.” Microwave Journal (December): 70-84. http://www.av.it.pt/nbcarvalho/docs/Revista7.pdf