Self Interference(SI) and 5G-4G Interworking Challenge in NSA [Why EN-DC of B3 is not favourable with N78]

We are not operating on single RAT!!!!!!! Invitation of IMD[intermodulation distortion]

When multiple air interface technologies coexist, mutual interference will inevitably occur between different working frequency bands (for the same device, we call this interference “self-interference (SI)”)

As a typical heterogeneous network, the fifth generation (5G) mobile communication system must be compatible with existing technologies (such as Long-Term Evolution (LTE)) and support multiple types of 5G New Radio (5G NR) so SI will came in picture everywhere 

As device is same and its competible for multi RAT then it creates Harmonics as a result of SI

Even/Odd Harmonics 

Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency components will form additional components at frequencies that are not just at harmonic frequencies (integer multiples) of either, like harmonic distortion, but also at the sum and difference frequencies of the original frequencies and at sums and differences of multiples of those frequencies

When any input frequency signal is applied to the nonlinear device, it will generate unwanted frequencies other than the input signal. This unwanted frequencies if present at integer multiple of input frequency then they are referred as Harmonics.

The frequencies which are 2, 4, 6, 8 times and so on multiplications of the main/root frequency are even harmonics. ODD harmonics are 3, 5, 7 times and so on multiplications of the main/root frequency

so if input signal multiplied by non integer value then its known as Spurious

5th generation mobile communication, a scheme (EN-DC) of dually connecting LTE and NR is underway to ensure communication stability. In a state in which a downlink carrier using LTE and a downlink carrier using NR are aggregated, transmission of an uplink signal may cause a harmonic component and an intermodulation distortion (IMD) component to impact on a downlink band of the UE itself

UE must be set to satisfy a reference sensitivity power level (REFSENS), which is minimum average power for each antenna port of the UE. However, in case that the harmonic component and/or the IMD component occurs, the REFSENS for the downlink signal may not be satisfied. That is, the REFSENS must be set such that throughput thereof is at least 95% of maximum throughput of a reference measurement channel, but the occurrence of the harmonic component and/or the IMD component may cause the throughput to fall below 95%.

Thus, it is determined whether the harmonic component and/or the IMD component of the EN-DC terminal (or EN-DC user equipment (UE)) has occurred, and when the harmonic component and the IMD component of the EN-DC terminal has occurred, a maximum sensitivity degradation (MSD) value for a corresponding frequency band may be defined to allow relaxation for the REFSENS in a reception band of the EN-DC terminal based on a transmission signal of the EN-DC terminal. Here, the MSD is maximum allowable degradation of REFSENS, and in a certain frequency band, the REFSENS may be relaxed by the defined amount of MSD

Taken refrences from well described EUROPEAN PATENT SPECIFICATION[EP 3 496 301 B1]

Below were summary 

Observation 1: In a TDD-TDD synchronization network, the harmonic/IMD problems does not occur

Observation 2: In an FDD-TDD NSA terminal, the harmonic problem may have a fatal impact on NR reception frequency of the terminal. 

Observation 3: Regarding the FDD-TDD NSA terminal, the IMD may impact on an LTE reception frequency of the terminal

Observation 4: In the FDD-TDD NSA terminal, the IMD may fall on the NR reception frequency of the terminal

Observation 5: In the dual uplink LTE CA, an MSD level according to IMD4 and IMD5 may be approximately 10 dB or less

Proposal 1: For the harmonic problem, a harmonic trap filter may be considered to reduce an interference signal level and the MSD level may be defined.

Proposal 2: When the IMD falls on the existing LTE band, a 0 dB MSD must be guaranteed using the A-MPR scheme or next generation NodeB (gNB) scheduling in the NR band.

Proposal 3: When the IMD falls on the NR band, an MSD level may be defined as exceptional requirements for reference sensitivity (REFSENS).

Proposal 4: Based on observation 4 and observation 5 above, the MSD level may be determined to be 10 dB as a reference point regarding whether NSA dual-connection operation is allowed

NSA UE Supported Band [Possible Combination of NSA]

Harmonics/IMD Problem with N77/N78 and LTE Bands 

So here is table from 3GPP which clearly tell you :- You should not go with EN-DC [ B3 & N78]

N78 is frequency ranges from 3300MHz to 3800MHz

NR Frequency Bands