## 4 Implementation Details
### Interference Detector and SNR\({}_{fe}\) Estimator (IDSE)
#### 4.1.1 Interference Detector  
Proximity of the adjacent channel to the desired channel makes it more harmful to the signal than the alternate channels. As can be seen from Figure 8, IDSE has three inputs: I and Q inputs from ADC and a signal that indicates if detection or estimation should be done. This input signal has three states: detect alternate, detect adjacent and estimate SNR\({}_{FE}\). For all three states, setting of NCO is changed to down-convert adjacent or alternate or desired signal. IDSE consists of two arms, one each for one adjacent or alternate channel. Only one arm is active during SNR\({}_{FE}\) estimation. Both arms have a CORDIC NCO unit to down-convert the interference or signal. Output of detectors/estimator goes to a comparator that compares it with threshold. For interference detection, output of comparators is 1-bit to indicate presence of interferences. In estimator mode, comparator finds the range in which the measured SNR\({}_{FE}\) falls. LUT has SNR steps with difference of \(1\,\)dB. Since SNR variation can be up-to \(60\,\)dB so it has \(60\) SNR steps, requiring 6-bit index. There are four possible combinations from interference detection: Alternate present/absent and Adjacent present/absent, it is indicated by \(2\) bits. So, LUT is indexed by \(8\)-bits. Figure 8: Non-coherent interference detection procedure. Figure 9(a) and 9(b) are frequency responses of FIR filters used in IDSE and data-path of the receiver. The filter used for IDSE has a sharper roll-off. Interference from each interfering channel is down-converted to baseband and filtered by this filter. It can be seen from the filter's frequency response that the attenuation at \(5\) MHz and \(10\) MHz distance is approximately \(80\) dB. When measuring the desired signal power, due to attenuation by the matched filter, adjacent signal level falls to \(-162\) dB and alternate signal level falls to \(-132\) dB. These levels of interference are quite low and do not corrupt the signal power estimation. Whereas, while measuring interference power, signal power from desired band can affect the interference power measurement. This is due to the fact that the maximum possible signal power is \(-20\) dBm and it can spill to neighboring bands. At such high signal level even after the attenuation by the matched filter, its strength in neighboring channels is high enough to affect interference power measurement. Let \(Y_{Iadj1}\) and \(Y_{Iadj2}\) be the in-phase and \(Y_{Qadj1}\) and \(Y_{Qadj2}\) are the quadrature phase adjacent channels. These terms are analogously defined for alternate channels too. \(g\) is the gain of VGA [18, 19]. Measured power in adjacent and alternate channels (\(\hat{P}_{adj},\hat{P}_{alt}\)) can be defined as:  
\[\hat{P}_{adj} = \frac{1}{g^{2}}\sum_{n}^{N}Y_{adj}(n), \tag{4}\] \[\hat{P}_{alt} = \frac{1}{g^{2}}\sum_{n}^{N}Y_{alt}(n) \tag{5}\]  
where,  
\[Y_{alt}(n) = Y_{Ialt1}^{2}(n)+Y_{Qalt1}^{2}(n)+Y_{Ialt2}^{2}(n)+Y_{Qalt2}^{2} (n) \tag{6}\] \[Y_{adj}(n) = Y_{Iadj1}^{2}(n)+Y_{Qadj1}^{2}(n)+Y_{Iadj2}^{2}(n)+Y_{Qadj2}^{2} (n) \tag{7}\]  
If \(\hat{P}_{adj}\) exceeds a-priori calculated threshold, \(\hat{P}_{adj}^{thresh}\), then adjacent interference is detected. Similarly, \(\hat{P}_{alt}^{thresh}\) is the threshold what is compared with \(\hat{P}_{alt}\). Figure 10 shows the effect of desired signal power on adjacent channel interference detection. The figure is obtained for front end noise figure (NF\({}_{FE}\)) of \(29\) dB [20]. \(\hat{P}_{adj}^{thresh}\) is the normalized threshold for detecting presence of adjacent interference. When  
Figure 9: Frequency Response of FIR Filters. signal power is large, then even in absence of adjacent interference, \(\hat{P}_{adj}\) can exceed \(\overline{\hat{P}_{adj}^{thresh}}\). \(P_{FID\_adj}\) in figure is probability of false adjacent interference detection. \(P_{FID\_adj}\) increases with increase in desired signal strength.