^{1}

^{1}

^{1}

^{1}

Wideband receivers for multistandards operation can simplify the system and lower the cost. In a wideband receiver, the tolerance of large interference signal within the operating band is important. Traditional frequency-domain filtering suffers from lacking in filtering capability for in-band interference signals. This paper describes a receiver system exploiting nonlinear transfer function. Based on the fundamental nonlinear theory, the receiver with nonlinear method can provide frequency-independent filtering for large blockers and linear amplification for weak desired signals simultaneously. The interference suppression performance depends on the amplitude discrimination between the envelope of the large and small signal. The operation of the nonlinear receiver is based on the amplitude of the interferer envelope. A feedforward path is designed to extract the envelope information of the interferer and a feedback path is added to keep track of the environment. With frequency-independent filtering, the nonlinear receiver system enhances both in-band and out-of-band linearity, thus enabling wideband multimode operation.

Wireless communication systems are developing to provide higher speed with reliability under the increasing amount of daily usage [

The multiradio coexistence scenario [

Multiradio coexistence scenario. The receiver of standard A in terminal #1 is plagued by the strong signal transmitting by either terminal #1, terminal #M, or terminal #N.

The lack of RF filtering after antenna generates problems for wideband operation. The problems can be divided into three categories: distortion, phase noise, and power consumption. It can be extremely harmful if the strong interferer is located close to the desired signal or at harmonic frequency of the desired signal. Firstly, if the interferer is too large, it leads to desensitization of the receiver. Secondly, when the interferer mixes with LO phase noise, it poses additional noise in the receiver band. That noise is proportional to the interferer power [

This paper presents a nonlinear receiver topology with frequency-independent interference tolerance. Based on the information of envelope amplitude of the interferer, the receiver is able to provide suppression at RF frequency for large interferers. It can achieve both good IB and OOB linearity, thus making it suitable for the multiradio coexistence scenario. The suppression at RF frequency also alleviates the requirements for the following receiver circuit blocks and saves the overall power consumption. The paper is organized as follows. Section

There are several wireless standards operating in a mobile handset device. Figure

Frequency allocation of different wireless standards from 1800 MHz to 2700 MHz.

The interference scenarios can be divided into two classes in terms of physical distance, namely, collocation and proximity. The collocation scenario refers to that multiple radios are placed in the same physical unit that the interferers are generated locally inside the device. The transmitting power of LTE user equipment is 24 dBm. The measured antenna coupling for collocated 915 MHz patch antennas is roughly 20 dB in worst case [

The proximity scenario happens when multiple devices are placed very close. The transmitting signal from device #A generates interference for receivers in other devices. Therefore, the interferers are generated externally, for example, from the use of LTE small cell access points and low-power WiFi routers [

The interference scenarios of interest can be summarized into three cases.

In a conventional narrowband receiver as shown in Figure

Simplified system architecture of a conventional narrowband receiver.

In FDD communication systems shown in Figure

Simplified system architecture of (a) a FDD system with duplexer and (b) an analog cancellation method of local interferer.

Figure

Instead of the conventional LNA-first approach, recent works [

Model of 4-phase passive mixer with sampling capacitor, load resistor, and LO driving waveforms.

The simplified system architecture of a two-path feedforward cancellation receiver [

Simplified system architecture of (a) a two-path feedforward receiver and (b) a frequency-translational noise-cancelling receiver.

The simplified system architecture of a frequency-translational noise-cancelling receiver [

Nonlinear transfer functions [

The input and output signals in frequency and time domain for various conditions are illustrated in Figure

Input and output of a large (single-tone) signal in frequency and time domain when passing through (a) ideal linear system, (b) conventional nonlinear system, (c) proposed nonlinear system, and (d) proposed nonlinear system accompanied with a weak signal.

Furthermore, the nonlinear transfer function does not obey the rule of superposition. Therefore, the large signal accompanied with a (much weaker) signal passing through the nonlinear system can undergo different operations. The situation is illustrated in Figure

Therefore, based on the envelope amplitude of the large interferer, the specially tailored nonlinear transfer function enables large interference suppression. When the amplitude of the large interferer changes, for example, modulated interferers, the nonlinear transfer function should be altered correspondingly to maintain the suppression. The nonlinear interference suppression can be considered as a notch filter in amplitude domain. The adaption of the nonlinear transfer function in amplitude domain is equivalent to the adaption of a frequency-domain notch filter. When the interferer amplitude is similar to or smaller than the wanted signal, the transfer function can be switched to a linear one.

The adaption is shown in Figure

The adaption of nonlinear transfer function to maintain large signal suppression when the amplitude of large signal changes.

To derive the general requirements of a nonlinear transfer function for interference suppression, the input signal

The effective gain of the fundamental component of the weak signal

Given the expression of effective gain of strong signal and weak signal, the output of the nonlinear transfer function using method described in [

To further analyze the nonlinear operation and consequences, a specific nonlinear transfer function is chosen here. As shown in Figure

Zig-zag transfer function (grey) for an interferer with envelope amplitude of

Based on (

The influence of zero-transition region

The third-order small signal gain _{3} of the nonlinear system using the ideal zig-zag transfer function is approximately 10 dB higher than the amplitude of strong interferer signal.

Based on (

However, for output noise power, all regions lead to additive behavior because the noise is white. So the output noise power can be calculated by

Based on (

If the transfer function is a noiseless zig-zag nonlinear function as shown in (

The influence of zero-transition region

A 1.8 GHz RF amplifier with linear mode and nonlinear mode operation was implemented in a 140 nm CMOS technology. The nonlinear mode operation is enabled for frequency-independent interference suppression, while the linear mode is for linear amplification when no large interference is present. In the presence of a 0 to 11 dBm interferer, the interferer is suppressed by more than 39 dB [

PCB including the nonlinear interference suppression (NIS) IC implementation.

Figure

The system diagram of (a) the proposed NIS operation in a multiradio platform and (b) a conventional narrowband receiver with off-chip SAW filter.

On the other hand, a feedback path is also needed to model the coupling changes between the transmitter antenna and the receiver antenna. Therefore, a mixer is placed around the NIS subblock to provide cross-correlation between the input and output of the NIS subblock. Assuming the interferer is the dominant signal, the cross-correlation measures how much the residue interference remains after nonlinear suppression, representing the errors in control signal

A conventional narrowband receiver with off-chip SAW filter is shown in Figure

The nonlinear receiver system is modeled in Advanced Design System (ADS). The NIS, cross-correlation mixer, NIS Control, and Magnitude subblocks are modeled with symbolically defined devices. The downconversion mixer uses ideal mixer component with ideal

16-QAM modulation scheme is used for both the interferer and the desired signal with raised cosine pulse shaping and a roll-off factor of 0.5. The basebands

The frequency spectrum of (a) input at receiving antenna, (b) output of NIS subblock, and (c) baseband output.

The output spectrum of the NIS subblock is shown in Figure ^{−3} [

Baseband output constellation diagram of the nonlinear receiver system.

As pointed out before, the interference suppression at RF stage by nonlinear transfer function is based on the amplitude discrimination between the interferer and the wanted signal. To illustrate the influence of the relative power ratio, the input signals are kept the same except the interferer power is swept from −30 dBm to 10 dBm. The results of RF suppression, EVM, and SNR at baseband output are shown in Table

Interference suppression limitations.

Interference power (dBm) | Interference suppression at RF (dB) | EVM (%) | SNR (dB) |
---|---|---|---|

−30 | 20 | 14.4 | 17 |

−20 | 42 | 3.7 | 29 |

−10 | 56 | 1.9 | 34 |

0 | 66 | 1.9 | 34 |

10 | 80 | 2.9 | 31 |

The probability densify function (PDF) of the instantaneous power of the modulated interferers for input power and the PDF of the wanted signal are shown in Figure

Probability density function of the instantaneous power of the interferer from −30 dBm to 10 dBm and probability density function of the instantaneous power of the wanted signal with −50 dBm power (light blue).

The limitation for complete interference suppression also comes from baseband filtering for signals outside the baseband bandwidth. The baseband filtering is determined by the baseband filter design such as order and power.

The cross-correlation mixer output spectrum is shown in Figure

The frequency spectrum of cross-correlation mixer output.

For simplicity, the 3rd-order harmonic generated by the nonlinear receiver with nonlinear transfer function is not shown here. It can be removed by frequency-domain filters and harmonic rejection mixers to avoid harmonic mixing.

The system diagrams of a nonlinear receiver system with proposed nonlinear interference suppression and a conventional narrowband linear receiver are shown previously in Figure

The signal-to-interference ratio (SIR) is used here to characterize the interference tolerance and influences on the linear and nonlinear receiver. Initially the SIR at the input of the receiver is −60 dB. The SIR at the input of baseband ADCs should be at least higher than zero, so that the signal is amplified while the interferer is largely suppressed. The suppression of interference signal before ADC is beneficial since it alleviates signal aliasing. Besides, the residue interference also needs extra ADC resolution bits to quantize the total input at baseband A DCs. According to [

The comparison of SIR at baseband output of the linear receiver and the nonlinear receiver versus frequency separation between large interference and wanted signal is shown in Figure

SIR of baseband output of the linear receiver (blue) and the nonlinear receiver (red) versus the frequency separation between input signals.

SNR of baseband output of the nonlinear receiver versus the frequency separation.

To extend the nonlinear method to the suppression of general large interference, the envelope amplitude of the interference needs to be extracted. The system architecture for general large interference suppression is shown in Figure

The system diagram of the proposed NIS operation for general interference suppression.

The feedforward path starts from the receiving antenna and consists of an envelope extraction subblock followed by LPF to derive the amplitude information. The envelope extraction subblock can be implemented as self-mixing mixers or diodes. The extracted envelope contains noise received by antenna, the envelope information of the desired signal, and the envelope information of interferer. However, as the focus of this work is the coexistence of large interferer and weak desired signal, the envelope of the wanted signal behaves as noise and small disturbance to the control signal

Here the desired signal is assumed as a 16-QAM modulated signal with raised cosine pulse shaping and a roll-off factor of 0.5. The baseband

The frequency spectrum of (a) input at receiver antenna, (b) output of NIS subblock, and (c) baseband output.

The output spectrum of NIS subblock is shown in Figure

Baseband output constellation diagram of the nonlinear receiver system for general large interference suppression.

The limited suppression at RF is a result of inaccuracy of the extracted interference envelope. Figure

The frequency spectrum of envelope of the interferer (blue) and control signal for NIS subblock (red).

The amount of interference suppression versus LPF bandwidth (

Trade-off of

| Intermodulation (dBm) | Delay (ns) | Suppression without delay (dB) | Suppression with delay (dB) |
---|---|---|---|---|

5 | −90 | 36 | 30 | 52 |

10 | −69 | 18 | 38 | 50 |

20 | −60 | 8 | 46 | 45 |

In Sections

The interference scenario is shown in Figure

Illustration of NIS operation principle with multiple large interferers accompanying weak desired signal (red). The interferers include local interferers (black) and external interferers (blue).

The NIS operation principle under multiple interferers is illustrated in Figure

Illustration of NIS operation principle with NIS blocks for each local large interferer.

The influence of IM3 product on signal distortion is not alleviated as it happens before the large interferers are suppressed and the NIS operation relies on nonlinear transfer function. On the other hand, for receiver or RF circuit, once IIP_{3} is known, IM3 at any other power level can be calculated. For every 1-dB increase of the IIP_{3} point, the corresponding IM3 product drops by 3 dB [_{3} 10 dB higher than the interferer envelope amplitude

Besides, the nonlinear receiver can implement frequency-translational filtering techniques at mixer and baseband stage, as shown in Figures

A final NIS circuit block can be enabled if still large external interferers exist. Since the external interferers are usually smaller in power compared with the local ones, it is only necessary to deal with the dominant external interferer. The envelope extraction circuit block will extract the envelope of the dominant external interferer and feed it to the NIS circuit block to partially suppress the external interferer, as discussed in Section

The nonlinear receiver with adaptive nonlinear transfer function has been proposed for multiradio coexistence problems in wideband receivers. It relies on the amplitude information of the interference signal and enables frequency-independent filtering, thus improving in-band and out-of-band linearity for wideband operation. With the nonlinear method, the interference suppression is achieved at the RF stage, which relieves the requirement and power consumption for the following circuitry in the receiver chain. With this method, the interference envelope should be tracked continuously to adjust the nonlinear transfer function accordingly. An adaption method for envelope extraction is proposed and cosimulated with the RF receiver. The limitations for interference suppression are identified. From the analysis, the main limitation of interference suppression is the amplitude discrimination between large and weak signals. In the situation of external interference suppression, the accuracy of the extracted envelope is affected by the LPF filter. Therefore the input frequency separation and bandwidth limit the performance of interference suppression. From system level simulation, a large interference suppression is achieved, and positive SIR can be achieved at the input of baseband ADCs. Therefore the ADC resolution requirement is relaxed and the aliasing product is alleviated.

The authors declare that there are no competing interests.

The authors would like to acknowledge the financial contribution of the CORTIF (CA116) project to this work.