Let's take a look at RF front-end receiver solutions for defense, aerospace, and 5G wireless infrastructure. This blog post is a practical guide to help reduce design complexity while meeting the stringent noise figure requirements of 5G infrastructure, defense, and aerospace applications.
Receiver Noise Figure Overview
Many RF front-end (RFFE) systems are unique, but receivers are quite similar in many ways. Generally, RF sensitivity is a key specification parameter for all radio receivers. An RF receiver's ability to receive desired radio signals while ignoring unwanted signals allows it to operate more efficiently in its application.
There are several methods for measuring the RF sensitivity of a receiver:
Noise Figure (NF) – The NF of a system is the logarithmic form of the noise factor. It specifies the noise performance of the receiver, individual system components, and the entire system.
Signal-to-noise ratio (SNR) - This is the ratio between a given signal power level and the internal noise of the system.
Bit Error Rate (BER) – This is a metric used in digital systems. It measures the increase in the number of errors, or bit errors, during transmission when signal levels decrease or link quality deteriorates. BER can reflect Signal-to-Noise Ratio (SNR), but its format is generally more useful in the digital domain.
Error Vector Magnitude (EVM) – EVM is a metric used to quantify the performance of digital radio transmitters and receivers. Signals transmitted by an ideal transmitter or received by an ideal receiver would place all EVM constellation points precisely in their ideal positions. However, defects such as noise, distortion, and phase noise can cause actual constellation points to deviate from their ideal positions. Ideally, the transmitter should generate digital data as close as possible to these points. EVM measures the distance between the actual received data elements and their ideal positions. Furthermore, the higher the linearity of the amplifier, the better the EVM.
Power amplifier (PA) and low-noise amplifier (LNA) technologies generally have no problem amplifying the signal within the receiver. Instead, the limiting factor often lies in noise limiting, as noise can mask the desired signal. For applications such as wireless communications, radar, instrumentation, and satellite, two key performance considerations are receiver sensitivity and signal-to-noise ratio (SNR).
In terms of receiver noise, this refers to any losses in the first stage or LNA and subsequently, which is crucial for determining the overall performance of the entire radio receiver. The overall receiver performance can be improved by optimizing the SNR and NF of the LNA. Furthermore, this performance must be optimized over the entire system bandwidth.
In 5G, defense, and aerospace, the bandwidth of LNAs and other system components is constantly increasing to achieve the higher data capacity required to handle today's applications. This increased bandwidth means that noise level optimization must be adapted to the same bandwidth range. This is obviously challenging, but essential to meet today's capacity and throughput requirements, as well as achieve high levels of receiver sensitivity.
5G RF receiver
Network densification is essential for the effective implementation of 5G. Density is increased by adding more access points per area and deploying more transmitters and receivers at each access point. This increased density improves the overall capacity and throughput of the wireless network, and these systems also enable 5G through the use of more sensitive high dynamic range transceivers. Increasing the number of base stations and access points per area also alters the radio frequency front-end requirements (RFFE). Because the average distance from the user equipment (UE) to the base station is shorter, it reduces the required transmit power. Furthermore, these access points will allow for the addition of more antennas to help increase spatial flow, thereby improving capacity and signal reliability.
Furthermore, multiple-input multiple-output (MIMO) has been added to further improve signal reliability, thereby increasing uplink system capacity. Utilizing multiple antennas and MIMO to increase spatial flow can improve SNR, and the effect is very good, because advanced radio systems like 5G require higher SNR to support higher data rates.
Many 4G LTE systems have transitioned to 5G. These systems feature massive MIMO capabilities, an extension of traditional MIMO that allows for more antennas (e.g., 32, 64, 128) and more antenna arrays on the base station antenna system. These massive MIMO antennas help concentrate power to improve network throughput and efficiency. These 5G networks also have very high bandwidth capabilities. For example, the frequency range FR1 (410 MHz – 7125 MHz) can achieve transmission bandwidths up to 100 MHz. Therefore, LNA designers are creating ultra-wideband LNAs to support multiple 5G RF chains, simplifying product design. To achieve these broadband capabilities, LNAs must have excellent noise figure and EVM characteristics across the entire bandwidth. Furthermore, they need to be small in size, as these RFFE components are currently located on the antennas at the top of the tower.
Figure 1: Components of the RF front end
Because these components are typically located atop base station towers, they require high power handling capabilities. They must be able to withstand high input power surges and, if surged, recover and restart very quickly. Therefore, components such as LNAs, being the first components in the link after the receiver input switch, need to have an input power handling capability of 20 dBm or higher to meet this requirement.
Defense and space receivers
The defense and aerospace RFFE field has also seen many changes, particularly in military radar, satellite communications, electronic warfare communications, and digital receivers. Below are some basic block diagrams. As you can see from numerous embedded module designs, this clearly drives the adoption of small, lightweight, and highly integrated products, integrating the receive and transmit chains into a single package (such as in 5G applications). And unsurprisingly, these characteristics are equally attractive to the defense and aerospace sectors, aligning with SWaP's (size, weight, and power) objectives.
Figure 2: Use cases of RFFE in the defense and aerospace fields
Receiver products in the defense and aerospace (D&A) sector not only require high power capabilities to achieve excellent amplification performance, but also the ability to operate normally under extreme conditions, such as in infrastructure applications. However, at higher input levels (in the kilowatt range), these receiver products typically need to possess robustness and interference immunity. This is primarily for military, aerospace radar, and military communications applications, where electronic countermeasures (ECM) may be used as a defensive strategy to suppress the receiver.
Therefore, receivers with robustness and resistance to electronic interference (such as radio interference) need to be able to withstand high-power surges. They should be able to withstand a high-power surge at the input and quickly restore communication. These devices must also be able to operate over a wider bandwidth than ever before.
In the past, D&A digital receivers were limited by technology and were always narrowband. But this has changed, as advancements in new technologies such as gallium arsenide, gallium nitride, and silicon allow for the use of much greater, sustainable bandwidth. This enables many entirely new defense and aerospace applications and brings entirely new capabilities to existing products.
Many military applications require this type of wideband and multi-band communication with a low probability of interception/radar detection. Wide bandwidth and spectrum can be used for transmission and reception by increasing frequency hopping to reduce signal detection. These aspects can increase noise on the receiver and reduce protection capabilities. If the receiver is exposed to high power levels for extended periods, component performance can degrade rapidly, leading to performance problems or component failure. Therefore, designers must take necessary measures to ensure reliability and receiver sensitivity.
Optimize noise performance
Ultimately, each individual application in these areas drives system design and requirements. However, at higher levels, some RF front-end requirements remain unchanged.
The noise performance of a receiver is typically considered starting from the first stage of the RFFE (Radio Frequency Identifier). The signal level at the RFFE is the lowest, and if noise is present in the signal, it is difficult to distinguish between noise and incoming signals. Moving past the switches, LNA (Low-Noise Amplifier), and then into the driver stage, all signals are amplified. Identifying the incoming signal becomes even more difficult. Therefore, before and at the LNA, it is essential to ensure that noise in the components is minimized. In the LNA, separating the preferred signal from input noise as early as possible is crucial, as this performance affects the entire receiver chain.
Optimal parameter trade-offs can achieve optimized performance.
Designers must make crucial trade-offs between parameters such as gain, gain flatness, input/output matching, linearity, power consumption, and size, while ensuring the inherent stability of the LNA. Designers must ensure a balance between these parameters, maintain system stability, and examine the system's stability across the entire range of operating conditions.
A lower receiver noise figure can indeed improve performance and coverage, but system designers must make trade-offs, as a better NF may lead to a reduction in receiver performance gains. Therefore, a nominal improvement made in one application may not be worthwhile in another. Qorvo's cascading analysis calculator can provide a starting point for system-level design trade-offs.
Figure 3: Qorvo Design Center
In the application shown in Figure 3, an important consideration is the ratio between the LNA and the subsequent insertion loss (the filter in the example above). If the filter after the LNA introduces loss, the NF (Not Required) will increase. For example, in the scenario above, if the first-stage gain of the LNA were 15 dB instead of 19 dB, the NF would be 0.47 dB instead of the 0.37 dB shown in the figure. Furthermore, if the LNA gain were 19 dB and the second-stage filter had an insertion loss of -4.0 dB, the NF would be 0.39 dB, meaning the NF increases again.
Receiver application and temperature
One obvious way to reduce input noise is to select an LNA with optimal NF parameters. Another important consideration for a receiver LNA is its performance as a function of temperature. Temperature has a significant impact on gain flatness and LNA stability across the entire frequency range. Both of these parameters can affect NF. Cooling the LNA or front-end using heatsinks or other thermal technologies can improve thermal noise. Matched design also helps reduce front-end temperature and thermal noise. Some applications in radio astronomy employ cryogenic cooling to maintain low NF. Furthermore, LNA stability is crucial because if the LNA is unstable, the system NF will increase.
Noise temperature
Each noise source has a corresponding noise temperature. Noise temperature is used to describe the noise performance of a device, not NF (Noise Level), and is primarily used as a system parameter. This makes the concept of input noise temperature more meaningful and convenient to use. It appears at the input of the receiver, where the signal level is low, and represents the minimum noise level that any circuit can achieve at a given temperature. It is also uniformly distributed across the entire system spectrum. Thermal noise is also a function of system bandwidth. Matching the bandwidth to the frequency response and the input signal can reduce thermal noise. To help you calculate NF and NF temperature, Qorvo provides an online calculator, as shown below.
Figure 4: Qorvo Design Center
Some additional noise reduction design strategies
Use top-tier LNAs with minimal noise in the design.
When designing a system, the actual nominal temperature of the application must be considered.
By shielding or eliminating noise sources, external noise can be isolated or prevented from affecting the performance or input of the receiver.
Reduce the characteristic impedance of DC power distribution circuits to reduce noise coupling.
Avoid using lossy components along the signal path up to the LNA input.
Maintain the RF impedance of the LNA input and output, and isolate noisy traces or circuits from the LNA or receiver path.
Furthermore, using GaN instead of a limiter helps reduce noise, as limiters introduce noise into the system. GaN can also improve the receiver's robustness.
The impact of limiters and circulators on D&A receivers
As mentioned earlier, high input power performance is crucial for an LNA. Adding a limiter or circulator at the input can reduce the potential impact of high input power on the receiver. This does help protect the receiver, but it increases noise at the LNA. This approach also reduces receiver sensitivity, thus narrowing signal coverage and decreasing throughput and performance. Therefore, if you choose an LNA with very high input power, you don't need to use a limiter or circulator, which helps improve the overall performance of the receiver.
Finally, noise figure and system linearity also affect receiver sensitivity. To achieve optimal receiver sensitivity performance, trade-offs must be struck between several key parameters, such as gain, matching, linearity, and bandwidth, while closely monitoring interference, temperature, and the receiver's ability to withstand shock.
