“Base station receiver design is a difficult task. Typical receiver components include mixers, low-noise amplifiers (LNA), and analog-to-digital converters (ADC), etc. These devices continue to improve over time. However, the changes in the architecture are not significant. The limitations of architecture choices hinder the efforts of base station designers to introduce differentiated products to the market. Recent product developments, especially integrated transceivers, have significantly reduced some of the limitations of the most challenging base station receiver designs. The new base station architecture provided by this type of transceiver allows base station designers to have more choices and methods to achieve product differentiation.
Base station receiver design is a difficult task. Typical receiver components include mixers, low-noise amplifiers (LNA), and analog-to-digital converters (ADC), etc. These devices continue to improve over time. However, the changes in the architecture are not significant. The limitations of architecture choices hinder the efforts of base station designers to introduce differentiated products to the market. Recent product developments, especially integrated transceivers, have significantly reduced some of the limitations of the most challenging base station receiver designs. The new base station architecture provided by this type of transceiver allows base station designers to have more choices and methods to achieve product differentiation.
The integrated transceiver series discussed in this article is the industry’s first to support all current cellular standards (2G to 5G) and cover the full tuning range below 6 GHz. Using these transceivers, base station designers can make a single compact radio design suitable for all frequency bands and power changes.
First look at some base station categories. The well-known standards organization 3GPP has defined several base station categories. These base station categories have different names. Broadly speaking, the largest base station or wide area base station (WA-BS) provides the largest geographic coverage and number of users. It also has the highest output power and must provide the best receiver sensitivity. As the base station becomes smaller, the required output power also decreases, and the receiver sensitivity decreases at the same time.
Table 1. Various base station sizes
In addition, 3GPP also defines different modulation schemes. Broadly speaking, the practical subdivision of modulation schemes is divided into non-GSM modulation (including LTE and CDMA type modulation) and GSM-based modulation-especially multi-carrier GSM (MC-GSM). Among these two types of solutions, GSM has the highest requirements in terms of radio frequency and analog performance. In addition, as higher-throughput radios become more and more common, MC-GSM has replaced single-carrier GSM as the standard. Generally speaking, a base station radio front end that supports MC-GSM performance can also handle non-GSM performance. Operators supporting MC-GSM have greater flexibility in seizing market opportunities.
Historically, base stations consisted of discrete devices. We believe that today’s integrated transceivers can replace many discrete devices while providing system advantages. But first, we need to discuss the challenges of base station receiver design.
The wide area or macro base station has historically been the main force of wireless communication networks, and its receiver design has traditionally been the most challenging and expensive. Why is it so difficult? In a word, sensitivity.
Base station receivers must achieve the required sensitivity under certain conditions. Sensitivity is a quality factor that measures the ability of the base station receiver to demodulate the weak signal sent by the mobile phone. Sensitivity can determine the longest distance that the base station can receive mobile phone signals while maintaining the connection. Sensitivity can be classified in two ways: 1) static sensitivity without any external interference; 2) dynamic sensitivity with interference.
Let’s talk about static sensitivity first. In engineering terms, sensitivity is determined by the system noise figure (NF). The lower the noise figure, the higher the sensitivity. By increasing the gain to achieve the required system noise figure, the required sensitivity can be achieved, and the gain is produced by an expensive device called a low-noise amplifier (LNA). The greater the gain, the higher the cost and power consumption of the LNA.
Unfortunately, dynamic sensitivity requires a trade-off. Dynamic sensitivity means that the static sensitivity will be worse due to interference. Interference refers to any unwanted signal that appears on the receiver, including signals from the outside world or signals unintentionally generated by the receiver, such as intermodulation products. In this context, linearity describes the ability of the system to deal with interference.
In the presence of interference, the sensitivity of the system we have worked so hard to achieve will be lost. This trade-off will become worse as the gain increases, because high gain is usually accompanied by a decrease in linearity. In other words, too large gain will reduce linearity performance, resulting in reduced sensitivity under strong interference.
When designing a wireless communication network, the burden of network performance is placed on the base station side, not on the mobile phone side. WA-BS is designed to cover a large area and achieve excellent sensitivity performance. WA-BS must have the best static sensitivity to support cell phones at the edge of the cell, where the cell phone signal is very weak. On the other hand, in the presence of interference or blocking, the dynamic sensitivity of the WA-BS receiver must still be very good. Even if the strong signal of the mobile phone near the base station produces interference, the receiver must still show good performance to the weak signal sent by the mobile phone.
The following signal chain is a simplified typical system receiver based on discrete components. The LNA, mixer, and variable gain amplifier (VGA) are called the RF front end. The noise figure of the RF front-end design is 1.8 dB, and the noise figure of the ADC is 29 dB; in the analysis in Figure 1, the RF front-end gain is scanned on the x-axis to show the system sensitivity.
Figure 1. Schematic diagram of a typical discrete receiver signal chain.
Now let’s compare a simplified transceiver receiver signal chain. It can be seen that the bill of materials of the transceiver signal chain is less than that of the similar discrete device signal chain. In addition, the transceiver chip contains two transmitters and two receivers. The seemingly simple integration hides the sophistication of the receiver design, which can usually achieve a noise figure of 12 dB. The following analysis shown in Figure 2 illustrates how the system achieves high sensitivity.
Figure 2. Schematic diagram of a typical transceiver/receiver signal chain.
Figure 3 shows the relationship between the RF front-end gain and static sensitivity of the above two implementations. WA-BS works in areas where the sensitivity almost meets the most stringent requirements. In contrast, small cells work in the region with the steepest slope of the sensitivity curve while still meeting the standard and having a small margin. For WA-BS and small cells, the transceiver achieves the required sensitivity with a much smaller RF front-end gain.
Figure 3. The sensitivity comparison of a discrete receiver and a transceiver/receiver.
How about dynamic sensitivity? In the RF front-end gain area, we will use transceivers to design wide-area base stations, and dynamic sensitivity is much better than discrete solutions. This is because a lower gain RF front end usually has higher linearity at a given power consumption. In discrete solutions that typically use high gain, linearity is often determined by the RF front end. In the transceiver design, the sensitivity drop caused by interference is significantly reduced compared to a discrete solution.
It is worth mentioning that when there is too much interference, the system will reduce the gain to the extent that it can tolerate the interference, and increase the gain when the interference decreases. This is automatic gain control (AGC). Decreasing gain will also reduce sensitivity. If the system can tolerate interfering signals, it is usually best to keep the gain as high as possible to maximize sensitivity. AGC is the subject of future discussion.
In short, this type of transceiver has two outstanding characteristics: excellent noise figure and higher anti-interference. Using a transceiver in the signal chain means that you can achieve the required static sensitivity with a much smaller front-end gain. In addition, lower interference levels mean you can achieve better dynamic sensitivity. If an LNA is required, its cost and power consumption will be lower. You can also make different design trade-offs elsewhere in the system to take advantage of these features.
Nowadays, there are configurable transceiver products on the market, which are suitable for both wide area base station design and small cell base station design. ADI plays a leading role in the development of this new approach, and the ADRV9009 and ADRV9008 products are very suitable for wide-area base stations and MC-GSM performance levels. In addition, the AD9371 series provides non-GSM (CDMA, LTE) performance and bandwidth options, but focuses more on power consumption optimization.
This article is far from a comprehensive overview. The topic of sensitivity will be discussed in more depth in subsequent articles. In addition, other challenges in base station receiver design include automatic gain control (AGC) algorithms, channel estimation, and equalization algorithms. We plan to write a series of technical articles following this article, the purpose is to simplify the design process and improve everyone’s understanding of the receiver system.