The base station receiver design is a daunting task. Typical receiver assemblies include mixers, low noise amplifiers (LNAs), and analog-to-digital converters (ADCs), etc., which are constantly improving over time. However, the changes in the architecture are not large. The limitation of architecture selection hinders the efforts of the base station designer to launch differentiated products to the market. Recent product development, especially integrated transceivers, significantly reduces some of the limitations of the most challenging base station receiver design. The new base station architecture provided by such transceivers enables base station designers to have more choices and methods to achieve product differentiation.
The integrated transceiver series discussed herein is a product in the industry to support all current honeycomb standards (2G to 5G) and cover all 6 GHz below tuning ranges. With these transceivers, base station designers can make single compact radio design suitable for all bands and power changes.
First, look at some base stations categories. The well-known standard organization 3GPP defines several base stations categories. These base stations have different names. Widely, the largest base station or wide area base station (WA-BS) provides the largest geographic coverage and the number of users. Its output power is also the highest, and the best receiver sensitivity must be provided. As the base station gradually becomes small, the output power required is also reduced, and the sensitivity of the receiver is reduced.
In addition, 3GPP also defines different modulation schemes. As a broad, the practical segment of the modulation scheme is divided into non-GSM modulation (including the modulation of LTE and CDMA types) and GSM-based modulation-especially multi-carrier GSM (MC-GSM). In both types of programs, GSM requires the highest in radio frequency and simulation performance. In addition, as higher throughput rates become more common, MC-GSM has replaced single carrier GSM a standard. In general, the radio front end of the base station that supports MC-GSM performance can also handle non-GSM performance. Operators supporting MC-GSM have greater flexibility in grasping market opportunities.
Historically, the base station consists of discrete components. We believe that today's integrated transceivers can replace a lot of discrete devices while providing system advantages. But first, we need to discuss the challenge of the base station receiver design.
Wide or Acer Stations have always been the main force of wireless communication networks, and its receiver design is traditional most challenging. Why is it so difficult? One sentence, sensitivity.
The base station receiver must reach the desired sensitivity under certain conditions. Sensitivity is a quality factor that measures the ability of the base station receiver to demodulate the weak signal emitted by the phone. By sensitivity, it is determined that the base station can receive the farthest distance from the phone signal while maintaining the connection. Sensitivity can be classified in two ways: 1) Static sensitivity without any external interference; 2) Dynamic sensitivity with interference.
First talk about static sensitivity. In engineering terms, sensitivity is determined by the system noise factor (NF). The lower the noise coefficient, the higher the sensitivity. By increasing the gain to achieve the desired system noise coefficient, the desired sensitivity can be achieved, and the gain is generated by an expensive device called a low noise amplifier (LNA). The greater the gain, the higher the cost and power consumption of LNA.
Unfortunately, dynamic sensitivity needs to be tried. Dynamic sensitivity means that static sensitivity is disturbed. Interference refers to any unwanted signals that appear on the receiver, including signals from an external signal or receiver, such as an intermodulation product. In this context, linearity describes the ability of the system to process interference.
In the case of interference, our laborious system sensitivity will be lost. This trade-off will become worse as the gain is increased, because high gain is usually reduced by linearity. In other words, excessive gain will reduce linearity performance, resulting in a decrease in sensitivity under strong interference.
When designing a wireless communication network, the burden of network performance is placed on the base station, not on the phone. WA-BS is designed to cover larger regions and achieve excellent sensitivity performance. WA-BS must have the best static sensitivity to support cell-edge mobile phones, and the mobile phone signal here is very weak. On the other hand, in the case of interference or blocking, the dynamic sensitivity of the WA-BS receiver must still be very good. Even though the strong signal of the mobile phone near the base station is disturbed, the receiver still must show a good performance to the weak signal emitted by the phone.
The following signal chain is a simplified a typical system receiver based on discrete devices. LNA, mixer and variable gain amplifier (VGA) are called RF front ends. The noise coefficient of the RF front end is 1.8 dB, and the noise coefficient of the ADC is 29 dB; in the analysis of Figure 1, the RF front end gain is scanned on the X-axis to display system sensitivity.
Now let's compare a simplified transceiver to receive the signal chain. It can be seen that the transceiver receives the material chain of the material chain less than a similar discrete device signal chain. In addition, the transceiver contains two transmitters and two receivers. It seems a simple integration hidden the exquisiteness of the receiver, and the latter can usually achieve the noise coefficient of 12 dB. The following analysis shown in Figure 2 illustrates how the system achieves high sensitivity.
Figure 3 shows the relationship between the RF front end gain and static sensitivity of the above two implementations. WA-BS works in the area where sensitivity is almost satisfied with the most stringent requirements. In contrast, small honeycombs work in a region where the slope of sensitivity curves, while still meeting standards and smaller margins. For WA-BS and small honeycombs, the transceivers achieve the desired sensitivity with a small RF front-end gain.
What is the dynamic sensitivity? In the radio front end gain area, we will use the transceiver to design a wide area base station, and dynamic sensitivity is much better than discrete solutions. This is because the lower gain RF front end usually has a high linearity at a given power consumption. The linearity is often determined by the RF front end in a discrete solution that usually uses high gain. In transceiver design, the sensitivity caused by interference is significantly reduced compared to discrete solutions.
It is worth mentioning that in the case of excessive interference, the system will reduce the gain to tolerate the degree of interference, and increase the gain when interference is lowered. This is the automatic gain control (AGC). Gain reduction also reduces sensitivity. If the system can tolerate the interference signal, it is usually the highest possible gain to maximize sensitivity. AGC is the subject of the discussion in the future.
In summary, such transceivers have two prominent characteristics: excellent noise coefficients and higher anti-interference. Using transceivers in the signal chain means that you can achieve the desired static sensitivity by small front-end gain. In addition, lower interference levels means you can achieve better dynamic sensitivity. If LNA is required, the cost and power consumption will be lower. You can also make different design weighing in other parts of the system to use these features.
Today, there is a configurable transceiver product in the market, which is suitable for a wide range of base station design, and is also suitable for small cellular base stations. ADI has played a leadership role in developing this new approach, and the ADRV9009 and ADRV9008 products are ideal for wide-area base stations and MC-GSM performance levels. In addition, the AD9371 series provides non-GSM (CDMA, LTE) performance and bandwidth option, but more focuses on power optimization.
This article is far from comprehensive review. Sensitivity topics will be discussed in a later article. In addition, other challenges for the base station receiver design include automatic gain control (AGC) algorithm, channel estimation, and equalization algorithm.
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