2G to 5G base station receiver design is too complicated? Here’s how to simplify it

The family of integrated transceivers discussed in this article is the industry’s first to support all current cellular standards (2G to 5G) and cover the full sub-6 GHz tuning range. Using these transceivers, base station designers can fit a single compact radio design across all frequency bands and power variations.

First, let's look at some base station classes. The well-known standards body 3GPP has defined several base station classes. These base station classes have different names. Broadly speaking, the largest base stations, or wide area base stations (WA-BS), provide the greatest geographic coverage and number of users. They also have the highest output power and must provide the best receiver sensitivity. As base stations get smaller, the required output power decreases, and the receiver sensitivity decreases.

Table 1. Dimensions of various base stations

In addition, 3GPP defines different modulation schemes. Broadly speaking, a useful breakdown of modulation schemes is into non-GSM modulations (including LTE and CDMA-type modulations) and GSM-based modulations—specifically multi-carrier GSM (MC-GSM). Of the two broad categories, GSM is the most demanding in terms of RF and analog performance. In addition, as higher throughput radios become 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 that support MC-GSM have greater flexibility in addressing market opportunities.

Historically, base stations have been made up of discrete components. We believe today’s integrated transceivers can replace many discrete components while providing system advantages. But first, we need to discuss the challenges of base station receiver design.

Wide-area or macro base stations have historically been the workhorse of wireless communications networks, and their receiver designs have traditionally been the most challenging and expensive. Why is it so difficult? In a word, sensitivity.

Base station receivers must achieve a desired sensitivity under certain conditions. Sensitivity is a figure of merit that measures how well a base station receiver can demodulate weak signals from mobile phones. Sensitivity determines the maximum distance at which a base station can receive a mobile phone signal while maintaining a connection. Sensitivity can be classified in two ways:

1. Static sensitivity without any external interference;

2. There is disturbing dynamic sensitivity.

In the presence of interference, the system sensitivity we worked so hard to achieve is compromised. This trade-off gets worse as gain increases, since high gain is usually accompanied by reduced linearity. In other words, too much gain degrades linearity performance, resulting in reduced sensitivity in the presence of strong interference.

When designing wireless communication networks, 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 mobile phones at the edge of the cell, where the mobile phone signal is very weak. On the other hand, the dynamic sensitivity of the WA-BS receiver must still be good in the presence of interference or obstruction. Even if the strong signal of the mobile phone near the base station causes interference, the receiver must still show good performance for the weak signal from the mobile phone.

The following signal chain is a simplified, typical discrete-based system receiver. The LNA, mixer, and variable gain amplifier (VGA) are called the RF front end. The RF front end is designed with a noise figure of 1.8 dB, while the ADC has a noise figure of 29 dB; in the analysis of Figure 1, the RF front end gain is swept on the x-axis to show the system sensitivity.

Figure 1. Typical discrete receiver signal chain diagram.

Now let’s compare a simplified transceiver receive signal chain. As you can see, the transceiver receive signal chain has a smaller bill of materials than a similar discrete device signal chain. In addition, the transceiver contains two transmitters and two receivers on-chip. The seemingly simple integration hides the sophistication of the receiver design, which typically achieves a 12 dB noise figure. The following analysis, shown in Figure 2, illustrates how the system achieves high sensitivity.

Figure 2. Typical transceiver/receiver signal chain diagram.

Figure 3 shows the relationship between RF front-end gain and static sensitivity for the two implementations described above. WA-BS operates in a region where the sensitivity almost meets the most stringent requirements. In contrast, small cells operate in a region where the slope of the sensitivity curve is the steepest while still meeting the standard with a small margin. For both WA-BS and small cells, the transceiver achieves the required sensitivity with much less RF front-end gain.

Figure 3. Sensitivity comparison between discrete receiver and transceiver/receiver.

动态灵敏度如何呢？在射频前端增益区域，我们会使用收发器设计广域基站，动态灵敏度也比分立解决方案好得多。这是因为较低增益的RF前端在给定功耗下通常具有较高的线性度。在通常使用高增益的分立解决方案中，线性度常常由RF前端决定。在收发器设计中，与分立解决方案相比，干扰导致的灵敏度降幅显著降低。

It is worth mentioning that in the presence of excessive interference, the system will reduce gain to a point where the interference can be tolerated, and increase gain as the interference decreases. This is called automatic gain control (AGC). This reduction in gain also reduces sensitivity. If the system can tolerate the interfering signal, it is usually best to keep the gain as high as possible to maximize sensitivity. AGC is a topic for a future discussion.

In summary, these transceivers have two outstanding features: excellent noise figure and improved immunity to interference. Using a transceiver in the signal chain means you can achieve the required static sensitivity with much less 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 also be lower. You can also make different design trade-offs elsewhere in the system to take advantage of these features.

Today, there are configurable transceiver products on the market that are suitable for both wide area base station designs and small cell base station designs. Analog Devices is playing a leading role in developing this new approach, with the ADRV9009 and ADRV9008 products being well suited for wide area base stations and MC-GSM performance levels. In addition, the AD9371 family offers non-GSM (CDMA, LTE) performance and bandwidth options, but with a greater focus on power optimization.

This article is far from a comprehensive review. 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 to follow this article with the goal of simplifying the design process and improving everyone's understanding of the receiver system, so stay tuned~