Frequency & Microwave
2G to 5G station receiver design simplified by integrated transceivers
By Jon Lanford and Kenny Man, Analog Devices B
ase station receiver design can be a daunting task. Typical receiver components such as mixers, low
noise amplifiers (LNAs), and analog-to- digital converters (ADCs) have progressively improved over time. However, architectures have only changed slightly. The limitation in architectural choices have held back base station designers from differentiating their products in the marketplace. Recent product developments, particularly integrated transceivers, have significantly relaxed some of the constraints of even the most challenging base station receiver designs. The new base station architecture offered by these transceivers allows base station designers more choices and ways to differentiate their product. The family of integrated transceivers
discussed in this article are the industry’s first to support all existing cellular standards, 2G to 5G, and cover the full sub-6 GHz tuning range. These transceivers allow base station designers to adopt a single, compact radio design across all band and power variants. First, let’s review several base station
classes. The well-known standards body 3GPP has several defined base station classes. These base station classes go by various names. In broad terms, the largest base stations, or wide area base stations (WA-BS), offer the most geographical coverage and number of users. They also output the highest power and must provide the best receiver sensitivity. Each progressively smaller base station requires less output power and a relaxed receiver sensitivity.
Table 1. Various Base Station Sizes
handle non-GSM performance. Carriers that handle MC-GSM will have more flexibility in market opportunities.
A lower noise figure means better sensitivity. The desired sensitivity is achieved by increasing gain to achieve the desired
Figure 1. Typical discrete receiver signal chain, simplified
Figure 2. Typical transceiver/receiver signal chain, simplified In addition, 3GPP also defines different
modulation schemes. Broadly speaking, a practical breakdown of modulation schemes is into non-GSM (including LTE and CDMA types of modulation) and GSM-based modulation - particularly multicarrier GSM (MC-GSM). Of the two broad schemes, GSM is the most demanding in terms of RF and analog performance. Also, as higher throughput radios have become more common, MC-GSM has become the norm over the single carrier GSM case. Generally, a radio front end in a base station that can support MC-GSM performance can also
34 June 2019 Components in Electronics
Historically, base stations have been
composed of discrete components. We believe today’s integrated transceivers can replace many discrete components and offer system advantages as well. But first, we need to discuss the challenges of base station receiver design. The wide area or macrobase station has
traditionally been the most challenging and expensive receiver design, and historically has been the workhorse of our wireless communications networks. What makes it so challenging? In a word, sensitivity. A base station receiver must achieve
desired sensitivity under specific conditions. Sensitivity is a figure of merit of how well a base station receiver can demodulate a desired weak signal from handsets. Think of sensitivity as determining the farthest a base station can get from a handset while maintaining a connection. Sensitivity can be categorised in two ways: 1) static sensitivity without any external interference and 2) dynamic sensitivity with interference. Let’s focus on static sensitivity, first. In
engineering parlance, sensitivity is determined by the system noise figure (NF).
system NF, and gain is generated by an expensive component called a low noise amplifier (LNA). The higher the gain, the more an LNA costs in dollars and power. Unfortunately, there’s a trade-off with
dynamic sensitivity. Dynamic sensitivity means that static sensitivity can get worse with interference. Interference is any unwanted signal that appears at the receiver, including signals from the outside world or signals generated unintentionally by the receiver, such as intermodulation products. Linearity in this context describes how well a system can handle interference. In the presence of interference, our
system loses the sensitivity we worked so hard to achieve. This trade-off gets worse with higher gain, because gain typically comes with lower linearity. In other words, too much gain degrades linearity performance, which leads to sensitivity degradation under strong interference. Wireless communication networks are
designed such that the burden of network performance is on the base station side as opposed to the handset side. WA-BSs are designed to cover a
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