Base station receiver design is a daunting task. Typical receiver components include mixers, low-noise amplifiers (LNAs), and analog-to-digital converters (ADCs), which have improved over time. However, the architectural changes are not much. Limitations in architectural choices hinder base station designers’ efforts to bring differentiated products to market. Recent product developments, especially integrated transceivers, have significantly lowered some of the constraints of the most challenging base station receiver designs. The new base station architecture offered by these transceivers gives base station designers more options and ways to differentiate their products.
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 make a single compact radio design suitable for all frequency bands and power variations.
Let’s first look at some base station categories. The well-known standards body 3GPP defines several classes of base stations. These base station classes 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 gets 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, a practical breakdown of modulation schemes is into non-GSM modulations (including LTE and CDMA types of modulations) and GSM-based modulations – in particular Multi-Carrier GSM (MC-GSM). Of the two categories, GSM has the highest RF and analog performance requirements. Additionally, as higher throughput rate radios have become more commonplace, MC-GSM has replaced single-carrier GSM as the standard. In general, base station radio front-ends that support MC-GSM performance can also handle non-GSM performance. Operators supporting MC-GSM have greater flexibility in capturing market opportunities.
Historically, base stations consisted of discrete components. We believe that today’s integrated transceivers can replace many discrete components while providing system benefits. 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 communication networks, and their receiver designs have traditionally been the most challenging and expensive. Why is it so difficult? In a word, sensitivity.
The base station receiver must achieve the required sensitivity under certain conditions. Sensitivity is a figure of merit that measures the ability of a base station receiver to demodulate weak signals from mobile phones. Sensitivity determines the farthest distance a base station can receive a cell phone signal while maintaining a connection. Sensitivity can be classified in two ways: 1) static sensitivity without any external disturbance; 2) dynamic sensitivity with disturbance.
Let’s talk about static sensitivity first. In engineering terms, sensitivity is determined by the system noise figure (NF). A lower noise figure means higher sensitivity. The desired sensitivity is achieved by increasing the gain to achieve the desired system noise figure, which is generated by an expensive device called a low noise amplifier (LNA). The higher the gain, the higher the cost and power dissipation of the LNA.
Unfortunately, dynamic sensitivity comes with a trade-off. Dynamic sensitivity means that static sensitivity is degraded by disturbances. Interference is any unwanted signal present at the receiver, including signals from the outside world or unintentionally generated by the receiver, such as intermodulation products. In this context, linearity describes the ability of a system to handle disturbances.
In the presence of interference, there is a loss of system sensitivity that we laboriously achieved. This trade-off gets worse as the gain increases, since high gain is usually accompanied by a decrease in linearity. In other words, excessive gain will degrade linearity performance, resulting in reduced sensitivity under strong disturbances.
When designing a wireless communication network, the burden of network performance is placed on the base station side, not the mobile phone side. The WA-BS is designed to cover large areas and achieve excellent sensitivity performance. The WA-BS must have the best static sensitivity to support cell-edge cell phones, where cell phone signals are very weak. On the other hand, in the presence of interference or blocking, the dynamic sensitivity of the WA-BS receiver must still be good. Even if a strong signal from a cell phone near the base station interferes, the receiver must still perform well against the weak signal from the cell 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 RF front-end design has a noise figure of 1.8 dB, while the ADC has a noise figure of 29 dB; in the analysis in Figure 1, the RF front-end gain is swept across the x-axis to show system sensitivity.
Figure 1. Typical Discrete Receiver Signal Chain Schematic
Now let’s compare a simplified transceiver receive signal chain. As can be seen, the bill of materials for the transceiver receive signal chain is less than for a similar discrete signal chain. In addition, the transceiver chip contains two transmitters and two receivers. The seemingly simple integration hides the sophistication of receiver design, which typically achieves a noise figure of 12 dB.The following analysis shown in Figure 2 illustrates how the system achieves high sensitivity
Figure 2. Typical Transceiver/Receiver Signal Chain Schematic
Figure 3 shows the RF front-end gain versus static sensitivity for the two implementations described above. The WA-BS operates in an area where the sensitivity almost meets the most stringent requirements. In contrast, small cells operate in the region with the steepest slope of the sensitivity curve, while still meeting the criteria with a small margin. For both WA-BS and small cells, the transceiver achieves the required sensitivity with much smaller RF front-end gain.
Figure 3. Discrete Receiver vs. Transceiver/Receiver Sensitivity Comparison
How about dynamic sensitivity? In the RF front-end gain area, we use transceivers to design wide-area base stations with much better dynamic sensitivity than discrete solutions. This is because lower gain RF front ends typically have higher linearity for a given power consumption. In discrete solutions that typically use high gain, linearity is often dictated by the RF front end. In transceiver designs, the sensitivity drop due to interference is significantly reduced compared to discrete solutions.
It is worth mentioning that in the presence of too much interference, the system will reduce the gain to a level where the interference can be tolerated, and increase the gain when the interference is reduced. This is automatic gain control (AGC). Gain reduction also reduces 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 summary, these transceivers have two outstanding features: excellent noise figure and improved immunity to interference. Using transceivers in the signal chain means you can achieve the desired static sensitivity with much less front-end gain. Additionally, 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 tradeoffs 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 has taken 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. Additionally, the AD9371 family offers non-GSM (CDMA, LTE) performance and bandwidth options, but is more focused on power optimization.
This article is far from a comprehensive overview. The topic of sensitivity will be discussed in more depth in a follow-up article. In addition, other challenges of base station receiver design include automatic gain control (AGC) algorithms, channel estimation and equalization algorithms, etc. We plan to write a series of technical articles following this article with the aim of simplifying the design process and improving your understanding of receiver systems.
About the Author
Jon Lanford is Manager of System and Firmware Validation at Analog Devices’ Transceiver Products Group in Greensboro. After receiving a master’s degree in electrical engineering from North Carolina State University in 2003, he worked at Analog Devices. His previous engineering positions included GSPS pipeline ADC design and calibration algorithm design, and transceiver test development.
Kenny Man’s 25-year career spans high-speed instrumentation and wireless base station system design, system applications, and system architecture for wireless infrastructure, with telecommunications equipment companies and semiconductor companies. Currently responsible for product engineering, he hopes to better contribute to the building blocks of communications infrastructure. Hobbies include hiking, skiing and reading history.
Base station receiver design is a daunting task. Typical receiver components include mixers, low-noise amplifiers (LNAs), and analog-to-digital converters (ADCs), which have improved over time. However, the architectural changes are not much. Limitations in architectural choices hinder base station designers’ efforts to bring differentiated products to market. Recent product developments, especially integrated transceivers, have significantly lowered some of the constraints of the most challenging base station receiver designs. The new base station architecture offered by these transceivers gives base station designers more options and ways to differentiate their products.
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 make a single compact radio design suitable for all frequency bands and power variations.
Let’s first look at some base station categories. The well-known standards body 3GPP defines several classes of base stations. These base station classes 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 gets 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, a practical breakdown of modulation schemes is into non-GSM modulations (including LTE and CDMA types of modulations) and GSM-based modulations – in particular Multi-Carrier GSM (MC-GSM). Of the two categories, GSM has the highest RF and analog performance requirements. Additionally, as higher throughput rate radios have become more commonplace, MC-GSM has replaced single-carrier GSM as the standard. In general, base station radio front-ends that support MC-GSM performance can also handle non-GSM performance. Operators supporting MC-GSM have greater flexibility in capturing market opportunities.
Historically, base stations consisted of discrete components. We believe that today’s integrated transceivers can replace many discrete components while providing system benefits. 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 communication networks, and their receiver designs have traditionally been the most challenging and expensive. Why is it so difficult? In a word, sensitivity.
The base station receiver must achieve the required sensitivity under certain conditions. Sensitivity is a figure of merit that measures the ability of a base station receiver to demodulate weak signals from mobile phones. Sensitivity determines the farthest distance a base station can receive a cell phone signal while maintaining a connection. Sensitivity can be classified in two ways: 1) static sensitivity without any external disturbance; 2) dynamic sensitivity with disturbance.
Let’s talk about static sensitivity first. In engineering terms, sensitivity is determined by the system noise figure (NF). A lower noise figure means higher sensitivity. The desired sensitivity is achieved by increasing the gain to achieve the desired system noise figure, which is generated by an expensive device called a low noise amplifier (LNA). The higher the gain, the higher the cost and power dissipation of the LNA.
Unfortunately, dynamic sensitivity comes with a trade-off. Dynamic sensitivity means that static sensitivity is degraded by disturbances. Interference is any unwanted signal present at the receiver, including signals from the outside world or unintentionally generated by the receiver, such as intermodulation products. In this context, linearity describes the ability of a system to handle disturbances.
In the presence of interference, there is a loss of system sensitivity that we laboriously achieved. This trade-off gets worse as the gain increases, since high gain is usually accompanied by a decrease in linearity. In other words, excessive gain will degrade linearity performance, resulting in reduced sensitivity under strong disturbances.
When designing a wireless communication network, the burden of network performance is placed on the base station side, not the mobile phone side. The WA-BS is designed to cover large areas and achieve excellent sensitivity performance. The WA-BS must have the best static sensitivity to support cell-edge cell phones, where cell phone signals are very weak. On the other hand, in the presence of interference or blocking, the dynamic sensitivity of the WA-BS receiver must still be good. Even if a strong signal from a cell phone near the base station interferes, the receiver must still perform well against the weak signal from the cell 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 RF front-end design has a noise figure of 1.8 dB, while the ADC has a noise figure of 29 dB; in the analysis in Figure 1, the RF front-end gain is swept across the x-axis to show system sensitivity.
Figure 1. Typical Discrete Receiver Signal Chain Schematic
Now let’s compare a simplified transceiver receive signal chain. As can be seen, the bill of materials for the transceiver receive signal chain is less than for a similar discrete signal chain. In addition, the transceiver chip contains two transmitters and two receivers. The seemingly simple integration hides the sophistication of receiver design, which typically achieves a noise figure of 12 dB.The following analysis shown in Figure 2 illustrates how the system achieves high sensitivity
Figure 2. Typical Transceiver/Receiver Signal Chain Schematic
Figure 3 shows the RF front-end gain versus static sensitivity for the two implementations described above. The WA-BS operates in an area where the sensitivity almost meets the most stringent requirements. In contrast, small cells operate in the region with the steepest slope of the sensitivity curve, while still meeting the criteria with a small margin. For both WA-BS and small cells, the transceiver achieves the required sensitivity with much smaller RF front-end gain.
Figure 3. Discrete Receiver vs. Transceiver/Receiver Sensitivity Comparison
How about dynamic sensitivity? In the RF front-end gain area, we use transceivers to design wide-area base stations with much better dynamic sensitivity than discrete solutions. This is because lower gain RF front ends typically have higher linearity for a given power consumption. In discrete solutions that typically use high gain, linearity is often dictated by the RF front end. In transceiver designs, the sensitivity drop due to interference is significantly reduced compared to discrete solutions.
It is worth mentioning that in the presence of too much interference, the system will reduce the gain to a level where the interference can be tolerated, and increase the gain when the interference is reduced. This is automatic gain control (AGC). Gain reduction also reduces 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 summary, these transceivers have two outstanding features: excellent noise figure and improved immunity to interference. Using transceivers in the signal chain means you can achieve the desired static sensitivity with much less front-end gain. Additionally, 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 tradeoffs 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 has taken 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. Additionally, the AD9371 family offers non-GSM (CDMA, LTE) performance and bandwidth options, but is more focused on power optimization.
This article is far from a comprehensive overview. The topic of sensitivity will be discussed in more depth in a follow-up article. In addition, other challenges of base station receiver design include automatic gain control (AGC) algorithms, channel estimation and equalization algorithms, etc. We plan to write a series of technical articles following this article with the aim of simplifying the design process and improving your understanding of receiver systems.
About the Author
Jon Lanford is Manager of System and Firmware Validation at Analog Devices’ Transceiver Products Group in Greensboro. After receiving a master’s degree in electrical engineering from North Carolina State University in 2003, he worked at Analog Devices. His previous engineering positions included GSPS pipeline ADC design and calibration algorithm design, and transceiver test development.
Kenny Man’s 25-year career spans high-speed instrumentation and wireless base station system design, system applications, and system architecture for wireless infrastructure, with telecommunications equipment companies and semiconductor companies. Currently responsible for product engineering, he hopes to better contribute to the building blocks of communications infrastructure. Hobbies include hiking, skiing and reading history.
