Frequency & Microwave


Figure 3. Discrete receiver vs. transceiver/receiver sensitivity


large area and achieve excellent sensitivity performance. A WA-BS must have the best static sensitivity to support handsets at the cell edge where the signal from handset is very weak. On the other hand, under interference or blocking conditions a WA-BS receiver’s dynamic sensitivity still needs to be good. The receiver still must exhibit good performance on a weak signal from a handset, even while a strong signal from a handset near the base station generates interference. The following signal chain is a simplified


typical discrete component-based system receiver. The LNA, mixer, and variable gain amplifier (VGA) are referred to as 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, and in the analysis in Figure 1, the RF front-end gain is swept on the x-axis to show the system sensitivity. Now let’s compare a simplified


transceiver receive signal chain. One can see the transceiver receive signal chain bill of materials is smaller than the comparable discrete component signal chain. Additionally, the transceivers are designed with two transmitters and two receivers on chip. The apparently simple integration hides the elegance of the receiver design, which typically achieves a 12 dB noise figure. The following analysis shown in Figure 2 will show how the sensitivity pays off in a system. Figure 3 shows the RF front-end gain vs. static sensitivity for the above two implementations. A WA-BS works in the region where the sensitivity is almost to meet tightest sensitivity requirement. In contrast, a small cell works where the sensitivity curve slope is steepest, while still meeting the standard with a small margin. The transceiver achieves the desired sensitivity with much less RF front-end gain for both the WA-BS and small cell. What about dynamic sensitivity? In the


RF front-end gain region, where we would design wide area base stations using a transceiver, dynamic sensitivity is also much better than a discrete solution. This is because lower gain RF front ends typically have higher linearity at a given power consumption. In discrete solutions, which typically use high gain, linearity is often


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dominated by the RF front end. In transceiver designs, degradation in sensitivity due to interference is dramatically reduced compared to a discrete solution. It’s worth mentioning that in the


presence of too much interference, systems are designed to reduce gain to a point where the interference can be tolerated and increase the gain when the interference is reduced. This is referred to as automatic gain control (AGC). Any reduction in gain is also going to reduce the sensitivity. If a system can tolerate the interferers, it is often best to keep the gain as high as possible to maximise sensitivity. AGC is a topic for a future discussion. In summary, two outstanding features


of this class of transceivers are excellent noise figure and higher immunity to interference. Using a transceiver in your signal chain means you can achieve a desired static sensitivity with much less front-end gain. In addition, the lower level of interference means you can achieve better dynamic sensitivity. If you need a LNA at all, it will be a less costly LNA and consume less power. You can also make different design trade-offs elsewhere in the system to take advantage of these features. Today, there are configurable transceiver


products in the market that fill a role in both wide area and small cell base station designs. Analog Devices is taking a leadership role in this new approach, with ADRV9009 and ADRV9008 products are well-suited for wide area base stations and MC-GSM levels of performance. Additionally, the AD9371 family offers options with non-GSM (CDMA, LTE) performance and bandwidth, but more power optimization. This article is far from a thorough


overview. The topic of sensitivity will receive a deeper treatment in our follow- up articles. Additionally, other challenges in base station receiver design include automatic gain control (AGC) algorithms, channel estimation, and equalization algorithms, etc. We plan to follow this article with a series of technical articles with the aim of simplifying your design process and improving your receiver system understanding.


Components in Electronics June 2019 35


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