by Brad Brannon, Systems Engineer, Analog Devices, Inc.
O-RAN was created as a catalyst to transform the wireless community, to enable new channels of wireless equipment, and to enable innovation to fulfill the promises made by 3GPP about 5G.¹ To be successful and cost-effective, open sources of radio equipment and optimized 5G technology devices must be available. This article will explore building a power efficient design.
There are several techniques that radio and network engineers are using to achieve these goals. In addition to moving data services to the edge of the network, utilization of both massive MIMO and small cell technology aids in increasing both capacity and throughput. Massive MIMO technology utilizes numerous radios in an array to achieve not only capacity but also coverage for a central location. Like its predecessor the macrocell, a massive MIMO radio would provide relatively broad coverage around that location. However, massive MIMO radios are placed at higher frequencies, typically 2.6 GHz and above, that do not penetrate buildings very well. To service indoor locations and other difficult to reach outdoor areas, small cells will be utilized. Given the number of indoor and outdoor locations ranging from households to enterprise installations to commercial shopping areas and even arenas, small cell utilization will be vital to the success of 5G. Given the vast numbers of small cells and diversity of deployments required in a network, they must be inexpensive to install and operate; this will be a key enabler of 5G.
Over the last few years, multiple technologies have evolved in a direction that enables solutions for 5G. First, from a baseband perspective, Moore’s law continues to not only reduce the cost of silicon per gate but also enable more complex functionality to be integrated into radio technology. For example, it is now possible to integrate many of the control algorithms required directly into the radio, including functionality like digital predistortion (DPD). Many other possibilities exist as new generations of radios become available.
Second, industry alliances like O-RAN2 are working across the wireless industry to enable economies of scale not only to reduce cost, but also to improve supply chain security and to offer new ways to monetize these wireless networks. Specifically, “the O-RAN Alliance was founded by operators to clearly define requirements and help build a supply chain eco-system to realize [its] objectives. To accomplish these objectives, the O-RAN Alliance’s work will embody” the principles of openness and intelligence.3 As such, their activities focus on defining the physical interfaces specified by 3GPP so that they can be standardized and implemented across the industry as interoperable white box solutions. Additionally, O-RAN also defines the hardware requirements and provides reference designs for the O-CU, O-DU, and O-RU (Open Centralized Unit, Open Distributed Unit, and Open Radio Unit, respectively—as defined by O-RAN). Together these will enable standardization of fronthaul and baseband processors to further reduce solution cost. Together with other integrated 5G devices like integrated radios, these can serve to define what small cells are to become as well as enable the implementation of those standards. The work of these bodies is a critical step.
Third, radio technology has rapidly evolved in the last few years. High performance radios are now available in several formats capable of meeting the required performance standards demanded by 3GPP in 38.104 and related documents.1 These radios are highly integrated and include not only the analog and RF components but also critical algorithms like DPD and crest factor reduction (CFR). While these radios are built on fine-line CMOS, other evolutions have taken place in the RF front ends where low cost RF processes (SiGe, SOI, GaN, GaAs, etc.) are turning out highly integrated LNAs and high power, high performance PAs that can meet the challenges these standards demand.
Finally, highly integrated and efficient power components—including power over Ethernet (PoE), standard power device, supervisory and monitoring, and protection components—are available that can provide compact power delivery. These components offer very high efficiency and very low noise in the radio environment and include options that provide protection of key devices like power amplifiers.
Together, these technologies are enabling low-cost, high-performance small cell platforms that can effectively be deployed throughout an operator’s network for both low- and high-power systems.
Figure 1 shows a typical 4T4R (four transmitters and four receivers) 5G small cell block diagram. There are many permutations possible, including 2T2R and a range of power classes from 24 dBm and higher. This figure will be the basis for the remainder of the discussion, focusing on 5G technology devices that are easily scaled for band and power level variations within the O-RU.
Key Radio Elements
Over the last decade, the integrated transceiver has matured into a high-performance platform. For example, the ADI RadioVerse family includes a wide range of integrated transceivers that support up to 200 MHz of occupied bandwidth, integrating advanced features like DPD. Devices like these not only meet the needs of 5G technology devices but also continue to support LTE and multicarrier GSM RF requirements. While new generations of these devices are always in development, one of the latest is shown in Figure 2, the ADRV9029, a 4T4R configuration. Other products are available that include devices with and without integrated DPD and other configurations including 2T2R.
Today’s integrated devices include everything required to construct a complete radio except for the LNA and PA. This includes all functionality for transmit and receive, synthesizers, and clocking. It also includes the state machine and VGA required to run the AGC and gain control amplifier. Not that must be specified by band or frequency range. Therefore, to complete the radio design, a suitable LNA and PA must be paired with the transceiver. The following sections will describe the signal chain for both the receive and transmit for a 5G NR small cell design and give some insight into selection of those devices.
Example Receiver Signal Chain
When the transceiver is combined with a receiver front end such as the ADRF5515 or ADRF5545A (shown in Figure 3), a 2-chip receiver is easily constructed. Combined with only a few other passive components, this results in a compact, high-performance receiver, as shown in the signal chain in Figure 4. The key advantage to an architecture like this is the high level of integration possible, which leads not only to a very low cost implementation but also to the lowest power dissipation possible.4
The architecture of integrated transceivers leads to the elimination of many elements typically associated with a classic receiver design, including some of the RF amplification, filtering, and integration of much of the remaining radio functionality, including channel filters (analog and digital) and baseband amplifiers. These are typically some of the largest and highest power devices in the system, which results in significant savings over other architectures like direct RF sampling.
As shown in Figure 4, the small cell receiver lineup consists of a circulator (for TDD applications), ADRF5545A, SAW/BAW (surface acoustic wave/bulk acoustic wave) or monoblock filter, balun, and transceiver. Additional amplifiers or VGAs are not required given the good noise performance and low input IP1dB of the ADRV9029 and other members of the RadioVerse family. Using this signal chain, it is possible to support noise figures as low as 2 dB for the complete system from the antenna to bits. While this design includes an integrated RF front-end module (FEM), many designs will still benefit from a discrete design not represented here. The integrated FEM trades off integration for slightly increased filter requirements in the antenna filter but still offers a compelling design for many highly integrated solutions such as massive MIMO and other TDD deployments. Typically, discrete front ends are used for FDD designs.
Assuming a loss prior to the LNA of about 0.5 dB, and if the loss of the band filter is 1 dB, given the data sheet specs of the two active devices, the nominal NF for the complete receiver signal chain should be about 2 dB. Assuming a 0 dB signal-to-noise-and-distortion ratio consistent with MCS-4, the reference sensitivity will be about –104.3 dBm for a G-FR1-A1-1 5G carrier (~5 MHz). This should be more than adequate to meet even the wide area conducted requirements shown in section 7.2.2 of 38.104 with room for margin, and much more than enough for a local area/small cell design that requires –93.7 dBm for this condition, as summarized in Table 1. Some low performance small cell applications may be able to utilize a single stage LNA such as GRF2093 followed by a SAW filter.
Additionally, 38.104 section 7.4.1 requires that under –52 dBm (wide area) ACS blocking, the receiver not desense more than 6 dB. Based on the NF vs. input level shown in Figure 5, very little additional noise occurs at –52 dBm than at lower levels. In fact, the noise floor doesn’t tilt upward until just after –40 dBm, which is ideal for the local area ACS that requires –44 dBm tolerance.
General blocking requirements (7.4.2) call for an aggressor of –35 dBm (local area) to be applied to the receiver within the band of interest at an offset of ±7.5 MHz with no more than 6 dB desense allowed. From Figure 5 showing the Analog Devices signal chain performance, only about 0.9 dB desense has occurred. Narrow-band blocking is a slightly lower power CW-like stimulus but is not a problem either.
Perhaps a more interesting challenge will be the out-of-band blocking from section 7.5.2. Here a signal of –15 dBm is passed to the antenna input. For a small cell with less than 200 MHz, the closest this signal can be to the band edge is 20 MHz. The test requires a sweep from 1 MHz up to 12.75 GHz, excluding the band within 20 MHz of the operational frequency. There are several things working to the signal chain’s advantage here. First, the circulator has a finite bandwidth and will reject many out-of-band signals, but close in it is not a big contributor. Second, the filter shown after the ADRF5545A will provide some filtering—typically, 20 MHz out-of-band ~20 dB rejection is reasonable. Finally, one of the unique and most useful features of ADI’s transceiver family, inherent to the transceiver architecture, is built-in out-of-band rejection. In Figure 20 from Analog Devices’ application note AN-1354 (available here: https://bit.ly/3zDKVcq), inherent out-of-band rejection is demonstrated as an increasing signal level to desense the receiver. In this application note, sweeping frequency in either direction around the pass band shows that a larger signal is tolerated for the same level of desense. In the application note, we see that near the band edge about 10 dB is possible for 6 dB desense. Beyond this, the integrated filter rolls off significantly out-of-band signals, which do not alias back in-band and are largely attenuated both by on-chip and external filtering.
Together these blocks filter the –15 dBm out-of-band aggressor to approximately –40 dBm to –45 dBm up to the 20 MHz exclusion band. Further out, even greater rejection would be assumed. At this level, Figure 5 shows very little desense would be expected.
Perhaps the bigger problem would be the linearity of the front-end module. At this level, a significant IM3 product could be anticipated. Depending on the actual FEM selected, it may be desirable to move the band selection filter before the second LNA to protect it from out-of-band signals, which typically produce large IM products. It is not possible to place a filter between the stages on this these types of FEMs, so an alternate option is implemented.
To aid in limiting the impact of intermods under large out-of-band blockers, a typical FEM includes second stage bypass switches to reduce gain and to protect the second stage from being driven into nonlinearity, as shown in Figure 3. Toggling the LNA gain reduces the signal chain SNR by 1 dB but aids in preserving the overall dynamic range by limiting intermodulation distortion caused by these large blockers, more than offsetting the loss of noise performance. Overall, this would result in a worst-case NF of about 5.7 dB, which is still within the local area (small cell) footprint requirement on reference sensitivity. Any remaining filter requirements are supplied by an antenna filter, and rejection can be determined based on the low gain compression point and IP3 of the receiver FEM.
Example Transmitter Signal Chain
When the ADRV9029 is combined with a suitable RF drive amplifier, or RFVGA (visit analog.com/rf for more options), and a suitable PA, a compact indoor picocell, outdoor picocell, or outdoor microcell5 are easily constructed. With only a few other passive components, these 5G technology devices can be combined to form a very compact and efficient transmitter design, as shown in the signal chain in Figure 6. The key advantage to this architecture is the high level of integration possible, which leads not only to a very low cost implementation but also to the lowest power dissipation possible by utilizing the integrated DPD functionality available on select ADI transceivers.
As shown in Figure 6, the small cell transmitter lineup consists of a circulator, PA, filter, and transceiver. Additionally, the circuit includes a coupler on the output of the PA that is used to monitor the output distortion (and can also be used to monitor the VSWR of the antenna as well as forward power) and can be used with DPD to improve the operational efficiency of the transmit function and improve spurious performance. While an external DPD can be used, select ADI transceivers include fully integrated DPD, which operate on 350 mW or less incremental power depending on the amount of correction a given PA requires. Lower power PAs will require less correction and therefore less power consumed by the DPD. In addition, the integrated DPD reduces the number of SERDES lanes by one-half to the external baseband chip as the observation receiver SERDES lanes are eliminated completely and the transmitter payload is reduced, given the bandwidth expansion for DPD is handled completely within the transceiver. An equivalent DPD in an FPGA typically has 10× higher power and is not effective or power efficient for low power small cells and massive MIMO. However, by integrating the DPD into the transceiver, the very low power and small cost effectively enable DPD to be used even for low power small cells and thus reap the benefit in efficiency and improved transmit linearity without a heavy external computation burden.
Figure 7 and Figure 8 show examples of ADI’s DPD working in low and medium power small cell applications. The stimulus shown is for five adjacent 20 MHz LTE carriers with a total of 100 MHz. Typically, LTE requires a minimum of 45 dB ACLR, with most deployments expected to get more than this. ADI runs an ongoing testing lab always reviewing new PAs of all power classes. Check the Power Amplifier Test Report, or consult the factory for the latest details on available DPD technology from ADI as well as a list of the latest qualified PAs.
In Part 2 of this article series, we bring everything together into the full signal chain. We’ll explore various design considerations in more detail, including the clock tree, power dissipation, and tradeoffs that can be made to optimize design for a particular application.
2 O-RAN Alliance.
3 “O-RAN: Towards an Open and Smart RAN.” O-RAN Alliance, October 2018.
4 Brad Brannon. “Where Zero-IF Wins: 50% Smaller PCB Footprint at 1/3 the Cost.” Analog Dialogue, Vol. 50, No. 3, September 2016.
5 Specifications. O-RAN Alliance.
About the Author
Brad Brannon has worked at ADI for 37 years following his graduation from North Carolina State University. At ADI he has held positions in design, test, applications, and system engineering. Currently Brad is developing reference designs for O-RAN and supporting those customers. Brad has authored several articles and application notes on topics that span clocking data converters, designing radios, and testing ADCs. He can be reached at firstname.lastname@example.org.