This article presents a comprehensive solution for distributed antenna systems (DAS), which are essential for extending cellular coverage and capacity within buildings. It outlines the benefits of highly integrated system designs that include an RF transceiver coupled with bidirectional amplifier (BDA) or remote access unit (RAU) equipment. By exploring this solution through the proposed block diagrams, readers can better understand how these elements work together.
Modern environments, such as commercial buildings and sports venues, often require excellent cellular coverage to provide seamless connections. However, the steel, concrete and energy-efficient glass walls in today’s large commercial buildings, hospitals and sports venues can easily prevent cellular signals from reaching the mobile devices of the people inside. The fortified construction and highly-tinted windows, among other construction materials, make buildings act like an RF shield.1 High-rise structures may also experience elevated levels of RF interference from nearby cellular towers, which can further degrade service. Overcapacity, with too many people occupying a small space, is another cause of poor mobile device reception. These factors combine to cause poor cellular device reception. An integrated DAS solution is essential to enabling quality cellular service and accelerating the future growth of wireless networks.
WHAT IS A DAS?
A DAS is an in-building wireless enhancement system that provides building occupants with reliable cellular device coverage. A DAS is a network of spatially separated antenna nodes that expand the cellular range and boost signal strength. The DAS helps to achieve superior cellular connectivity in high-density indoor or outdoor venues. Although no two DAS implementations are the same, a typical deployment may involve direct connections between a donor antenna, an RF signal BDA or booster, a wireless carrier’s base transceiver station (BTS), a fiber distribution headend, RAUs and many strategically placed in-building ceiling antennas. In some cases, multiple BTSs are installed with each carrier that serves the building supplying one.
Often, multiple RF feeds are combined and then transmitted to the headend, which is the primary distribution unit. The donor antenna, placed on the roof of the building, sends and receives signals from a cell carrier and brings the wireless signal into the building through an optimally-located RF signal BDA. The headend equipment then feeds the RAUs via a variety of fiber-optic cabling. The RAUs, in turn, feed the antenna systems via coax cables. Multiple ceiling antennas can be fed from a single RAU. This provides voice and data services to devices inside the building, much like a cell site provides coverage in a cellular network. Figure 1 shows a typical full implementation of a hybrid DAS architecture in a building.
Figure 1 A hybrid DAS architecture.
There are two main methods of improving in-building wireless coverage. The first uses only an RF booster or BDA, which are simple repeaters for signals. This is known as passive DAS. The second method is to use a fully active DAS system. Both passive and active DAS signal distribution systems are used to improve wireless coverage and capacity inside a commercial building, depending on the situation. When a distribution system has both passive and active aspects, it is known as a hybrid DAS architecture.
THE BIDIRECTIONAL AMPLIFIER
The RF signals will grow weaker in response to the coaxial cable attenuation as the distance these signals travel from the donor antenna increases. To mitigate this loss, a passive DAS offers a wide variety of multi-band RF repeaters to amplify and rebroadcast the signals. The BDA front-end consists of a filter, LNA and sometimes an automatic gain control (AGC) circuit. The AGC components are designed to limit the RF power level and protect the BDA from damage or distortion. The BDA amplifies the RF signals in two directions simultaneously. They do not modulate, modify or otherwise distort the actual radio signal. Their main purpose is to keep the RF signal strong throughout the building. Most BDA modules are designed to amplify multiple carriers at the same time and their use does not require an agreement with the carriers. Figure 2 shows the high-level block diagram with some suggested electronic components for a BDA for RF signal amplification and rebroadcasting.
Figure 2 A BDA/RF booster block diagram.
THE DAS REMOTE ACCESS UNIT
The DAS headend equipment performs analog-to-digital conversions and can convert RF signals from one or multiple carriers. This is why carrier approval is usually needed from each provider to install an active DAS. Digitizing the RF signal and transporting it on fiber-optic cabling results in a high bandwidth signal that can be transmitted over much longer distances with minimal losses. This allows the signal to be distributed to all the RAUs strategically placed on each floor throughout the commercial building.1 With this process, signals are much less susceptible to interference.
The RAUs convert the digital fiber signals back to analog RF and feed them to the DAS ceiling antennas. The RAU is connected to the remote ceiling antennas via coaxial cables providing more coverage and range, which allows all users to experience good levels of cellular connectivity. Figure 1 shows the optical fiber cabling between the headend and all the RAUs.
The RAU in a DAS facilitates the expansion of RF capacity and this is a very important function. The main purpose of the RAU is the digital-to-RF and RF-to-digital conversions. To support the requirements of the RAU, Analog Devices, Inc. (ADI) supplies highly integrated and agile RF transceiver solutions, like the ADRV902x family. These integrated circuit components enable the RAU to take on complex tasks.
Figure 3 A block diagram of a typical RAU with the ADRV9029 RF transceiver.
Figure 3 shows a high-level block diagram of a typical DAS RAU. Table 1 lists some possible ADI part numbers for functional components that can be used in an RAU design. While the RAU function can be realized with the discrete part number recommendations shown in Figure 2, the rest of this article addresses the integrated ADRV9029 RF transceiver and a few of the external power components that are necessary to complete the design.
ENHANCING DAS PERFORMANCE WITH THE ADRV9029
The ADRV9029 is a highly integrated zero-IF sampling analog transceiver, capable of synthesizing and digitizing wideband signals. The device can be programmed for usage in both frequency-division duplex (FDD) and time-division duplex (TDD) applications. The device provides the performance demanded by DAS cellular infrastructure applications, especially within the RAU. The device includes a digital predistortion (DPD) adaptation engine and a crest factor reduction (CFR) engine. For cases where the DAS system has stringent latency requirements, the CFR can be bypassed. The functional block diagram of the ADRV9029 is shown in Figure 4.
Digital Predistortion Function
The DPD function allows a wireless system to drive its power amplifiers (PAs) closer to saturation to enable higher efficiency in the PA, while still maintaining the amplifier linearity. The DPD function enables the RAUs to achieve higher PA efficiency by extending the linear operating region of the PA, while still meeting adjacent channel leakage ratio (ACLR) requirements in the transmit signal chain. A PA in the remote DAS node also helps to reduce its overall power consumption. The ADRV9029’s observation receiver paths connect to the DPD actuator and the coefficient calculation engine to help the system PAs operate at high efficiency levels.
Figure 4 ADRV9029 functional block diagram.
The DPD algorithm in the ADRV9029 supports a carrier bandwidth of up to 200 MHz. ADI has calculated that the integration of the DPD function into the ADRV9029 results in significant system-level cost, space and power savings when compared to a discrete implementation that uses an RF transceiver with a field-programmable gate array-based DPD solution. When a specific application calls for it, the DPD engine in the ADRV9029 can be completely bypassed through GPIO control.
In addition to the savings mentioned above, the DPD engine improves the ACLR performance. ACLR is the ratio of the transmitted power on the assigned channel to the power leaked into the adjacent radio channel. Figure 5 shows how ACLR performance improves following the application of DPD to a 20 MHz LTE signal baseband. These power spectral density plots illustrate how the out-of-band nonlinearities, caused by intermodulation products of the LTE 20 MHz signal, are reduced from 15 to 20 dB after the application of DPD.
Figure 5 Power spectral density before and after DPD application.
Crest Factor Reduction Block
Many of the current wireless multiplexing schemes, especially multicarrier waveforms such as orthogonal frequency-division multiplexing (OFDM), require a signal that can have a high peak-to-average power ratio (PAPR). A high PAPR can adversely impact the efficiency of the PAs. The main concern is that the signal may have peaks that exceed the linear operating range of the PA. A CFR scheme ensures that the range required by the signal is within the linear range of the PA. Meeting this requirement helps to mitigate or eliminate the issues created by a high PAPR in a system.
The ADRV9029 comes with an onboard CFR engine that helps reduce the PAPR. With reduced PAPR, the PAs in the RAU can operate at a higher output power, which increases the power-added efficiency in the transmit lineup. The ADRV9029 has three CFR engines to assist the DPD engine which will improve the linearity and efficiency of the RAU transmit architecture. The ADRV9029 implements CFR using a variation of the pulse cancellation technique by subtracting a precomputed pulse from the detected peaks to bring the signal within the PA’s linear range. Therefore, a pulse needs to be generated and loaded for each carrier combination. For these and other reasons, the CFR block adds latency. In many cases, DAS systems have stringent latency requirements. When this is the case, the CFR function can simply be bypassed or a companion part like the ADRV9026 that does not have the DPD and CFR functions can be used.
Power Supplies
After taking all the precautions to achieve the highest possible EVM and ACLR performance, the RAU power supply design must also be considered. During operation, supply currents can vary significantly, especially when operating in TDD mode. If noise from the power supply is not controlled, it could affect the JESD204B/JESD204C link performance.
ADI has developed a switched-mode power supply and the packaging technologies necessary to support all its RF transceivers and other 5G RF SoC parts, like the ADRV9029. The Silent Switcher® 3 family of ICs features low RMS output noise, fast transient response, low EMI emissions and high efficiency. ADI has several products in this family that can provide benefits in the power management and control function of the DAS RAU shown in the block diagram in Figure 3. In many cases, the ADI third-generation Silent Switcher devices eliminate the need for the LDO regulator shown in Figure 3.
CONCLUSION
A DAS helps deliver effective RF coverage and capacity, which facilitates seamless connectivity to support today’s needs for reliable voice and data transmission. A fully active DAS or a passive DAS/BDA solution can improve cellular signals within building structures to ensure that users have robust wireless connections throughout the building. The RAU is an integral part of a fully active DAS communication solution and ADI’s ADRV9029 can be an integral part of the DAS node solution, helping to improve both the performance and the cost of the system.
Reference
- “Designing Distributed Antenna Systems (DAS),” Advantage Business Media, 2016.
