RF converters enable efficient multiband radios for next-gen wireless base stations
01 February 2019
To support rising wireless data demand, modern base station radios are being designed to support multiple E-UTRA bands, as well as carrier aggregation techniques. These multiband radios employ next-generation, GSPS RF ADCs & DACs that allow frequency-agile, direct RF signal synthesis and sampling techniques.
This article was originally featured in the February 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy.
To deal with the sparse nature of the RF wireless spectrum, sophisticated DSP is used to efficiently process the data bits to RF and back again. In this article, John Oates, Wireless System Engineer at Analog Devices, describes an example direct RF transmitter for multiband application, considering DSP configurations and power versus bandwidth trade-offs.
Introduction: 10 years, 10x bands, 100x data rates
It’s been 10 years since the smartphone revolution began, after Apple released the original iPhone in 2007. 10 years and two generations of wireless standards later – and much has changed. Perhaps not as glamourous as the headline-grabbing consumer smartphones, known as user equipment (UE), but the infrastructure base station (eNodeB) of the radio access network (RAN) has also gone through its own transformation to enable the data deluge of our now ever-connected world. Cellular bands have increased 10x, while data converter sample rates have increased 100x. So, where does this leave us?
Multiband radio and efficient use of spectrum
From 2G GSM to 4G LTE, the number of cellular frequency bands has exploded 10x – from just four to over 40. With LTE networks arriving on the scene, base station suppliers have found themselves multiplying radio variants. LTE-advanced increased the requirements of multiband radios by adding carrier aggregation into the mix, whereby noncontiguous frequency spectrum inside the same band, or more importantly, in different bands, could be aggregated in the baseband modem as a single stream.
However, the RF spectrum remains sparse. Figure 1 shows several carrier-aggregated band combinations highlighting the sparse spectrum problem. In green is interband spacing and in red is the band of interest. Information theory dictates the system should not waste power converting the undesired frequency spectrum. Therefore, multiband radios with an efficient means of converting sparse spectrum between analogue and digital domains are needed.
Base station transmitter evolution to direct RF
To facilitate the increased data consumption of 4G LTE networks, the wide area base station has undergone an evolution in radio architecture. Superheterodyne, narrow-band, IF-sampling radios with mixers and single-channel data converters have been replaced with I/Q-based architectures that double the bandwidth, such as complex-IF (CIF) and zero-IF (ZIF). ZIF and CIF transceivers require analogue I/Q modulators/demodulators with dual- and quad-channel data converters. However, these wider bandwidth CIF/ZIF transceivers also suffer from LO leakage and quadrature error images that must be corrected.
Fortunately, data converter sampling rates have also increased 30× to 100× in the last 10 years, from 100 MSPS in 2007 to 10 GSPS+ in 2017. This increase in sampling rate has ushered in GSPS RF converters with very wide bandwidths, enabling frequency-agile software-defined radio (SDR) to finally become a reality.
Perhaps the holy grail of sub-6 GHz radio BTS architecture has long been direct RF sampling and synthesis. Direct RF architectures eliminate the need for analogue frequency translation devices, such as mixers, and I/Q modulators and demodulators, which themselves are the source of many unwanted spurious signals. Instead, the data converter directly interfaces with RF frequencies, and any mixing can be done digitally by integrated digital up-/downconverters (DUCs/DDCs).
Multiband efficiency gain comes in the form of sophisticated DSP, included in ADI’s RF converters, that allow digital channelisation of only the desired spectrum bands, while simultaneously giving access to the full RF bandwidth. Using parallel DUCs or DDCs, which combine interpolating/decimating up-/down-samplers, half-band filters and numerically controlled oscillators (NCOs), the band(s) of interest can be digitally constructed/deconstructed before conversion between analogue and digital domains.
The parallel digital up-/downconverter architecture allows you to channelise multiple bands of desired spectrum (shown in red in Figure 1), and not waste valuable cycles converting unused interband spectrum (shown in green in Figure 1). Efficient multiband channelisation has the effect of lowering the required sample rates of the data converters, as well as the number of serial lanes required for transport across the JESD204B data bus. Reducing system sample rates reduces the cost, power and thermal management requirements on the baseband processor, saving CAPEX and OPEX of the total base station system. It remains true that implementing channelisation DSP in a highly optimised CMOS ASIC process is far more power efficient than implementation in generalised FPGA fabric – even if the FPGA is in smaller geometries.
Direct RF transmitter with DPD receiver: an example
The RF DAC has succeeded in replacing the IF DAC in these next-generation multiband BTS radios. Figure 3 shows an example direct RF transmitter with an AD9172, 16-bit, 12 GSPS RF DAC that supports tri-band channelisation with three parallel DUCs, allowing flexible placement of subcarriers across 1200 MHz bandwidth. Following the RF DAC, the ADL5335 Tx VGA provides 12 dB of gain and 31.5 dB of attenuation range up to 4 GHz. The output of this DRF transmitter can then drive a power amplifier of choice depending on the output power requirements of the eNodeB.
Consider the Band 3 and Band 7 scenario shown in Figure 4. Two different approaches can be employed to convert the data stream to RF directly. The first approach (a wideband approach) would synthesise the bands without channelisation, requiring a data rate of 1228.8 MHz. 80% of this bandwidth yields a DPD (digital predistortion) synthesised bandwidth of 983.04 MHz, sufficient to transmit both bands and their 740 MHz of interband spacing. The advantage of this approach is for DPD systems, which allows for predistortion not only of the intraband IMDs of each individual carrier, but also other unwanted nonlinear emissions between the desired bands.
The second approach is to synthesise channelised versions of these bands. Since each band is only 60 MHz and 70 MHz, respectively, and since operators will only have licences for a subset of this bandwidth, it is not necessary to transmit everything, incurring high data rates consequently. Instead, let us utilise a more appropriate, lower data rate of 153.6 MHz, 80% of which results in a DPD bandwidth of 122.88 MHz. If an operator owns licences for 20 MHz in each band, there is still enough DPD bandwidth for 5th-order correction of intraband IMDs for each band, respectively. This mode can save up to 250 mW of power in the DAC from the wideband approach above, and even more power/thermal savings in the baseband processor, as well as reducing serial lane count, allowing for smaller, lower cost FPGA/ASIC implementations.
Observation receivers for DPD have also evolved to DRF (direct RF) architectures. The AD9208 14-bit, 3 GSPS RF ADC also supports multiband channelisation through parallel DDCs. The combination of RF DAC and RF ADCs in the transmitter DPD subsystem has many benefits, including shared converter clocks, correlated phase noise cancellation and overall simplification of the system. One such simplification is the ability of the AD9172 RF DAC, with its integrated PLL, to generate up to a 12 GHz clock from a low frequency reference signal, removing the need to route high frequency clocks around the radio board. Additionally, the RF DAC can output a phase coherent, divided down version of its clock for the feedback ADC. Such system features truly enhance the BTS DPD system, by creating an optimised multiband transmitter chipset.
Ten years after the smartphone revolution, the cellular business is all about data throughput. Single band radios can no longer keep up with the capacity requirements of consumers. To increase data throughput, more spectrum bandwidth must be made accessible through carrier aggregation of multiple bands. RF data converters can access the entire sub-6 GHz cellular spectrum, being quickly reconfigured for various band combinations, making software-defined radio a reality. These frequency-agile, direct RF architectures reduce cost, size, weight and power. This fact has made the RF DAC transmitter and RF ADC DPD receiver the winning architecture of choice for sub-6 GHz, multiband base stations.
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