Ten years ago, smartphones typically supported only a few standards operating in the four GSM frequency bands, and perhaps a few WCDMA or CDMA2000 standards. With so few frequency bands to choose from, a certain degree of global uniformity has been achieved with “quad-band” GSM phones, which use the 850/900/1800/1900 MHz bands and can be used anywhere in the world (well, pretty much).
This is a huge benefit for travelers and creates huge economies of scale for device manufacturers, who only need to release a few models (or maybe just one) for the entire global market. Fast forward to today, GSM remains the only wireless access technology that provides global roaming. By the way, if you didn’t know, GSM is gradually being phased out.
Any smartphone worthy of the name must support 4G, 3G and 2G access with varying RF interface requirements in terms of bandwidth, transmit power, receiver sensitivity and many other parameters.
Additionally, due to the fragmented availability of global spectrum, 4G standards cover a large number of frequency bands, so operators can use them on any frequencies available in any given area – currently 50 bands in total, as is the case with LTE1 standards. A true “world phone” must work in all of these environments.
The key problem that any cellular radio must solve is “duplex communication”. When we speak, we listen at the same time. Early radio systems used push-to-talk (some still do), but when we talk on the phone, we expect the other person to interrupt us. First generation (analog) cellular devices used “duplex filters” (or duplexers) to receive the downlink without being “stunned” by transmitting the uplink on a different frequency.
Making these filters smaller and cheaper was a major challenge for early phone manufacturers. When GSM was introduced, the protocol was designed so that transceivers could operate in “half duplex mode”.
This was a very clever way to eliminate duplexers, and was a major factor in helping GSM become a low-cost, mainstream technology capable of dominating the industry (and changing the way people communicated in the process).
The Essential phone from Andy Rubin, the inventor of the Android operating system, features the latest connectivity features including Bluetooth 5.0LE, various GSM/LTE and a Wi-Fi antenna hidden in a titanium frame.
Unfortunately, the lessons learned from solving technical problems were quickly forgotten in the techno-political wars of the early days of 3G, and the currently dominant form of frequency division duplexing (FDD) requires a duplexer for each FDD band in which it operates . There is no doubt that the LTE boom comes with rising cost factors.
While some bands can use Time Division Duplex, or TDD (where the radio quickly switches between transmitting and receiving), fewer of these bands exist. Most operators (except mainly Asian ones) prefer the FDD range, of which there are more than 30.
The legacy of TDD and FDD spectrum, the difficulty of freeing up truly global bands, and the advent of 5G with more bands make the duplex problem even more complex. Promising methods under investigation include new filter-based designs and the ability to eliminate self-interference.
The latter also brings with it the somewhat promising possibility of “fragmentless” duplex (or “in-band full duplex”). In the future of 5G mobile communications, we may have to consider not only FDD and TDD, but also flexible duplex based on these new technologies.
Researchers at Aalborg University in Denmark have developed a “Smart Antenna Front End” (SAFE)2-3 architecture that uses (see illustration on page 18) separate antennas for transmission and reception and combines these antennas with (low performance) in combination with customizable filtering to achieve the desired transmission and reception isolation.
While the performance is impressive, the need for two antennas is a big drawback. As phones get thinner and sleeker, the space available for antennas is getting smaller and smaller.
Mobile devices also require multiple antennas for spatial multiplexing (MIMO). Mobile phones with SAFE architecture and 2×2 MIMO support require only four antennas. In addition, the tuning range of these filters and antennas is limited.
So global mobile phones will also need to replicate this interface architecture to cover all LTE frequency bands (450 MHz to 3600 MHz), which will require more antennas, more antenna tuners and more filters, which brings us back to the frequently asked questions questions about multi-band operation due to duplication of components.
Although more antennas can be installed in a tablet or laptop, further advances in customization and/or miniaturization are needed to make this technology suitable for smartphones.
Electrically balanced duplex has been used since the early days of wireline telephony17. In a telephone system, the microphone and earpiece must be connected to the telephone line, but isolated from each other so that the user’s own voice does not deafen the weaker incoming audio signal. This was achieved using hybrid transformers before the advent of electronic phones.
The duplex circuit shown in the figure below uses a resistor of the same value to match the impedance of the transmission line so that the current from the microphone splits as it enters the transformer and flows in opposite directions through the primary coil. The magnetic fluxes are effectively canceled out and no current is induced in the secondary coil, so the secondary coil is isolated from the microphone.
However, the signal from the microphone still goes to the phone line (albeit with some loss), and the incoming signal on the phone line still goes to the speaker (also with some loss), allowing two-way communication on the same phone line. . Metal wire.
A radio balanced duplexer is similar to a telephone duplexer, but instead of a microphone, handset, and telephone wire, a transmitter, receiver, and antenna are used, respectively, as shown in Figure B.
A third way to isolate the transmitter from the receiver is to eliminate self-interference (SI), thereby subtracting the transmitted signal from the received signal. Jamming techniques have been used in radar and broadcasting for decades.
For example, in the early 1980s, Plessy developed and marketed an SI compensation-based product called “Groundsat” to extend the range of half-duplex analog FM military communications networks4-5.
The system acts as a full-duplex single-channel repeater, extending the effective range of half-duplex radios used throughout the work area.
There has been recent interest in self-interference suppression, mainly due to the trend towards short-range communications (cellular and Wi-Fi), which makes the problem of SI suppression more manageable due to lower transmit power and higher power reception for consumer use. Wireless Access and Backhaul Applications 6-8.
Apple’s iPhone (with help from Qualcomm) arguably has the world’s best wireless and LTE capabilities, supporting 16 LTE bands on a single chip. This means that only two SKUs need to be produced to cover the GSM and CDMA markets.
In duplex applications without interference sharing, self-interference suppression can improve spectrum efficiency by allowing the uplink and downlink to share the same spectrum resources9,10. Self-interference suppression techniques can also be used to create custom duplexers for FDD.
The cancellation itself usually consists of several stages. The directional network between the antenna and the transceiver provides the first level of separation between the transmitted and received signals. Secondly, additional analog and digital signal processing is used to eliminate any remaining intrinsic noise in the received signal. The first stage may use a separate antenna (as in SAFE), a hybrid transformer (described below);
The problem of detached antennas has already been described. Circulators are typically narrowband because they use ferromagnetic resonance in the crystal. This hybrid technology, or Electrically Balanced Isolation (EBI), is a promising technology that can be broadband and potentially integrated on a chip.
As shown in the figure below, the smart antenna front end design uses two narrowband tunable antennas, one for transmit and one for receive, and a pair of lower-performance but tunable duplex filters. Individual antennas not only provide some passive isolation at the cost of propagation loss between them, but also have limited (but tunable) instantaneous bandwidth.
The transmitting antenna operates effectively only in the transmit frequency band, and the receiving antenna operates effectively only in the receive frequency band. In this case, the antenna itself also acts as a filter: out-of-band Tx emissions are attenuated by the transmitting antenna, and self-interference in the Tx band is attenuated by the receiving antenna.
Therefore, the architecture requires the antenna to be tunable, which is achieved by using an antenna tuning network. There is some unavoidable insertion loss in an antenna tuning network. However, recent advances in MEMS18 tunable capacitors have significantly improved the quality of these devices, thereby reducing losses. The Rx insertion loss is approximately 3 dB, which is comparable to the total losses of the SAW duplexer and switch.
The antenna-based isolation is then complemented by a tunable filter, also based on MEM3 tunable capacitors, to achieve 25 dB isolation from the antenna and 25 dB isolation from the filter. Prototypes have demonstrated that this can be achieved.
Several research groups in academia and industry are exploring the use of hybrids for duplex printing11–16. These schemes passively eliminate SI by allowing simultaneous transmission and reception from a single antenna, but isolating the transmitter and receiver. They are broadband in nature and can be implemented on-chip, making them an attractive option for frequency duplexing in mobile devices.
Recent advances have shown that FDD transceivers using EBI can be manufactured from CMOS (Complementary Metal Oxide Semiconductor) with insertion loss, noise figure, receiver linearity, and blocking suppression characteristics suitable for cellular applications11,12,13. However, as numerous examples in the academic and scientific literature demonstrate, there is a fundamental limitation affecting duplex isolation.
The impedance of a radio antenna is not fixed, but varies with operating frequency (due to antenna resonance) and time (due to interaction with a changing environment). This means that the balancing impedance must adapt to track impedance changes, and the decoupling bandwidth is limited due to changes in the frequency domain13 (see Figure 1).
Our work at the University of Bristol is focused on investigating and addressing these performance limitations to demonstrate that the required send/receive isolation and throughput can be achieved in real-world use cases.
To overcome antenna impedance fluctuations (which severely impact isolation), our adaptive algorithm tracks antenna impedance in real time, and testing has shown that performance can be maintained in a variety of dynamic environments, including user-handed interaction and high-speed road and rail travel.
Additionally, to overcome the limited antenna matching in the frequency domain, thereby increasing bandwidth and overall isolation, we combine an electrically balanced duplexer with additional active SI suppression, using a second transmitter to generate a suppression signal to further suppress self-interference. (see Figure 2).
The results from our testbed are encouraging: when combined with EBD, active technology can significantly improve transmit and receive isolation, as shown in Figure 3.
Our final laboratory setup uses low-cost mobile device components (cell phone power amplifiers and antennas), making it representative of mobile phone implementations. Moreover, our measurements show that this type of two-stage self-interference rejection can provide the required duplex isolation in the uplink and downlink frequency bands, even when using low-cost, commercial-grade equipment.
The signal strength a cellular device receives at its maximum range must be 12 orders of magnitude lower than the signal strength it transmits. In Time Division Duplex (TDD), the duplex circuit is simply a switch that connects the antenna to the transmitter or receiver, so the duplexer in TDD is a simple switch. In FDD, the transmitter and receiver operate simultaneously, and the duplexer uses filters to isolate the receiver from the transmitter’s strong signal.
The duplexer in the cellular FDD front end provides >~50 dB isolation in the uplink band to prevent overloading the receiver with Tx signals, and >~50 dB isolation in the downlink band to prevent out-of-band transmission. Reduced receiver sensitivity. In the Rx band, losses in the transmit and receive paths are minimal.
These low-loss, high-isolation requirements, where frequencies are separated by only a few percent, require high-Q filtering, which so far can only be achieved using surface acoustic wave (SAW) or body acoustic wave (BAW) devices.
While the technology continues to evolve, with advances largely due to the large number of devices required, multi-band operation means a separate off-chip duplex filter for each band, as shown in Figure A. All switches and routers also add additional functionality with performance penalties and trade-offs.
Affordable global phones based on current technology are too difficult to manufacture. The resulting radio architecture will be very large, lossy and expensive. Manufacturers have to create multiple product variants for different combinations of bands needed in different regions, making unlimited global LTE roaming difficult. The economies of scale that led to GSM’s dominance are becoming increasingly difficult to achieve.
Increasing demand for high data speed mobile services has led to the deployment of 4G mobile networks across 50 frequency bands, with even more bands to come as 5G is fully defined and widely deployed. Due to the complexity of the RF interface, it is not possible to cover all of this in a single device using current filter-based technologies, so customizable and reconfigurable RF circuits are required.
Ideally, a new approach to solving the duplex problem is needed, perhaps based on tunable filters or self-interference suppression, or some combination of both.
While we don’t yet have a single approach that meets the many demands of cost, size, performance and efficiency, perhaps the pieces of the puzzle will come together and be in your pocket in a few years.
Technologies such as EBD with SI suppression can open up the possibility of using the same frequency in both directions simultaneously, which can significantly improve spectral efficiency.
Post time: Sep-24-2024