Patent Publication Number: US-2019190609-A1

Title: Optical Transceiver for Radio Frequency Communication

Description:
TECHNICAL FIELD 
     This disclosure relates generally to electronic communications and, more specifically, to reducing power consumption and electromagnetic interference in radio frequency communication devices (e.g., millimeter-wave radio frequency communication devices) by using an optical transceiver. 
     BACKGROUND 
     Electronic devices play a crucial role in many aspects of modern society. In many cases, they are our primary computing platform and provide communication, entertainment, social media, email, calendar, and many other services that we use in our daily work and personal lives. Electronic devices are not merely our smartphones, for they are integrated in our cars, our televisions, and even our homes. 
     Electronic devices with wireless capabilities provide increased versatility. Historically, wireless devices were primarily used with analog cellular networks, which are good for voice communications, but are expensive to use. The development of digital networks reduced costs and allowed data communications in addition to voice calling. Higher-frequency digital networks (e.g., 3G and 4G) provided the capability to transmit more data and provide more services (e.g., smartphones that can deliver messaging, voice, video, and support applications such as social media, navigation, and so forth). 
     In today&#39;s interconnected world, the services provided by wireless electronic devices rely on ever-increasing amounts of data to deliver the best experiences. For example, virtual reality, on-demand video, self-driving vehicles, and technology for the internet-of-things transmit and receive vast amounts of data. More data requires faster data-transfer rates (bandwidth). Radio waves in the extremely high frequency (EHF) range have frequencies between approximately 25 gigahertz (GHz) and approximately 300 GHz. Radio waves in this range, also known as millimeter-wave (mmW) frequencies, can support data rates up to 10 gigabits per second, which is much higher than conventional radio frequency (RF) limits (e.g., a 4G cellular radio network is limited to about 1 gigabit per second). 
     Although more bandwidth is made available by using higher-frequency RF signals such as mmW RF signals, the devices that communicate over these frequencies use more power and suffer from increased electromagnetic (EM) interference. Consequently, engineers designing these devices are working to improve power efficiency and EM performance for EHF signals to enable increased bandwidth and more services to be provided to users. 
     SUMMARY 
     An optical transceiver for radio frequency communication is disclosed herein. Example implementations of the disclosed optical transceiver for radio frequency communication can consume less power, emit less electromagnetic radiation, and have a reduced component count, while being used to facilitate higher-bandwidth radio frequency communication. 
     In an example aspect, an apparatus is disclosed. The apparatus includes an antenna array, a radio frequency integrated circuit (RFIC), and a baseband integrated circuit that includes a light-receiving device. The RFIC is coupled to the antenna array and mounted to a printed circuit board (PCB). The RFIC includes a surface that faces the PCB. The RFIC is configured to receive a radio frequency (RF) signal from the antenna array. The apparatus additionally includes a fiber optic cable that has a first end disposed proximate to the RFIC and a second end disposed proximate to the light-receiving device. At least a portion of the fiber optic cable is secured to the PCB. A light-emitting device is disposed on the surface that faces the PCB, and the light-emitting device is configured to emit an optical signal that is modulated responsive to the RF signal. The light-emitting device is further configured to direct the modulated optical signal to the first end of the fiber optic cable so as to cause the modulated optical signal to propagate along the fiber optic cable to be received by the light receiving device. 
     In an example aspect, an apparatus is disclosed. The apparatus includes an antenna array, a printed circuit board (PCB), a fiber optic cable, and a radio frequency integrated circuit (RFIC) that is coupled to the antenna array. The RFIC is mounted to the PCB, and the RFIC includes a surface that faces the PCB. At least a portion of the fiber optic cable is secured to the PCB. The RFIC is configured to receive a radio frequency (RF) signal from the antenna array. The RFIC includes a light-emitting device disposed on the surface that faces the PCB. The light emitting device is configured to emit an optical signal that is modulated responsive to the RF signal and direct the modulated optical signal to the fiber optic cable to propagate along the fiber optic cable. 
     In an example aspect, a method for operating an optical transceiver for radio frequency communication is disclosed. The method includes receiving an RF signal via an antenna array. The method also includes routing the RF signal from the antenna array to a radio frequency integrated circuit (RFIC) that is electrically connected to a light-emitting device. The method additionally includes converting, using the light emitting device, the RF signal to an analog optical signal. The method further includes directing the analog optical signal from the light-emitting device to a fiber optic cable such that the analog optical signal propagates along the fiber optic cable. 
     In an example aspect, an apparatus is disclosed. The apparatus includes a printed circuit board (PCB), an antenna array mounted to the PCB, and a fiber optic cable, with at least a portion of the fiber optic cable secured to the PCB. The apparatus also includes a radio frequency integrated circuit (RFIC) mounted to the PCB and coupled to the antenna array. The RFIC includes a surface that faces the PCB. The RFIC also includes a light-emitting device that is disposed on the surface that faces the PCB. The RFIC is configured to receive a radio frequency (RF) signal from the antenna array and to emit an optical signal that is modulated responsive to the RF signal. The apparatus further includes means for coupling the modulated optical signal between the light-emitting device and the fiber optic cable in a manner that causes the modulated optical signal to propagate along the fiber optic cable. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example environment that includes an electronic device in which an optical transceiver for radio frequency communication can be implemented. 
         FIG. 2  illustrates a conventional mmW radio frequency system as a simplified block diagram. 
         FIG. 3A  illustrates, as a simplified block diagram, an example radio frequency communication system that includes an optical transceiver for radio frequency communication as described herein. 
         FIG. 3B  illustrates an example implementation of an optical transceiver for radio frequency communication. 
         FIG. 4  illustrates additional details of the example implementation of the optical transceiver for radio frequency communication shown in  FIG. 3B . 
         FIG. 5  illustrates an example of circuitry for implementation of an optical transceiver for radio frequency communication. 
         FIG. 6  is a flow diagram illustrating an example process for operating an optical transceiver for radio frequency communication. 
         FIG. 7  is a flow diagram that illustrates additional details of the example process shown in  FIG. 6 . 
         FIG. 8  illustrates an example electronic device that includes one or more integrated circuits in which an optical transceiver for radio frequency communication can be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     Radio frequency (RF) communication is based on using RF signals to transmit data. Electronic devices typically use government-assigned frequency bands of the electromagnetic (EM) spectrum, such as from about 500 KHz up to about 25 gigahertz (GHz), to transmit data and ensure compatibility between devices. These frequencies have a finite capacity to transmit data. As more devices communicate at particular frequencies, and as each device transmits more data, the various frequencies are approaching their maximum capacity. Thus, developing the ability to transmit data over additional ranges of the spectrum at higher frequencies is becoming more important to support our connected and data-driven economy and society. 
     Using higher-frequency ranges of the spectrum, however, has proven difficult because, historically, there were few devices that could transmit or receive mmW signals, which correspond to frequencies of 30-300 GHz. While there are more devices with mmW capability today, these devices are typically more expensive because they consume more power and because the higher frequencies require additional components and countermeasures to address EM interference. As the technology for antennas, transceivers, and integrated circuits that can receive, process, and transmit mmW signals has become more common, addressing the challenges related to cost has become more urgent. 
     With respect to integrated circuits, for example, the cost decreases as the total size of the circuit decreases. Consequently, the cost of integrated circuit chips, along with the electronic devices that use them, has been reduced over time. Unfortunately, the ability to rely on new manufacturing technologies to significantly shrink the size of the building blocks of circuits is becoming more difficult. However, there are alternative approaches to reducing the size and power usage of electronic circuitry. For example, the number of components used to implement a given functionality can be lowered. Example implementations that are described herein enable electronic devices to take advantage of the bandwidth provided by mmW radio frequency communication while reducing the power consumption and component cost usually associated with this technology. 
     Conventional mmW systems include one or more antenna modules that are coupled to a radio frequency integrated circuit (RFIC). The RFIC is connected to a baseband integrated circuit (BBIC) that includes a baseband modem. The RFIC and BBIC are connected via an intermediate frequency (IF) circuit that converts an IF signal to an analog BB signal, after the RFIC has converted the mmW signal to the IF signal. The RFIC is connected to the IF circuit via an IF cable that is typically inefficient with respect to power consumption and can be problematic with respect to stray EM emissions. In other words, existing IF cables usually consume appreciable levels of power while also producing EM radiation that interferes with the operation of other components. Additionally, designing the RFIC to accommodate the IF cable adds additional complexity to the system because multiple signals are generally multiplexed on the same IF cable, which further increases power usage. 
     In contrast, implementations that are described herein enable electronic devices that operate using mmW signals with reduced power consumption, reduced component count, and improved EM radiation emission. In an example approach, an RFIC includes an optical transceiver that converts an RF signal from at least one antenna module into an analog optical signal and transmits the analog optical signal to the BBIC (and the baseband modem) via a fiber optic cable. The optical transceiver may comprise an optical transmitter, an optical receiver, or both. 
     The RFIC includes a light-emitting device, such as a light-emitting diode (LED), that performs the signal conversion from RF signal to optical signal. The LED can be disposed on a side of the RFIC that is connected to a printed circuit board (PCB). Thus, the LED can emit the optical signal into a fiber optic cable secured to (e.g., adhered to or embedded in) the PCB. In an example arrangement, the fiber optic cable is effectively sandwiched between the RFIC and the PCB (e.g., the LED may be integrated into the underside of an RFIC in a flip-chip configuration). 
     Further, the RFIC and the LED are fabricated so that the LED provides the optical signal to the fiber optic cable in a manner that enables propagation of the signal along the fiber optic cable (e.g., within a range of angles of incidence that enable propagation). For example, the LED can be sufficiently close to a prepared, first end of the fiber optic cable to direct light from the LED to the first end of the fiber optic cable across a through-the-air connection (e.g., an air gap). Because the RFIC and the modem are typically manufactured or installed to be relatively close together (e.g., within a few meters of each other), losses resulting from an air gap between the LED and the first end of the fiber optic cable are acceptable. The system can then transmit the analog optical signal to the BBIC via the fiber optic cable. The fiber optic cable has a prepared, second end that is proximate to a light-receiving device of the BBIC, such as an optical diode. The light-receiving device is configured to convert the analog optical signal to an electrical signal. The BBIC can then downconvert from the mmW signal directly to a BB frequency, without using IF circuitry. 
     These implementations reduce the complexity of the circuitry used to convert a mmW signal to a BB signal by lowering the number of components employed for mmW communications, while also reducing power consumption. Using an analog optical signal also obviates the need to include analog-to-digital conversion circuitry at each antenna module, and antenna modules are replicated at multiple locations in a given device for mmW communications. Further, using a fiber optic cable instead of an IF cable saves power and reduces the potential for EM interference. Because of the relatively short distances in typical applications, there is little significant performance degradation. 
       FIG. 1  illustrates an example environment  100  that includes an electronic device  102  in which an optical transceiver for radio frequency communication can be implemented. The electronic device  102  communicates with a base station  104  through a radio frequency (RF) communication link  106  (RF signal  106 ). The RF signal  106  may be, for example, a mmW radio frequency signal. In this example, the electronic device  102  is implemented as a display device. However, the electronic device  102  may be implemented as any suitable computing or other electronic device, such as a modem, cellular base station, broadband router, access point, cellular phone (e.g., a smart phone), gaming device, navigation device, media device (e.g., a television), laptop computer, desktop computer, tablet computer, server, network-attached storage (NAS) device, smart appliance, vehicle, vehicle-based communication system, and/or an Internet-of-Things (IoTs) device. 
     The base station  104  communicates with the electronic device  102  via the mmW signal  106 , which may be implemented at any of a variety of suitable frequencies (e.g., 28 GHz, 39 GHz, 60 GHz, or 90 GHz). Although depicted as a base station tower of a cellular radio network, the base station  104  may represent or be implemented as any of a variety of mmW-capable devices, such as a satellite, cable television head-end, terrestrial television broadcast tower, access point, peer-to-peer device, set-top box, mesh network node, and/or GPS transceiver. Hence, the electronic device  102  may communicate with the base station  104  or another device via a wired connection, a wireless connection, or a combination thereof. 
     The RF signal  106  can include a downlink of data or control information communicated from the base station  104  to the electronic device  102  and an uplink of other data or control information communicated from the electronic device  102  to the base station  104 . The RF signal  106  may be implemented using any suitable communication protocol or standard, such as  5 G, WirelessHD, WirelessHD v1.1, IEEE 802.11ad, automotive radar (short-, medium-, and/or long-range), vehicle-to-infrastructure communication (V2I), and/or vehicle-to-vehicle communication (V2V). 
     The electronic device  102  includes a processor  108  and at least one computer-readable storage medium  110  (CRM  110 ). The processor  108  may include any type of processor, such as an application processor or multi-core processor, that is configured to execute processor-executable instructions (e.g., code) stored by the CRM  110 . The CRM  110  may include any suitable type of data storage media, such as volatile memory (e.g., random access memory (RAM)), non-volatile memory (e.g., Flash memory), optical media, magnetic media (e.g., disk or tape), and so forth. In the context of this disclosure, the CRM  110  is implemented to store instructions  112 , data  114 , and other information of the electronic device  102 , and thus does not include transitory propagating signals or carrier waves. 
     The electronic device  102  may also include input/output ports  116  (I/O ports  116 ) and/or a display  118  (as shown). The I/O ports  116  enable data exchanges or interaction with other devices, networks, or users. The I/O ports  116  may include serial ports (e.g., universal serial bus (USB) ports), parallel ports, audio ports, infrared (IR) ports, and so forth. The display  118  presents graphics of the electronic device  102 , such as a user interface associated with an operating system, program, or application. Alternatively or additionally, the display  118  may be implemented as a display port or virtual interface through which graphical content of the electronic device  102  is communicated or presented. 
     The electronic device  102  also includes at least one antenna array  120 , at least one radio frequency integrated circuit (RFIC)  122 , and at least one baseband integrated circuit (BBIC)  124 . The antenna array  120  can include multiple antenna elements per array, which may be arranged in subarrays. The antenna array  120  is communicatively coupled with the RFIC  122  and may be implemented as any of a variety of suitable active or passive antenna arrays that are capable of transmitting and receiving RF signals (including mmW signals), such as planar arrays, linear arrays, patch arrays, micro-strip arrays, and/or hybrid arrays. The antenna array  120  provides connectivity to respective networks and other electronic devices connected to those networks. 
     The RFIC  122  and the BBIC  124  are communicatively coupled one to another at least via a fiber optic cable  126 . The fiber optic cable  126  includes at least one optical fiber, which can be either multi-mode or single-mode. The optical fiber may be made from a variety of materials, such as glass, plastic, or plastic-clad silica (PCS), and may have different cross sectional shapes, such as circular, square, rectangular, and so forth. 
     The RFIC  122  includes at least one light-emitting device  128  and, for respective antenna elements of the antenna array  120 , a low-noise amplifier (LNA)  130 , a phase shifter  132 , and an input/output power amplifier  134  (I/O amplifier  134 ). Additional details of the LNA  130 , the phase shifter  132 , and the I/O amplifier  134  are described with reference to  FIG. 5 . In operation, the RFIC  122  receives the RF signal  106  from the various antenna elements of the antenna array  120  and converts at least a portion of the RF signal  106  into an analog optical signal. Additional details of the operations of the RFIC  122  are described herein. 
     The light-emitting device  128  may include any of a variety of devices, such as a light-emitting diode (LED), a laser diode, and/or an organic light-emitting diode (OLED). In implementations in which the light-emitting device  128  comprises an LED or an OLED, the LED or OLED may have a dynamic range between approximately 20 dB and approximately 50 dB, such as between approximately 30 dB and approximately 40 dB. 
     The BBIC  124  includes at least one light-receiving device  136  and at least one baseband modem  138 . The light-receiving device  136  may include any one or more of a variety of photodetectors that can receive an optical signal and convert the optical signal to an electrical signal (e.g., a photodiode, an LED, or a phototransistor). The baseband modem  138  may be implemented as a system on-chip (SoC) that provides a digital communication interface for data, voice, messaging, and other applications of the electronic device  102 . The baseband modem  138  may also include baseband circuitry to perform high-rate sampling processes that can include analog-to-digital conversion (ADC), digital-to-analog conversion (DAC), gain correction, skew correction, frequency translation, and so forth. 
     In some cases, components of the electronic device  102  are implemented on a single substrate, such as a printed circuit board (PCB) that can be formed from a rigid or flexible material to support components, circuits, and so forth. In other cases, various components may be implemented on separate substrates that are electronically coupled to one another. Example operations of, and interactions between, the RFIC  122  and the BBIC  124  are described with reference to  FIGS. 3-7 . 
       FIG. 2  illustrates a conventional mmW radio frequency system  200  as a simplified block diagram. The conventional system includes an antenna array  202 , a conventional radio frequency integrated circuit (conventional RFIC)  204 , an intermediate frequency integrated circuit (IFIC)  206 , and a conventional baseband integrated circuit (conventional BBIC)  208 . The conventional RFIC  204  and the IFIC  206  are connected via an intermediate frequency (IF) cable  210 . The conventional RFIC  204  includes low-noise amplifiers, phase shifters, and mixers. The conventional RFIC  204 , however, also includes a downconverter circuit  212  that downconverts the mmW signal to an IFIC frequency (e.g., from 28 GHz to 7 GHz or from 39 GHz to 10 GHz) for transmission of an IF signal over the IF cable  210  to the IFIC  206 . Although not depicted in  FIG. 2 , the IFIC  206  includes a buffer circuit to amplify the IF signal and a downconverter circuit to convert the IF signal to a BB signal. 
     In contrast with systems that employ one or more IF-related components, an example RF communication system that includes an optical transceiver for RF communication as described herein is depicted in  FIG. 3A . By using a system such as that illustrated in  FIG. 3A , the downconverter circuit  212 , the IFIC  206 , and the IF cable  210  (as shown inside the ellipse  214  of  FIG. 2 ) may be replaced or omitted. For example these components can be replaced with the light-emitting device  128  and the fiber optic cable  126  as described below with reference to  FIG. 3A . These replacements reduce the number of components, along with their associated costs, and cut the risk of EM interference created by employing an IF cable. The risk of EM interference can be reduced without significant performance degradation, especially for implementations in which the fiber optic cable  126  is below some threshold length (e.g., less than about 15 meters, such as less than 10 meters, long). 
       FIG. 3A  illustrates, as a simplified block diagram, an example RF communication system  300 -A that includes an optical transceiver for radio frequency communication, as described herein. From left to right, the system includes the antenna array  120 , the RFIC  122  (including the light-emitting device  128 ), the fiber optic cable  126 , and the BBIC  124 . Although not shown in  FIG. 3A , the RFIC  122  can also include, for respective antenna elements of the antenna array  120 , the LNA  130 , the phase shifter  132 , and the I/O amplifier  134 , as described with reference to  FIG. 1 . 
     By converting a mmW RF signal to an analog optical signal and propagating the analog optical signal via the light-emitting device  128  and the fiber optic cable  126 , the electronic device  102  can employ mmW radio frequency communication while reducing cost and power consumption. Further, using an analog optical signal and a fiber optic cable reduces EM interference often associated with conventional implementations of mmW communication systems that include intermediate frequency components and cables. 
       FIG. 3B  illustrates an example implementation  300 -B of an optical transceiver for radio frequency communication. The example implementation  300 -B includes the antenna array  120 , the RFIC  122 , the BBIC  124 , and the fiber optic cable  126 . In the example implementation  300 -B, these components are secured to, such as by being mounted on, a printed circuit board (PCB)  302 . The antenna array  120  is coupled to the RFIC  122  so that the RFIC  122  can receive from the antenna array  120  an RF signal  106 , such as a mmW RF signal (e.g., an RF signal having a frequency between approximately 20 GHz and approximately 90 GHz). 
     The RFIC  122  includes the light-emitting device  128 , which is disposed on a surface of the RFIC  122 , with the surface being on a side that faces the PCB  302 . The light-emitting device  128  may be mounted on the surface of the RFIC  122  or integrated with the RFIC  122  during manufacturing of the RFIC  122 . The light-emitting device  128  emits a modulated optical signal  304  that is modulated responsive to the RF signal  106 . The modulated optical signal  304  can be an analog optical signal (e.g., the intensity of the modulated optical signal  304  varies directly with the frequency of the signal from the mmW antenna modules). As described above, the RF signal  106  is associated with a radio frequency and may be, for example, a mmW signal. 
     In some implementations, the RFIC  122  can control the light-emitting device  128  to modulate the modulated analog optical signal  304  at the radio frequency. For example, the RFIC  122  may modulate the modulated analog optical signal  304  at the radio frequency by changing a current flowing through the light-emitting device  128  in response to the RF signal  106 . In other cases, the RFIC  122  may modulate the modulated analog optical signal  304  at the radio frequency by changing an intensity of the modulated analog optical signal  304  emitted by the light-emitting device  128  in response to the RF signal  106 . Additionally or alternatively, other encoding methods may be used, such as orthogonal frequency-division multiplexing (OFDM). In still other implementations, the light-emitting device  128  may emit a modulated digital optical signal. 
     In the example implementation  300 -B, the fiber optic cable  126  propagates the modulated analog optical signal  304 . As shown, a first end  306  of the fiber optic cable  126  is disposed proximate to the light-emitting device  128  of the RFIC  122 , and a second end  308  of the fiber optic cable  126  is disposed proximate to the light-receiving device  136  of the BBIC  124 . The fiber optic cable  126  is positioned between the surface of the RFIC  122  on which the light-emitting device  128  is disposed and a surface of the PCB  302 . The light-emitting device  128  can emit the modulated analog optical signal  304  such that the modulated analog optical signal  304  is directed to the first end  306  of the fiber optic cable  126  at an angle that causes the modulated analog optical signal  304  to propagate along the fiber optic cable  126  and exit from the second end  308  of the fiber optic cable  126  at an angle that causes the modulated analog optical signal  304  to be received by the light-receiving device  136  of the BBIC  124 . 
     In other implementations, the antenna array  120  may include multiple antenna elements that can receive signals at different frequencies. In this case, the RF signal  106  received from the antenna array  120  can include two or more signals with different frequencies (e.g., a 28 GHz signal and a 39 GHz signal). As noted above, the RFIC  122  may include multiple light-emitting devices. For example, the light-emitting device  128  may include two LEDs (e.g., having different band gaps that emit at different wave lengths) that are electrically connected in a parallel configuration. One LED can thus emit an analog optical signal modulated in response to the 28 GHz signal, and the other LED can emit another analog optical signal modulated in response to the 39 GHz signal. The two LEDs are positioned such that the two optical signals are directed to the first end  306  of the fiber optic cable  126  at the RFIC  122  at one or more angles that enable the two modulated optical signals to propagate along the fiber optic cable  126 . 
     Still other implementations may include additional LEDs, (or other types of light-emitting devices) connected in parallel, that can direct modulated analog optical signals  304  of different wavelengths (e.g., red, green, and blue LEDs) along the same or a separate fiber optic cable  126  to enable multiplexing of modulated analog optical signals along the fiber optic cable  126 . 
     The light-emitting device  128 , as shown in  FIG. 3B , directs the modulated analog optical signal  304  to the first end  306  of the fiber optic cable  126  across an air gap located between the light-emitting device  128  and the first end  306  of the fiber optic cable  126 . Similarly, the modulated analog optical signal  304  exits from the second end  308  of the fiber optic cable  126 , travels across a corresponding air gap, and then is incident on the light-receiving device  136 . In some implementations, the light-emitting device  128  can direct the modulated optical signal  304  across the air gap without propagating through a mechanical coupling device (e.g., an optical connector). In other cases, one or both ends of the fiber optic cable  126  may be terminated with a coupling device and/or either or both of the light-emitting device  128  and the light-receiving device  136  may include a mechanical optical coupling device. 
     As shown in  FIG. 3B , the fiber optic cable  126  can be implemented as a discrete cable that is at least partially embedded in the PCB  302  and secured to a surface of the PCB  302  with a cable bracket  310 . In other implementations, the fiber optic cable  126  may comprise a tube, a pipe, or a strip that can propagate an optical signal along a length of the fiber optic cable  126 . For example, the fiber optic cable  126  may include a flat strip (e.g., a ribbon or tape) that is mounted on the surface of the PCB  302  or a rectangular pipe that can be inserted into a groove on the PCB  302 . The ends of the fiber optic cable  126  are disposed at or near the surface of the PCB  302  to provide a coupling interface between the respective ends of the fiber optic cable  126  and the light-emitting device  128  and/or the light-receiving device  136 . Example cross-sectional shapes (circular, elliptical, and rectangular) are shown in detail view 3B-1, and other shapes may also be used. For example, the cross-section of the fiber optic cable  126  may be customized to a shape similar to a shape of an emitting surface of the light-emitting device  128  or a receiving surface of the light-receiving device  136 , which may increase the efficiency of the optical coupling. 
     The antenna array  120 , the RFIC  122 , the BBIC  124 , and the fiber optic cable  126  may be mounted on a single rigid or flexible PCB, as shown in  FIG. 3B . In other implementations, however, the antenna array  120 , the RFIC  122  or the BBIC  124  may be mounted on one or more separate substrates (e.g., separate rigid or flexible PCBs, such as PCBs that are formed from a glass epoxy or a flexible circuit material) that are communicatively coupled. The RFIC  122  and the BBIC  124  can be attached to the PCB  302  using a variety of technologies. For example, the RFIC  122  or the BBIC  124  may be implemented as respective flip-chip components that are connected to the PCB  302  via electrical connections created between one or more solder bumps  312  and corresponding bond pads  314 . 
     Consider  FIG. 4 , which illustrates generally at  400  additional details of the example implementation of the optical transceiver for radio frequency communication shown in  FIG. 3B . In  FIG. 4 , a surface of the RFIC  122  includes the light-emitting device  128  and multiple solder bumps  312 . The PCB  302  includes multiple bond pads  314 . During the assembly process, as shown by the arrow  402 , the RFIC  122  is flipped on to the PCB  302 . When the RFIC  122  is flipped, the light-emitting device  128  aligns over an opening  406  that exposes the fiber optic cable  126  (e.g., at the first end  306 ), and the solder bumps  312  contact the corresponding bond pads  314 . The solder bumps  312  are melted to form respective electrical connections with the bond pads  314  and thereby form electrical connections between the various components mounted to the PCB  302 . 
     Conductive traces  404  can be used to make additional connections between and among components (e.g., between the antenna array  120  and the RFIC  122  (including the light-emitting device  128 )). For clarity,  FIG. 4  does not show the traces  404  as making every individual connection between the elements of the antenna array  120  and the bond pads  314 . The fiber optic cable  126  is shown, as described above with reference to  FIG. 3B , extending between the light-emitting device  128  and the light-receiving device  136 . A portion of the fiber optic cable  126  that is embedded in the PCB  302  under the RFIC  122  and under the BBIC  124  (e.g., as in  FIG. 3B ) is shown in  FIG. 4  as a dashed line. Another portion of the fiber optic cable  126  is shown in  FIG. 4  on the surface of the PCB  302 , secured by the cable bracket  310 . 
     Alternatively or additionally, the RFIC  122  and/or the BBIC  124  may be manufactured using other technologies, such as wire bonding. The RFIC  122  and/or the BBIC  124  may also be made in other formats, such as a quad-flat package (QFP), a dual inline package (DIP), or a dual in-line pin package (DIPP). 
       FIG. 5  illustrates generally at  500  an example of circuitry for implementation of an optical transceiver for radio frequency communication. Specifically, examples of circuitry for the RFIC  122  and the BBIC  124  are shown. As illustrated, RFIC  122  includes, for each respective antenna element of the antenna array  120 , an LNA  130 , a phase shifter  132 , and an I/O amplifier  134 . The RF signal (e.g., a mmW signal at 28 GHz or 39 GHz) is received through the antenna array  120 . The respective antenna elements of the antenna array  120  are coupled to the LNA  130 , which amplifies the received RF signal. The amplified RF signal is routed through the phase shifter  132 . The shifted signals from the antenna elements are combined, and the combined RF signal  502  is amplified by the I/O amplifier  134  and provided to the light-emitting device  128 . The combined RF signal  502  drives the light-emitting device  128  to produce an optical signal, such as the modulated analog optical signal  304 . 
     The modulated analog optical signal  304  propagates along the fiber optic cable  126  and is received at the light-receiving device  136 , as described herein. The light-receiving device  136  converts the modulated analog optical signal  304  to an electrical signal  504 . An amplification circuit  506  amplifies the electrical signal  504 . A downconverting circuit  508  receives the amplified electrical signal and modulates the amplified electrical signal to a baseband frequency. The downconverting circuit  508  provides the modulated baseband signal to the baseband modem  138 . 
     As shown in  FIG. 5 , the example circuitry for the RFIC  122  and the example circuitry for the BBIC  124  are designed to allow the components to operate in either or both of a transmit mode and a receive mode to implement the optical transceiver for radio frequency communication. For example, as shown at detail view  510 , the BBIC  124  may also include another light-emitting device (e.g., analogous to the light-emitting device  128 ) and other components that the BBIC  124  can use to receive an electrical signal from the baseband modem  138  and convert the electrical signal to an optical signal that can be propagated along another fiber optic cable (with other first and second ends) to the RFIC  122 . Similarly, as shown in the detail view  510 , the RFIC  122  may also include another light-receiving device (e.g., like the light-receiving device  136 ) and other components, such as amplifiers and filters, that the RFIC  122  can use to receive another optical signal sent from the BBIC  124  and convert the other optical signal to a mmW RF signal that can be transmitted by the antenna array  120 . In some implementations, the light-emitting devices and the light-receiving devices may work jointly as a system that can convert an electrical signal to an optical signal and convert an optical signal to an electrical signal. In these implementations, other components may also be included, such as silicon-based couplers (e.g., integrated with the RFIC  122  and/or the BBIC  124 ). 
     In still other implementations, a bidirectional fiber optic cable may be used to propagate signals from the RFIC  122  to the BBIC  124  and from the BBIC  124  to the RFIC  122 . In some cases of this implementation, both the RFIC  122  and the BBIC  124  may include a light-emitting device and a light-receiving device (e.g., the light-emitting device  128  and the light-receiving device  136 ). In other cases, both the RFIC  122  and the BBIC  124  may include a device or system that can perform the functions of the light-emitting device  128  and the light-receiving device  136  (e.g., an LED configured as a photodiode or a combination of photodiodes and LEDs integrated with the RFIC  122  and the BBIC  124 ). 
       FIG. 6  is a flow diagram illustrating an example process  600  for operating an optical transceiver for radio frequency communication. The process  600  is described in the form of a set of blocks  602 - 608  that specify operations that can be performed. However, operations are not necessarily limited to the order shown in  FIG. 6  or described herein, for the operations may be implemented in alternative orders or in fully or partially overlapping manners. Operations represented by the illustrated blocks of the process  600  may be performed by a radio frequency integrated circuit that includes a light-emitting device, such as the RFIC  122  described herein. More specifically, the operations of the process  600  may be performed by the components illustrated in  FIG. 3-5 , including the antenna array  120 , which can send and receive mmW RF signals. The operations below are described in terms of handling a mmW RF signal that is received by the antenna array  120 , but analogous operations may also be implemented in the reverse to handle a mmW RF signal that is transmitted by the antenna array  120 . 
     At block  602 , an antenna array receives an RF signal. For example, an antenna array  120  can receive one or more mmW signals (e.g., an RF signal with a frequency of 28 GHz or 60 GHz). 
     At block  604 , the RF signal is routed to a radio frequency integrated circuit (RFIC) that includes a light-emitting device. For example, the RF signal may be routed to the RFIC  122 , which includes the light-emitting device  128 . As noted, the light-emitting device  128  is electrically connected to the RFIC  122  (e.g., integrated with the RFIC  122  or secured to the RFIC  122  in an electrically conductive manner). 
     At block  606 , the RF signal is converted to an analog optical signal using the light-emitting device. For example, the light-emitting device  128  can emit an analog optical signal, such as the modulated analog optical signal  304 . As noted, the modulated analog optical signal  304  can be modulated responsive to the RF signal and may be an analog optical signal (e.g., the intensity of the modulated optical signal  304  varies directly with the frequency of the signal from the mmW antenna modules). 
     In some cases, the RFIC  122  may be used to control the light-emitting device  128  to modulate the modulated analog optical signal  304  at the radio frequency of the RF signal. For example, the RFIC  122  may modulate the modulated analog optical signal  304  at the radio frequency by changing a current flowing through the light-emitting device  128  in response to the RF signal. In other cases, the RFIC  122  may modulate the analog optical signal  304  at the radio frequency by changing an intensity of the analog optical signal  304  emitted by the light-emitting device  128  in response to the RF signal. 
     At block  608 , the modulated analog optical signal is directed from the light-emitting device to a fiber optic cable such that the analog optical signal propagates along the fiber optic cable. For example, a first end  306  of the fiber optic cable  126  can be disposed proximate to the light-emitting device  128  of the RFIC  122 . The light-emitting device  128  emits the modulated analog optical signal  304  such that the modulated analog optical signal  304  is directed to the first end  306  of the fiber optic cable  126  at one or more angles (e.g., angles of incidence) that cause the modulated analog optical signal  304  to propagate along the fiber optic cable  126 . 
     The modulated analog optical signal  304  may emit from the light-emitting device  128 , propagate across an air gap, and may be incident on the fiber optic cable  126 . The angles of incidence may be determined in a variety of manners. For example, the angles of incidence may be determined by one or more of the geometry of the light-emitting device  128 , the geometry of the first end  306  of the fiber optic cable  126 , the position or orientation of the light-emitting device  128 , and the position of the first end  306  of the fiber optic cable  126 , or the RFIC  122  may include a controller and/or additional hardware that can be used to control the angles of incidence (e.g., a controller may control a focal point of a lens that is disposed between the light-emitting device  128  and the fiber optic cable  126 ). In some cases, the light-emitting device  128  can direct the modulated analog optical signal  304  across the air gap without propagating through a mechanical coupling device (e.g., without an optical connector) connected to either or both of the light-emitting device  128  or the first end  306  of the fiber optic cable  126 . In other cases, the fiber optic cable  126  and/or the light-emitting device  128  may include an optical connector that enables the propagation with or without an air gap. 
       FIG. 7  is a flow diagram that illustrates additional details  700  of the example process  600  shown in  FIG. 6 . At block  702 , the analog optical signal is propagated along the fiber optic cable. For example, the modulated analog optical signal  304  may be propagated along the fiber optic cable  126 . 
     At block  704 , the analog optical signal is received at a baseband integrated circuit via the fiber optic cable. For example, the second end  308  of the fiber optic cable  126  can be disposed proximate to the light-receiving device  136  of the BBIC  124 . In this example, the fiber optic cable  126  is positioned to cause the modulated analog optical signal  304  to exit from the second end  308  of the fiber optic cable  126  at one or more angles (e.g., angles of incidence) that cause the light-receiving device  136  of the BBIC  124  to receive the modulated analog optical signal  304 . Similar to the process described with reference to block  608  of  FIG. 6 , the modulated optical signal  304  may exit from the second end  308  of the fiber optic cable  126  to the light-receiving device  136  across an air gap without propagating through a mechanical coupling device (e.g., without an optical connector) connected to either or both of the light-receiving device  136  or the fiber optic cable  126 . In other cases, the fiber optic cable  126  and/or the light-receiving device  136  may include an optical connector that enables the propagation with or without an air gap. 
     At block  706  the analog optical signal is converted to a baseband signal via a light-receiving device that is electrically connected to the baseband integrated circuit. Continuing the example above, the modulated analog optical signal  304  may be received by one or more of the light-receiving devices  136  of the BBIC  124  (e.g., a photodiode, an LED, or a phototransistor) that can convert the modulated analog optical signal  304  to an electrical signal, which is then converted to a baseband signal as described herein. 
       FIG. 8  illustrates generally at  800  an example electronic device  802  that includes one or more integrated circuits in which an optical transceiver for radio frequency communication can be implemented. As shown, the electronic device  802  includes an antenna array  804 , a radio frequency (RF) transceiver  806 , a user input/output (I/O) interface  808 , and an integrated circuit (IC)  810  having multiple cores. Illustrated examples of the IC  810 , or cores thereof, include a microprocessor  812 , a graphics processing unit (GPU)  814 , a memory array  816 , and a modem  818 . In one or more example implementations, an optical transceiver  820  for radio frequency communication as described herein (including, for example, the RFIC  122 , the BBIC  124 , and/or the fiber optic cable  126 ) can be implemented in the electronic device  802 , such as with the RF transceiver  806  and/or the IC  810 . Thus, the electronic device  802  can communicate using mmW RF signals with fewer components and reduced risk of EM interference as compared to conventional approaches. 
     The electronic device  802  can be a mobile or battery-powered device or a fixed device that is designed to be powered by an electrical grid. Examples of the electronic device  802  include a server computer, a network switch or router, a blade of a data center, a personal computer, a desktop computer, a notebook or laptop computer, a tablet computer, a smart phone, an entertainment appliance, a display device such as a television or monitor, or a wearable computing device such as a smartwatch, intelligent glasses, or an article of clothing. An electronic device  802  can also be a device, or a portion thereof, having embedded electronics. Examples of the electronic device  802  with embedded electronics include a passenger vehicle, industrial equipment, a refrigerator or other home appliance, a drone or other unmanned aerial vehicle (UAV), or a power tool. 
     For an electronic device with a wireless capability, the electronic device  802  includes an antenna array  804  that is coupled to the RF transceiver  806  to enable reception or transmission of one or more wireless signals. The IC  810  may be coupled to the RF transceiver  806  to enable the IC  810  to have access to received wireless signals or to provide wireless signals for transmission via the antenna array  804 . The electronic device  802  as shown also includes at least one user I/O interface  808 . Examples of the user I/O interface  808  include a keyboard, a mouse, a microphone, a touch-sensitive screen, a camera, an accelerometer, a haptic mechanism, a speaker, a display screen, or a projector. The RF transceiver  806  can correspond to, for example, the RFIC  122  and the BBIC  124  (e.g., of  FIGS. 1 and 3A-6 ) that implement an optical transceiver for radio frequency communication. 
     The IC  810  may comprise, for example, one or more instances of a microprocessor  812 , a GPU  814 , a memory array  816 , a modem  818 , and so forth. Alternatively or additionally, the IC  810  can correspond to, for example, the RFIC  122  or the BBIC  124  (e.g., of  FIGS. 1 and 3A-6 ) that implement an optical transceiver for radio frequency communication. 
     The microprocessor  812  may function as a central processing unit (CPU) or other general-purpose processor. Some microprocessors include different parts, such as multiple processing cores, that may be individually powered on or off. The GPU  814  may be especially adapted to process visual-related data for display, such as video data images. If visual-related data is not being rendered or otherwise processed, the GPU  814  may be fully or partially powered down. The memory array  816  stores data for the microprocessor  812  or the GPU  814 . Example types of memory for the memory array  816  include random access memory (RAM), such as dynamic RAM (DRAM) or static RAM (SRAM); flash memory; and so forth. If programs are not accessing data stored in memory, the memory array  816  may be powered down overall or block-by-block. The modem  818  demodulates a signal to extract encoded information or modulates a signal to encode information into the signal. If there is no information to decode from an inbound communication or to encode for an outbound communication, the modem  818  may be idled to reduce power consumption. The IC  810  may include additional or alternative parts than those that are shown, such as an I/O interface, a sensor such as an accelerometer, a transceiver or another part of a receiver chain, a customized or hard-coded processor such as an application-specific integrated circuit (ASIC), and so forth. 
     The IC  810  may also comprise a system on a chip (SOC). An SOC may integrate a sufficient number of different types of components to enable the SOC to provide computational functionality as a notebook computer, a mobile phone, or another electronic apparatus using one chip, at least primarily. Components of an SOC, or an IC  810  generally, may be termed cores or circuit blocks. Examples of cores or circuit blocks include, in addition to those that are illustrated in  FIG. 8 , a voltage regulator, a main memory or cache memory block, a memory controller, a general-purpose processor, a cryptographic processor, a video or image processor, a vector processor, a radio, an interface or communications subsystem, a wireless controller, or a display controller. Any of these cores or circuit blocks, such as a central processing unit or a multimedia processor, may further include multiple internal cores or circuit blocks. 
     Unless context dictates otherwise, use herein of the word “or” may be considered use of an “inclusive or,” or a term that permits inclusion or application of one or more items that are linked by the word “or” (e.g., a phrase “A or B” may be interpreted as permitting just “A,” as permitting just “B,” or as permitting both “A” and “B”). Further, items represented in the accompanying figures and terms discussed herein may be indicative of one or more items or terms, and thus reference may be made interchangeably to single or plural forms of the items and terms in this written description. Finally, although subject matter has been described in language specific to structural features or methodological operations, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or operations described above, including not necessarily being limited to the organizations in which features are arranged or the orders in which operations are performed.