Conceptual diagrams of MMW-over-fiber communication systems with (a) a common optical local oscillator (LO) MMW source shared by different base stations and (b) different electrical LO MMW sources installed at each base station.
In this paper, recent progress in millimeter-wave (MMW) photonic gigabit wireless communication is reviewed. This technique is attractive partly because the MMW signal can be easily distributed from central to base stations through the use of a low-loss optical fiber. This radio-over-fiber approach facilitates the transmission of MMW signals. An MMW photonic transmitter, comprised of high-power photodiodes with integrated antennas for MMW signal broadcasting, is needed for signal generation only over the last mile. The development of several different low-noise optical MMW sources and high-power photonic transmitters and photodiodes for optical MMW wireless links is summarized. The performance of photonic wireless links with extremely high data rates (over 10 Gbit per second) developed based on these key components and using different modulation schemes is also reviewed. Finally, some advanced commercially available products and the prospects of a future gigabit wireless communication era are discussed.
* 10 Gbit per second wireless line-of-sight data transmission at a center frequency of 120 GHz has been demonstrated
* the 16-QAM or OFDM modulation formats have been used to achieve wireless data transmission of around 30 Gbit per second at a 60 GHz center frequency
* In our recent work, we demonstrated error-free 20 Gbit per second OOK wireless data transmission in the W-band using an NBUTC photodiode-based photonic transmitter–mixer
Several key front-end components have been developed employing the already mature silicon-based complementary metal–oxide–semiconductor (CMOS) integrated-circuit (IC) technology for wireless communication in the V (50–75 GHz) and W (75–110 GHz) bands or even higher operating frequencies for wireless communication. Recently, a 60 GHz CMOS transceiver module and system comprised of antennas, 60 GHz power amplifiers, local oscillators and baseband ICs has been commercially developed by SiBEAM (USA). In addition, advanced InP-based high electron mobility transistors (InP-HEMTs) and heterojunction bipolar transistors have been used to develop some high-speed ICs and key components that operate at 125 GHz or greater than 300 GHz for over 1 Gbit s–1 wireless communication systems.
However, MMW signals suffer substantial propagation loss in free space. This problem and their inherent straight-line path of propagation affect connections and synchronization between the different parts of the whole communication system. A promising solution to overcome this problem is the radio-over-fiber (RoF) technique , in which the MMW local-oscillator signal and data are both distributed through a low-loss optical fiber and only radiated over the last mile to the end user.
(a) Conceptual schematic diagrams of a broadside patch antenna (top) and an end-fire tapered slot antenna (bottom) for rectangular waveguide excitation (adapted from Ref. 30 © 2004 IEEE). (b,c) Conceptual cross-sectional diagram of a UTC photodiode-based photomixer with integrated patch antenna (b) and tapered slot antenna (c) for WR-10 and WR-08 waveguide excitation, respectively (adapted from Ref. 76 © 2004 IEEE).
Practical applications of photonic MMW wireless linking can be found in the fields of HDTV broadcasting as demonstrated by the NTT and NHK corporations in Japan. An uncompressed high-definition serial digital interface (HD-SDI) signal is desirable during live broadcasting, requiring a data rate of 1.5 Gbit s–1 per channel. However, this is much faster than the rate achievable using state-of-the-art microwave field pick-up units. MPEG or JPEG2000 encoders are therefore adopted in current microwave wireless links to compress the HD-SDI signal, but this can cause time delays and signal distortion during live broadcasting. The use of photonic MMW wireless linking in such applications involves connecting several HDTV cameras, each with a data rate of 1.5 Gbit s–1, to a broadcast van via optical fiber. In the broadcast van, the multi-channel HDTV optical data are converted to an MMW signal centered at around 100 GHz and then radiated to a relay point using a photonic transmitter with a high-directivity antenna.
Another important commercial application of MMW wireless linkage for consumer electronics is in the 60 GHz band (e.g. SiBEAM). Applications include wireless linking for high-definition multimedia interface (HDMI) and high-speed data transfer between cell phones and digital cameras. However, to date, photonic technology has not been essential in order to realize 60 GHz indoor wireless linking. The major challenge for MMW photonic wireless linking is the cost of the technology and strong competition with the all-electronic approach. As the CMOS IC technology matures, it is now possible to install MMW CMOS local-oscillator chips in each base station without requiring a synchronized local oscillator signal. Furthermore, if the MMW output power from the base station is sufficiently strong, the links could be made directly with the base station without using optical fiber. However, MMW (over 60 GHz) has optical behavior, such as straight-line propagation and subject to shading by obstacles in the transmission path. Numerous remote antenna units would therefore be needed at the user end to increase network coverage. This is a particularly important issue for outdoor wireless linking systems. Compared with the all-electronic approach, the most important advantage of the photonic technique is that the use of a fiber backbone to interconnect and synchronize the units minimizes the problems of interference and multi-path effects between these units, providing better immunity to bad weather conditions. Overall, lowering the cost of photonic MMW technology will be one of the dominating factors in making the technology commercially viable for gigabit wireless linking. Recently, some of the key components and systems, which include photonic transmitters for 75–180 GHz, 60 GHz electro-absorption modulators, and compact 60 GHz photonic-wireless linking systems, have been commercialized (e.g. IPHOBAC, www.ist-iphobac.org) to meet the challenges of the coming wireless gigabit era. Figure 10 shows a photograph of a commercial UTC photodiode-based photonic transmitter from NTT (Japan). As can be seen, the delicate MMW waveguide output and optical input package of such a module allows easy integration with an MMW antenna or amplifier for RoF applications.