The Path towards 100 Gbit/s Wireless Communications

Wireless communication links with capacities above 100 Gbit/s require sub-THz carrier frequencies, pencil beams, usage of novel modulation techniques and novel photonic-electronic devices. We review recent high-speed wireless link demonstrations and discuss steps to make such systems more viable. Wireless transmission is forecast to grow by a 53% annual compound growth for the next five years [1]. Such growth is only possible if fundamental changes in the design of future wireless networks are adopted. A possible path to vastly extend the capacity is to shift the RF carrier to the millimeter waves with frequencies above 100 GHz [2]. As an example, the frequency range between 200 GHz and 300 GHz is characterized by low atmospheric losses where wireless links can be operated even under averse weather conditions [3]. Moving towards higher frequencies will also make it necessary to increase the antenna gain significantly to offset the increased link losses. Thus, narrower beams, or pencil beams will be needed. In return, ultra-fast, continuously tunable true-time delays (TTDs) will be necessary as building blocks in phased array feeder networks to keep such systems flexible and to distribute the information spatially [4]. Additionally, multiple-input multiple-output (MIMO) will ultimately be needed in order to enhance the capacity of base stations. Fortunately, with an increase in frequency, the constraints due to limited space for the number of antennas that goes along with MIMO will relax. This is particularly important on the user side – where space is scarce [2]. MmWave systems also have to overcome distinct hardware constraints [2]. Major constraints come from the high power consumption of mixed signal components, chiefly the analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). In a conventional approach each antenna would be connected to a single high-rate ADC/DAC system. Equipping every antenna would not only lead to an immense surge in cost but also require large computation powers together with a huge power consumption. MmWave communications is thus unlikely to happen unless there would be a leap forward in semiconductor technology. However, such constraints can be overcome by clever hybrid approaches based on photonics and microwave communications [4]. Key microwave-photonic elements include true-time delay elements [5], high-speed photo detectors that directly translate an optical signal into a microwave signal [6] or microwave-to-photonic receivers, see Fig. 1 [7]. In this talk we will review some recent approaches on wireless communication links with data rates up to 100 Gbit/s [6] and we will review some of the photonic-electronic hardware implementations.