Development of 1-bit Digital Radio-Frequency Transmitter

1-1 Development background To cope with the recent rapid increase in communication traffic, various measures are being implemented for communication systems such as distributing traffic pathways using wireless LANs or other systems and increasing transmission capacity by introducing multiple-input/multiple-output (MIMO) technology or array antenna systems. The appearance of smartphones has accelerated the increase in communication traffic, pressing radio communication systems to further increase both capacity and communication speed. Reflecting the situation surrounding radio communication, the introduction of various new technologies has been studied. A typical example of these is carrier aggregation, which intelligently combines multiple radio networks in a cooperative manner. Therefore, future radio equipment is required to be further integrated and downsized while responding flexibly to the progress of various radio systems and their performance. In response to the above technological trend in radio communication, Sumitomo Electric Industries, Ltd. has developed a 1-bit digital radio frequency (RF) signal transmitter comprising a bandpass delta-sigma modulator (BP-DSM(1),(2)). Since the BP-DSM outputs radio signals generated only by a digital circuit instead of an analog circuit, development of the new digital RF transmitter benefits directly from silicon semiconductor nanofabrication. In other words, the size and power consumption of the new digital RF transmitter is expected to decrease according to Moore’s Law. Meanwhile, the common view has been that the advantages of a radio signal generated only by a digital circuit are obtained only when the radio signal has an ideal waveform (square wave). Therefore, with a time wave that changes its form so smoothly as shown in Fig. 1, it is difficult to achieve high wireless performance. The digital radio equipment used in wireless base stations must meet a severe requirement for adjacent channel leakage power ratio (ACLR) in order to avoid the interference with other wireless networks. The technical problem preventing practical use of digital radio equipment is its non-square signal waveform. In developing the new digital RF transmitter, we carried out quantitative analysis of the time waveform(3),(4) shown in Fig. 1 by introducing a waveform separation method(5) in order to clarify the effects of a time waveform on the ACLR performance of the transmitter. As a result, we discovered the conditions for a time waveform that ensures high ACLR performance even if the waveform contains ringing, and achieved ACLR performance close to the ideal level. This achievement encouraged us to develop a 1-bit digital RF transmitter that could ensure high communication quality. 1-2 Features of the new 1-bit digital RF transmitter The newly developed 1-bit digital RF transmitter performs digital signal processing without using any high frequency analog circuit and outputs radio signals directly from a digital device. Therefore, this transmitter eliminates mutual interference between circuits that has been a longstanding problem with high frequency circuits, opening prospects for further integration of the circuits in digital chips. Further, the new digital RF transmitter can be controlled by software to set the carrier frequency of the high frequency circuit to the value required for specific countries where the transmitter is used. This will enable communalization of radio transmitter components and their subsequent mass production, thereby reducing the cost of signal processors. Furthermore, the technology used for the new Development of 1-bit Digital Radio-Frequency Transmitter