Designing phase adjustable PA for 5G front-ends

A huge amount of research effort is currently being devoted to developing 5G technology with the aim of roll out by the year 2020. The details of the 5G standard have yet to be defined, but a common vision is that as well as providing much higher data rates this new standard must also allow for extremely low latency (less than one millisecond) and uniform coverage over a wide area. In addition to providing improved performance for existing applications, for example allowing the download of several HD movies in a second, the technology will enable and encourage the development of new markets, technologies and applications.

Although there is still much debate about the precise form that 5G will take, there is a degree of consensus that the standard will frequently require large chunks of contiguous spectrum. This can only be found by utilising much higher frequencies than those used for current cellular systems operating below 3GHz. It is therefore envisaged that, as well as making use of current cellular frequencies, a key component of the new 5G radio interface will be the use of mm-wave frequencies where there is greater spectral availability.

Until recently the use of mm-waves for mobile applications has been viewed as a rather inappropriate choice due to their unfavourable propagation characteristics. However a number of research programmes into the implementation of 5G systems have recently reported the results of extensive mm-wave propagation measurements. These were conducted around metropolitan areas in both the United States [1] and South Korea and have shown that the issues can be addressed and overcome. Such research included the investigation of more sophisticated antenna schemes employing phased arrays of antennas to optimise the transmitted and received beams at both the mobile device and the base-station. The fact that wavelengths are small at mm-wave frequencies allows such arrays to be incorporated into a small mobile form factor. It also allows the implementation of compact base-stations which will facilitate regular deployment around metropolitan areas.

Bands in the range 27 to 29.5GHz are strong candidates for the new 5G radio interface and much of the research undertaken to date has been conducted at around 28GHz [2]. This paper describes the design of a 4-channel transmitter IC with each channel containing a power amplifier (PA) with integral 4bit phase shifter. The IC is designed using a commercially available 0.15?m GaAs pHEMT process and is intended to be housed in a low cost SMT package suitable for volume production.

28GHz 5G transmit RF front end architecture
The architecture of a typical RF Front-End (RFFE) using the 28GHz transmit IC is depicted in figure 1. It shows a 4-element antenna array, with each element being driven by one of the four parallel phase adjustable power amplifiers. It is likely that some degree of filtering would be implemented immediately after each power amplifier for harmonic rejection and suppression of receive band noise and unwanted spurious outputs. A common RF input signal drives each of the 4 channels via an in-phase 4-way splitter at the input of the IC.

The IC itself is a 4-channel device but if a particular architecture was developed that required a higher number of elements in the antenna array, say 16, then multiple ICs could simply be used in parallel.

Figure 2 shows a layout plot of one channel of the transmitter IC; this is a stand-alone test-chip – the transmitter IC itself comprises 4 separate channels with an in-phase splitter at the input. The test chip measures 3.8-mm x 1.84-mm.