I have mentioned in several comments here that an area I think has room for more analog integration is in the RF signal path used in wireless communications. Recently I received a press release from Scintera Networks Inc., regarding a partnership with Freescale Semiconductor Inc. to develop optimized, mixed-signal, integrated circuit solutions for small basestations (a.k.a. small cells). I have discussed small cells, which are important to 4G build-out in high-usage areas, in a post on The Connecting Edge, a sibling community.
Scintera is, "a fabless semiconductor company delivering innovative signal-processing solutions to monitor, measure and modify RF signals," according to its website. Freescale makes a range of ICs from microcontrollers to analog and RF components. The press release from Scintera said that as part of the agreement, Freescale would design power amplifiers and evaluation boards for small cell designs using Scintera's RFPAL product line of analog pre-distortion ICs.
I talked with Mendy Ouzillou, vice president of marketing for Scintera, to understand more about how Scintera's products help in small cell design.
In nearly any wireless basestation design, the PA (power amplifier) and related circuitry generate unwanted out-of-band power (intermodulation [IM] products), distorting the output of the PA (i.e., the behavior of the PA is non-linear). Without any mitigation approach, the PA must be operated at low enough power level to remain in a linear performance region (this is known as "back-off"). Back-off reduces the system efficiency, increasing the total power requirement for a given RF signal.
Because these unwanted frequencies appear at known frequencies (e.g. 3rd order IM, 5th order IM, etc.) it is possible to reduce them by a technique called pre-distortion (PD). Figure 1 shows a highly simplified description of how pre-distortion works.
In the top half ("Uncompensated") the RF signal after the power amplifier (PA) contains unwanted power out of the desired signal band. With PD (bottom half), the signal is modified before it reaches the power amplifier so that the resulting amplified signal has less unwanted out-of-band distortion. Diagrams courtesy of Scintera.
In order to apply PD, the output of the PA must be sampled and used to generate the linearization signal. Mr. Ouzillou explained that the traditional approach has been to sample after the PA but to add the PD into the digital baseband signal. The advantage of so-called digital PD (DPD) is that it is very flexible, in that you can control and fine-tune the PD characteristics in firmware.
According to Ouzillou, however, this flexibility brings with it some important downsides, such as requiring a high level of skill and experience to code DPD systems. He explained further that since DPD introduces of out-of-band PD signals in the baseband, it increases the needed bandwidth of the entire digital-to-RF transmitter section, requiring higher clock rates and using much more power. The added cost, complexity, and power consumption of DPD has presented a barrier to using PD in small cells. "Nobody thought they could do anything but back off for small cells," Ouzillou told me.
Scintera "focused on a different set of parameters -- size, cost, power -- to achieve excellent performance in applications of lower power than macro basestations," he continued. It looked at the problem again, and developed an integrated, analog, mixed-signal solution that shifted portions of the computationally intensive signal processing from the digital domain into the more power-efficient analog/RF domain. Scintera's RFPAL product line is the result. Figure 2 shows an RFPAL system implementation.
Schematic representation of an RF PD solution for a small cellular base station. In the middle of the board is Scintera's IC. Figure adapted from Scintera, with permission.
By successfully integrating PD into the analog section at RF frequencies, Scintera enables lower cost and lower power consumption for small cells, especially in those that would have traditionally used back-off for linearization. In addition, for indoor DAS (distributed antenna systems) enhancement of cellular coverage, the lower power dissipation allows use of plastic enclosures and Power over Ethernet (PoE), thus further lowering cost, Ouzillou said.
The proof of any design comes from fielding units and satisfying customers. According to Mr. Ouzillou, Scintera has almost 500,000 units of its PD products in the field to date, without any field failures. Once again analog saves the day.