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RF Integration for Small Basestations

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.

Figure 1

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 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.

Figure 2

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.

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.

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16 comments on “RF Integration for Small Basestations

  1. eafpres
    June 20, 2013

    Getting power requirements down to where POE (Power over Ethernet) can power a cellular small cell inside a building makes deploying cellular in-building much easer. It should be interesting to see how this impacts the in-building market.

  2. Davidled
    June 20, 2013

    I am wondering what type application this technology is used in the field and where small basestations is. As my understanding, the module with Scintera's IC chip might be connected wirelessly to regular base station by searching the best signal strength around several basestations.  It sound like “excellent performance” provides robust connectivity in any time and any place.

  3. Brad Albing
    June 20, 2013

    Quite so – once the power requirements are down that low, designers have a pretty broad range of power supply ICs to choose from to make their supplies. But the devil is in the details….

  4. eafpres
    June 21, 2013

    @DaeJ–the application is where there are a large number of customers trying to access the network in a small space.  Then, there are not enough channels on the macro cell.  Instead, they deploy many of these small cells and set them up that they can reuse the same channels, becuase the range is limited.  The small cells are linked back to an aggregation point using ethernet or fiber.

    The technology described allows the small cell to operate at higher output power with less input power.  Therefore, there is less heat to deal with, and the potential to power over ethernet (PoE) means the same ethernet cable that links the small cell back to an aggregation node can supply the power, making the installation so much easier.

    Take a look at my article over on The Connecting Edge (sibling site to this one) about small cells and backhaul.

    Wireless Backhaul & Small Cells Connect Data-Hungry Users

     

  5. Davidled
    June 21, 2013

    What type network architecture are we discussing?  There are many network architectures such as CDMA, EV_DO and LTE.  In other hands, in the high frequency spectrum, small  basestation might be operated in the lower output power comparing with that of low frequency spectrum.  I might be missing something else. It seem like this chip is doing power amplifier. Or I am wondering if this chip can be used in other network architecture beside LTE.

  6. eafpres
    June 21, 2013

    Hi DaeJ–this is mainly LTE or so-called 4G.  The issue is that LTE and 4G promise “always on” data connectivity so the network demand is much higher than before with mainly voice.  Even at 3G the data rates are low and not as much a problem.  But in LTE, with high number of users, since everyone expects “always on” or “instant connection” then this poses a problem for the carriers.

    Regarding this particular chip, it can be used in any basestation design for analog pre-distortion in order to keep the power amp in the most efficient operating region, thereby reducing power requirements.  However, this design is really optimized for small cell, lower power applications.  The company Scintera has announced a partnership with Freescale where Freescale will design new PAs optimized to use this analog pre-distortion.  These will mainly be targeted at small cell designs.

  7. amrutah
    June 23, 2013

    I believe, the AISG standards are used for the base station and the Antenna.  And as per standard the PA should not deliver any power outside of its bands. So is the receiver not allowing anything outside its band.

       Is there any other standard used for defining the BS and the Antenna for small cells

  8. Netcrawl
    June 23, 2013

    @easpres you're right we're going to see a massive shift for mobile communications bast stations, with new technologies driving growth in the number of trnasceivers and base stations annually, and its going to happen within the next few years.

     

  9. eafpres
    June 23, 2013

    Hi amrutah–as I understand, AISG is really more about the antennas, and has a lot to do with beam shaping, side lobes, electrical downtilt, etc.  

    The issue is that the significant intermodulation products may still be within the band defined for the standard being used (say, LTE in the US) but could cause problems with channel to channel interference.  Intermodulation noise is a fact for a real system, the issue is what levels can the overall protocol tolerate and if the levels are higher, then some way must be found to reduce them.  Two possible approaches are to back off in power to get into a more linear range of the PA, but that hurts overall performance, or find a way to remove the IM products that are created.  The analog chip described here is applying the known technique of predistotion to reduce IM products.  What is innovative is that it is all in the RF band, not in the digital baseband.

     

  10. eafpres
    June 23, 2013

    Hi Netcrawl–well, the market forecasts are saying this too,  However, the industry has talked about picocells and femto cells, even cells in homes, for many years.  Perhaps 4G/LTE deployment will finally be the real force to drive this architecture change to the network.

  11. Davidled
    June 23, 2013

    In the parallel, 5G is being progressively developed from RF engineer and researcher. 5G is more faster than other network. But in the market, still 4G is not fully installed some area. Near future,  I guess that 5G/LTE will be deployed in either AT&T or Version.

  12. Netcrawl
    June 23, 2013

    @Daej the demand for high capacity data continues to increase every year at siginificant rates. Its an ongoing arm race, engineers have pushed into higher bandwidths, first with voice then the multimedia, this demand is evident in cellular technology with the rollout of 4G, and now the introduction of 5G, which aimed at the millions of application-rich mobile devices. The move to 5G would probably take about 2-3 years, we need to upgrade our systems and infrastructures. 

  13. Netcrawl
    June 23, 2013

    @Daej you're right, as new LTE networks deploy, many next-generation mobile devices will contain up to 14 frequency bands for quad-band 3G roaming or 4G data, will receive everything bluetooth and GPS, and more to come. But we have some serious challenges here, how do RF engineers squeeze more content into mobile devices without sacrificing perfomance or hitting battery life?

     

  14. Mendy Ouzillou
    June 24, 2013

    The family of RFPAL products can be found in a very wide range of deployed systems including: 20Wrms repeaters, 40Wrms (quad 10Wrms) TD-LTE Remote Radio Heads, picocells, microwave backhaul and others. Also will be in production with small cells applications down to 250mWrms.

  15. Brad Albing
    June 24, 2013

    @eafpres – that was a succinct, accurate explanation. That's the kind we like.

  16. Brad Albing
    June 24, 2013

    Mendy – thanks for jumping in and supplying a bit more info on these devices.

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