Developing Highly Integrated Solutions to Meet the Changing Demand of Multi-Mode Devices
As the number of 3G/4G wireless permutations increases, a multi-mode 2-point polar modulation scheme may point the way forward.
With an increasing number of travelers, both nationally and internationally, the world continues to become a smaller place and the desire for a single universal communication device grows. As the demand grows, so do the requirements – voice is still king, but the use of packet-based services, such as Web browsing, e-mail, file downloads and multi-media applications, is on the rise. These packet-based services require increased data rates which go beyond the capabilities of 2G technologies (GSM and CDMA) and require 3G and 4G standards. However, in order to maintain acceptable voice coverage, the devices must be backward compatible with established 2G networks. These multi-mode devices introduce a new set of challenges to the industry that will continue to increase as consumers demand more functionality.
A few years ago, it appeared that network carriers would continue to follow roadmaps based on their installed network base. For the carriers that had GSM networks, the progression to EDGE, WCDMA and LTE was relatively stable. For CDMA networks, the progression to EVDO Rev. A through to Rev. C also seemed to be well-defined. Then, a number of decisions occurred that really changed the dynamics. The WiMAX consortium developed traction in the cellular space when Sprint decided to use WiMAX as its 4G network, and a WiBro network was rolled out in Korea. Having traditionally followed the CDMA path, the Korean carriers have one of the largest established CDMA user bases; however, some of these carriers have migrated to WCDMA for 3G. Verizon Wireless, a carrier that is focused on CDMA, announced that the company would jointly develop LTE with Vodafone, which suggests that they will not migrate beyond EVDO Rev B. The last major resolution was the initial roll-out of TD-SCDMA by China Mobile just in time for the Olympics.
These initiatives led to the introduction of two new cellular standards, TD-SCDMA and WiMAX (WiBro). With these two additional standards, the number of potential 3G/4G technologies that still require 2G backward compatibility for voice coverage has grown. The possible combinations include GSM/EDGE with WCDMA, TD-SCDMA, WiMAX and/or LTE. For the CDMA networks, the options are CDMA2000 with EVDO, LTE and potentially WiMAX. This complexity is further compounded by the different options associated within each standard – WCDMA includes HSPA with various data rates, and LTE and WiMAX support various channel bandwidths.
The increasing number of modes and combinations challenge the capability of the industry to develop highly integrated solutions for each different combination. For products to meet the strict power, size and cost requirements that are expected by the consumer, alternative solutions are required that can address the changing demand of multi-mode devices.
The RF Problem
It is clear that the individual development of integrated circuits (IC) for all of these different modes is not practical. Beyond the IC development costs, evaluating and integrating an RF solution requires significant resources from a handset team in both the hardware and software engineering groups. The initial tasks are ensuring that the solution will theoretically meet the performance requirements based on the data sheet and other documentation. A physical evaluation of the hardware is then performed and can take many weeks of testing.
The next step is to integrate the RF with the baseband platform, which involves developing the hardware platform and the software to allow all of the devices to “talk” to each other. This often requires weeks of performance optimization beyond the integration phase. The entire process can take anywhere from three to nine months depending on the complexity and the teams’ familiarity with the standard. If this process has to be repeated for many devices, vast engineering resources would be required and the time-to-market for many of these products would extend considerably.
Developing an RF platform that can support multiple modes and combinations would mean that the efforts necessary to add an extra mode, such as TD-SCDMA, would be incremental to previous work. This can be compared to discrete products, where in each case all modes need to be tested and all of the integration problems must be resolved.
A Flexible RF Solution
For a single RF device to function for different modulation schemes, it needs to support narrow band modulation (GSM and EDGE) and wide band modulation schemes (WCDMA, LTE, WiMAX etc). They each have their own challenges. Until recently, the use of a narrow band polar modulator and a wide band linear modulator has proven to be the only solution that provides acceptable performance, despite being a suboptimal solution for size and cost. If a common architecture cannot be realized, then the bill of materials (BOM), die size and solution cost will never approach that of GSM/EDGE solutions.
Polar modulation has been proven to provide the most efficient transmit path for narrow band modulation schemes such as GSM and EDGE. It is the ideal architecture for GSM/EDGE as it provides a common low noise, high efficiency architecture for both modes. With the improved noise performance of the architecture, it is possible to remove the transmit SAW filters which is required in order to support multi-band power amplifiers. The basic architecture is shown in the block diagram in Figure 1.
Figure 1: GSM/EDGE Polar Modulator
The concept of polar modulation is simple. The transmit signal is split into AM and FM components – the FM component is digitally modulated onto the PLL and the AM path (for EDGE) is time aligned and recombined with the FM at the PA driver or the PA stage. The linearity requirements of the FM path have been effectively negated, which means that the architecture’s linearity is only really defined by the VGA or PA stage of the design. Hence, the support of higher peak to average ratio signals, such as those used in HSUPA, LTE and WiMAX, present less of a challenge compared to a standard linear modulator.
There are, however, significant challenges in extending polar modulation to support wideband modulation schemes, such as WCDMA and LTE. It is not possible to simply widen the bandwidth of the digital modulator as the increased loop bandwidth would degrade the overall noise performance and cause stability concerns. The timing alignment for the AM and FM path has significant effect on the spectral purity of the output signal. For EDGE, the signals need to be aligned to less than 10nS, but for WCDMA, this reduces to less than 2nS.
There is little doubt in the industry that as with current GSM/EDGE solutions, polar modulation also provides the best architecture for WCDMA and beyond; however, the difficulties have proven to be a deterrent for most companies. Sequoia Communications is one company that has conceived a solution to the issues surrounding polar modulation and developed a 2-point polar modulator that provides an architecture to support increased modulation bandwidths, as shown in Figure 2. In 2-point modulation there are two modulation paths: the digital path, which is similar to a standard GSM/EDGE path, and the analog path, which extends the bandwidth that the PLL can support. Maintaining the gain match between the analog and digital paths is critical in order to prevent higher EVM and degraded ORFS.
Figure 2: 2-Point Polar Modulation
This architecture has been proven in Sequoia Communications’ SEQ7400 multi-mode transceiver. The device has been tested and has proven its flexibility – it has been verified to support GSM, EDGE, WCDMA, HSUPA and TD-SCDMA. Further testing is under way to show support for CDMA, LTE and WiMAX.
A true single multi-mode device reduces the overall footprint that the RF section consumes, providing either improved form factors or allowing additional features to be added into the handset. For the development teams it means that more time can be spent evaluating a single device rather than multiple devices, and that they only need to develop a single RF platform in order to support all of the different requirements.
The cellular market is rapidly changing and will continue to do so as network operators determine the best solutions to meet subscriber performance and capacity needs. Regardless of their decision, many options and variants will need to be supported and it is not realistic for semiconductor suppliers to develop custom devices for each possible variant. Significant benefits can be realized by handset engineering teams with a single RF platform capable of meeting their varied needs.
For this to occur, a different approach to RF is required and Sequoia Communications has proven that multi-mode 2-point polar modulation provides a solution to many of these problems. A single device has been developed that supports both narrow band and wide band modulation schemes creating a flexible multi-mode product. This product (and architecture) will allow development teams to cost effectively sustain the myriad of devices that are expected to be released into the market over the coming five years.
San Diego, CA
This article originally appeared in the May, 2008 issue of Portable Design. Reprinted with permission.