Simplified design of RF front end for multi-mode and multi-band 3G mobile phones

Intense competition in the mobile phone market is driving manufacturers to find new design methods to reduce cost, printed circuit board (PCB) area and power consumption. At the same time, the rollout of third-generation (3G) networks has opened the door to a variety of new multimedia and experiment-based applications, from wireless network access and mobile video to text messaging and mobile TV.

As demand for these new applications rises and the market becomes increasingly global, handset manufacturers are faced with a dilemma. How can they maintain the growing number of frequency bands needed to support global platforms and the multiple broadband technologies required to deliver these new revenue-generating services without violating the market's stringent cost, coverage and power constraints? The latest 3GPP standards have increased the number of frequencies supported from 3 to 10 and are set to continue to expand.

One thing is clear: to succeed in today's market, handset designers need to design handsets with multi-band, multi-mode capabilities. Existing 2G GSM/GPRS networks continue to grow in subscribers and have the largest network coverage today. EDGE technology increases data rates by introducing a secondary modulation format to the GSM system, and shipments of handsets based on this technology are growing rapidly.

At the same time, network operators are continuing to roll out 3G Wideband CDMA (WCDMA) networks. Based on the Universal Mobile Telecommunications System (UMTS) network topology, this new technology is rapidly becoming the leading global mobile broadband solution. Industry analysts predict that WCDMA and EDGE will represent the two fastest growing segments of the mobile phone market in the next few years. In addition, to meet the demand for IP-based services, an increasing number of UMTS operators around the world are deploying High Speed ​​Downlink Packet Access (HSDPA) networks. High Speed ​​Uplink Packet Access (HSUPA) will also begin deployment soon. Figure 2 shows each mobile phone standard and the associated uplink and downlink data rates.

At the same time, network operators and service providers believe that now is the time to proactively accelerate the development of WCDMA in the direction of 3GPP Long Term Evolution (LTE). LTE is becoming the dominant technology for the next generation of wireless broadband networks. It enables data rates of 100Mbps for downlink and 50Mbps for uplink, and improves network coverage and efficiency through orthogonal frequency division multiplexing (OFDM) transmission mechanism using multiple-input multiple-output (MIMO) smart antenna technology.

LTE will lay the foundation for 4G technology, which requires network operators to support another modulation scheme. To take advantage of these new network technologies, network operators must overcome two huge obstacles: higher costs and greater power consumption. The BOM cost of a WCDMA phone is twice that of an EDGE phone and nearly three times that of a GSM/GPRS phone. At the same time, GSM phones have twice the talk time of WCDMA phones, and talk time is a key factor in consumers' mobile phone experience.

These differences are mainly due to the more complex WCDMA front-end architecture. WCDMA is a spread spectrum technology that uses a 5MHz transmission bandwidth. Because WCDMA uses full-duplex communication, the receive and transmit functions can be performed simultaneously, but this requires the front-end circuit to attenuate the wideband noise of the transmitter to avoid desensitization of the receiver. Usually, this is achieved by using a duplexer and additional bandpass filters on the transmit and receive paths. In addition, designers generally use external LNAs. The additional component count and area increase the cost of WCDMA phones compared to GSM/GPRS and EDGE phones.

Power efficiency is also a challenge. In wireless devices, the output power amplifier stage usually consumes most of the battery energy. Unlike the power amplifier (PA) of GSM mobile phones, which operates in saturation mode, the PA in WCDMA systems operates in linear mode. In addition, the complex quadrature phase shift keying (QPSK) modulation technology also requires the PA stage to have high linearity to avoid degrading the signal quality or interfering with adjacent channels. Therefore, WCDMA mobile phone designers often have to balance the high linearity required to ensure WCDMA performance with the high power efficiency required for longer battery life requirements.

Front-end circuit replication

Traditionally, to support multiple air interface standards in the same device, handset designers have used stacked radio architectures with separate radio transceivers. Typically, supporting multiple air interfaces has a significant impact on the number of components in a handset, as it requires the use of multiple surface acoustic wave filters (SAW), oscillators, filters, and dedicated mixers. Obviously, such a large number of components is a challenge for designers in the cost- and power-sensitive handset industry. In addition, functional duplication is in direct conflict with the requirement to minimize the product PCB area. Implementing this front-end functionality currently requires 4 PAs, 10 SAW filters, 3 duplexers, and 1 single-pole, nine-throw switch.

It is clear that engineers designing mobile phones for the global market need a new front-end architecture that can reduce the inherent redundancy of existing stacked RF front-end circuits. A single common transmit channel can maximize the reuse of circuits on the chip, reduce system BOM costs, save PCB area, and simplify the front-end design of the mobile phone. In addition, since linear PA consumes most of the energy of the mobile phone battery, a single transmit channel using a nonlinear PA can significantly reduce power loss and extend the battery life of the mobile phone.

Extended Polar Modulation

One way to achieve this front-end design is to use polar modulation in WCDMA and other high-bandwidth wireless technologies. Polar modulation is widely used in GSM and EDGE systems and eliminates the inherent conflict between power efficiency and amplifier linearity by allowing the PA input signal to be a fixed envelope or contain no components with different amplitudes.

In the polar modulation mechanism, the I and Q rectangular baseband signals that are usually sent to the transceiver with direct upconversion are converted to a polar format with amplitude and phase components. This allows designers to apply the two modulation elements differently and more effectively. The phase signal is supplied to the phase-locked loop (PLL) used as a phase/frequency modulator. The output signal of the PLL-VCO is then supplied to the VGA or PA operating in a near saturation/clipping state. Because the amplitude of the phase modulated signal generated by the PLL remains unchanged, it can be amplified by using a more efficient nonlinear class E or F amplifier. Power loss through the transmitter is greatly reduced, and ultimately battery life is extended.

The GSM system uses fixed envelope modulation with Gaussian minimum shift keying. Since the complex signal trajectory lies on the unit circle, the modulation can be completely described by its phase content. The EDGE system uses 8-phase shift keying (PSK) modulation that encodes 3π/8 in a different way to increase the GSM data rate by a factor of 3. AM is added to the signal so that the transmitted signal occupies the same 270 kHz bandwidth as GSM. These similarities simplify the extension of GSM polar transceivers to EDGE.

WCDMA presents a completely different set of challenges. This technology includes multiple data channels and uses spread spectrum hybrid PSK (HPSK) modulation to achieve higher data rates. The use of multiple channels results in a set of superimposed quadrature phase PSK (QPSK) modes with different gains caused by different spreading factors. A root raised cosine filter limits the sign smearing, and the bandwidth of the transmitted signal is constrained to 3.84MHz.

These differences place different demands on the design of the transmitter. GSM and EDGE systems require excellent phase linearity, low phase noise, and high efficiency. WCDMA systems require high accuracy over a wide bandwidth and amplitude range.

The polar architecture has been proven in GSM/EDGE solutions, providing the lowest noise performance without the need for SAW filters. This approach can be used in WCDMA solutions to eliminate the transmit SAW filter without the additional current consumption required by the linear architecture. Because the polar architecture supports all modulation formats, it can also support true multimode PAs. This architecture greatly reduces the overall size and complexity of next-generation solutions.

New front-end circuit architecture

To simplify the front-end circuit design of multi-mode mobile phones and reduce the cost and PCB area of ​​mobile phones, Sequoia Communications has developed an innovative architecture that uses polar modulation technology to provide a single transmit channel for all modes. The company's FullSpectra architecture provides the basis for the design of single-chip multi-mode RF transceivers. The second-generation SEQ7400 supports 7 frequency bands, including 3-band WCDMA/HSDPA, 4-band EDGE, GPRS and GSM, which can be applied to most major networks in the world. In order to reduce the number of components and cost, the transceiver integrates all LNAs and WCDMA interstage filters. The device provides standard analog interfaces and SCI or DigRF 2.5G control interfaces in a compact RF pin.

The advantages of this device are very obvious in the design of multi-mode and multi-band mobile phones. A single IC can significantly reduce the workload of engineers by eliminating the complexity and duplication of stacked design. By integrating LNA and eliminating the SAW filter of WCDMA between receiving stages, BOM cost can be reduced and PCB area can be reduced. With this new technology, designers can reduce the RF panel area by nearly 70% and the number of RF components by more than 40%.

In addition, by supporting quad-band EDGE and 3-band WCDMA interfaces in the same handset, this new approach gives design teams the flexibility to develop platform systems for different geographic regions and markets. This new architecture can increase factory throughput and further improve handset manufacturing costs, and ultimately allow designers to extend battery life in next-generation handset designs by reducing transmit and standby current requirements.

Conclusion

In today's highly competitive mobile phone market, traditional stacked radio architecture is no longer feasible for multi-mode multi-band mobile phones. Their repeated functional design, higher BOM cost and larger PCB area will reduce market competitiveness. To meet customer requirements, designers need a new and more effective front-end design method for multi-mode multi-band mobile phones.

Polar modulation provides an opportunity to develop the most promising transmit architecture. Polar modulation allows a single channel to be used in all modulation schemes, providing the smallest silicon implementation. It easily supports the next generation of multi-mode PAs. The inherent low noise performance of this solution provides an efficient way to use battery energy, eliminating the need for WCDMA transmit SAW filters. In addition, this efficiency advantage over other architectures will increase as the industry moves to higher-order bit modulation schemes such as HSUPA and LTE.