Simplified RF front-end design for multimode multiband 3G mobile phones

The fierce competition in the mobile phone market has driven manufacturers to find new design methods to reduce costs, printed circuit board (PCB) area and power consumption. At the same time, the first demonstration of the third generation (3G) network has opened the door to various new multimedia and experiment-based applications, from wireless network access and mobile video to text transmission processing and mobile TV.

As the demand for these new applications rises and the market becomes increasingly global, mobile phone manufacturers are in trouble. How can they maintain the ever-increasing number of frequency bands used to support global platforms and the need to provide these new revenue-increasing services without violating the strict cost, coverage and power constraints of the market? The number of frequencies supported by the latest 3GPP standard has increased from 3 to 10 and is set to continue to expand.

There is no doubt that: in order to succeed in today's market, mobile phone designers need to design mobile phones with multi-band and multi-mode capabilities. The users of the existing 2G GSM / GPRS network continue to grow and have the largest network coverage rate today. EDGE technology improves the data transmission rate by introducing a secondary modulation format in the GSM system, and the shipment speed of mobile phones based on this technology has grown rapidly.

At the same time, network operators are continuing to roll out 3G broadband CDMA (WCDMA) networks. Based on the network topology of the Universal Mobile Telecommunications System (UMTS), this new technology is rapidly becoming the solution to lead global mobile broadband. Industry analysts foresee that WCDMA and EDGE will represent the two fastest growing segments of the mobile phone market in the coming years. Moreover, to meet the needs of IP-based services, UMTS operators that are increasing worldwide are deploying high-speed downlink data packet access (HSDPA) networks. High-speed uplink data packet access (HSUPA) will also be deployed soon. Figure 2 shows each mobile phone standard and the associated uplink and downlink data transmission rate.

At the same time, network operators and service providers believe that it is now time to take the initiative to accelerate the development of WCDMA in the direction of 3GPP long-term evolution (LTE). LTE is becoming the dominant technology for next-generation wireless broadband networks. It enables downlink and uplink data transmission rates of 100 Mbps and 50 Mbps, respectively, and improves network coverage and efficiency through an 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 WCDMA mobile phones is twice that of EDGE phones, and nearly three times the cost of GSM / GPRS mobile phones. At the same time, the talk time of a GSM cell phone is twice that of a WCDMA cell phone, and the talk time is a key factor for consumers' experience of using the cell phone.

These differences are mainly due to the more complex front-end architecture of WCDMA. WCDMA is a spread spectrum technology, which uses a 5MHz transmission bandwidth. Because WCDMA adopts full-duplex communication, the receiving and transmitting functions can be performed at the same time, but this requires that the front-end circuit can attenuate the broadband noise of the transmitter to avoid a reduction in receiver sensitivity. Generally, this can be achieved by using duplexers and additional bandpass filters on the transmit and receive channels. In addition, design engineers generally use external LNAs. Compared with GSM / GPRS and EDGE mobile phones, the number and area of ​​additional components increase the cost of WCDMA mobile 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 operating in saturation mode, the PA works in linear mode in WCDMA systems. In addition, the complex four-phase shift keying (QPSK) modulation technique also requires the PA stage to have high linearity so as not to degrade the quality of the signal or interfere with adjacent channels. Therefore, WCDMA mobile phone design engineers often have to balance the high linearity required to ensure WCDMA performance with the high power efficiency required for longer battery life.

Front-end circuit replication

Traditionally, in order to support multiple air interface standards in the same device, cell phone design engineers have been using stacked radio architectures with separate radio transceivers. In general, supporting multiple air interfaces has a greater impact on the number of components of a mobile phone, because it requires the use of multiple surface acoustic wave filters (SAW), oscillators, filters, and dedicated mixers. Obviously, for design engineers in the mobile phone industry sensitive to cost and power, such a large number of components is not a small challenge. In addition, functional replication directly conflicts with the requirement to minimize product PCB area. Realizing this front-end function currently requires 4 PAs, 10 SAW filters, 3 duplexers, and a single-pole nine-throw switch.

Obviously, 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 transmission channel can maximize the multiplexing of the circuits on the chip, reduce the system BOM cost, save PCB area and simplify the front-end design of the mobile phone. In addition, since the linear PA consumes most of the mobile phone battery energy, a single transmission channel using a non-linear PA can significantly reduce power consumption 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. It eliminates the inherent conflict between power efficiency and amplifier linearity by allowing the PA input signal to be a fixed envelope or not containing component signals of different amplitudes.

In a polar modulation mechanism, the I and Q rectangular baseband signals, which are usually sent directly to the transceiver with an up-conversion, are converted into a polar format with amplitude and phase composition. This allows designers to differentiate and use two modulation elements more efficiently. The phase signal is supplied to a 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 near saturation / clipping. Because the amplitude of the phase-modulated signal generated by the PLL remains unchanged, it can be amplified by using a more efficient non-linear E or F amplifier. The transmitter greatly reduces power loss and ultimately extends battery life.

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 fully described by its phase composition. The EDGE system encodes 3Ï€ / 8 8-phase shift keying (PSK) modulation in different ways to increase the GSM data transmission rate by 3 times. AM is added to the signal to carry the same 270 kHz bandwidth as GSM. These similarities simplify the expansion 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 transmission rates. The use of multiple channels produces a set of superimposed four-phase PSK (QPSK) modes with different gains caused by different spreading factors. A root-raised cosine filter limits flag tailing, and the bandwidth of the transmitted signal is constrained to 3.84MHz.

These differences have different requirements for 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 range of bandwidth and amplitude.

The polar architecture has been proven in the GSM / EDGE solution, which provides the lowest noise performance, eliminating the need for SAW filters. This method can be used in the WCDMA scheme to eliminate the transmission SAW filter, and does not require the additional current loss 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 transmission channel for all modes. The company's FullSpectra architecture provides the basis for the design of single-chip multimode RF transceivers. The second-generation SEQ7400 supports 7 frequency bands, as well as 3-band WCDMA / HSDPA, 4-band EDGE, GPRS, and GSM, and can be applied to most major networks worldwide. In order to reduce the number of components and cost, the transceiver integrates all LNA and WCDMA interstage filters. The device provides standard analog interfaces and SCI or DigRF 2.5G control interfaces in compact RF pins.

In the design of multi-mode multi-band mobile phones, the advantages of this device are very obvious. A single IC can significantly reduce the workload of engineers by eliminating the complexity and repeated design of the stacked design. By integrating LNA and SAW filters that eliminate WCDMA between receiving stages, BOM cost can be reduced and PCB area can be reduced. Using 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 4-band EDGE and 3-band WCDMA interfaces in the same mobile phone, this new approach gives the design team great flexibility in developing platforms for different geographic regions and markets. This new architecture can increase factory production and further improve mobile phone manufacturing costs, and ultimately allow designers to extend battery life in next-generation mobile phone designs by reducing transmission and standby current requirements.

Summary of this article

In today's highly competitive mobile phone market, the traditional stacked radio architecture is no longer feasible for multi-mode multi-band mobile phones. Their repeated function 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 and multi-frequency mobile phones.

Polar modulation provides an opportunity to develop the most promising transmission architecture. Polar modulation allows a single channel to be used in all modulation schemes, thus providing the smallest silicon implementation. It is easy to support the next generation multi-mode PA. The inherently low noise performance of this solution provides a highly efficient method of using battery energy, eliminating the need for WCDMA transmit SAW filters. In addition, this efficiency advantage over other architectures will increase as the industry shifts to higher order modulation mechanisms (such as HSUPA and LTE).

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