Different designs for mobile broadband modems

In the evolution of 4G wireless baseband, there are currently two evolving technologies in the lead, which is LTE and WiMAX. WiMAX is positioned as the technology of choice for computing devices and M2M, as well as providing a fixed wireless network for untapped regions and supported by WiMAX Forums led by Intel and many other companies. LTE evolved along the "GSM" route towards mobile broadband, supported by the 3rd Generation Partnership Project (3GPP) and included Qualcomm, ST-Ericsson, Verizon Adopted by many baseband OEMs and operators including Vodafone. It is still unknown which standard will win in the end, but the result is very likely that the two coexist and serve different user groups in different regions. To ensure that both standards are supported by 4G modems, the market needs a flexible solution that can simultaneously meet the blueprint for both technologies.

The limitations faced by the development of mobile modems are not limited to increasingly complex wireless standards. Today's smartphones must support multiple wireless interfaces. In addition to WiMAX and/or LTE, 4G mobile devices also need to support a large number of wireless interfaces such as GSM, GPRS, EDGE, W-CMDA, HSPA and the latest HSPA+.

Due to the unpredictable future of the wireless baseband market, the environment faced by chip suppliers is very harsh. Development costs and multiple changing standards make the traditional hardwired design approach for terminal modems more risky. For example, a chip developed by a supplier may target the wrong standard, which may eventually lead to the elimination of the solution before it is released. More importantly, hard-wired is difficult to support the flexibility of all standards without having to redesign a large number of silicon technologies, so it is costly, bulky, and power-hungry. This naturally creates the need for a programmable solution that is flexible enough to support multiple standards and shorten the development cycle.

In the design of the new mobile baseband chip, there are three main options available:

Traditional (hardwired) solution - the entire modem is implemented in hardware. The main advantage of this solution is that it allows the first batch of chips to be launched quickly. In addition, hardware designed for a specific standard typically ensures the lowest power consumption. However, as already mentioned, this approach lacks flexibility and does not match the roadmap for future product upgrades.

Hybrid approach - a mix of hardwired design and programmable design processors. The parts of the modem that require flexibility are mapped into the programmable DSP core, using software for flexibility. The remaining computationally intensive and less flexible part of the modem, such as the Fourier Transform (FFT), can be implemented in hardware like a traditional hardwired solution.

SDR Solution - A complete "software modem" implementation that supports multiple wireless standards simultaneously in software on the same chip. This solution features a fully programmable design solution with full flexibility to handle multiple existing or future standards without the need to redesign new products. The main problem with this solution is that it is more complex and consumes more power than a hardwired solution optimized for the supported standards.

Because of the high risk of traditional solutions and the inability to meet current unpredictable market requirements, any supplier is unlikely to choose such an architecture. So we will focus on the latter two programmable designs.

CEVA-X is a high-performance, versatile DSP core family that is widely used in mobile and wireless applications and has been shipped in volume to a number of leading vendors.

The CEVA-X1641 is a quad-MAC unit in the CEVA-X DSP family consisting of four 16-bit data-width MAC units. At the worst conditions of 65nm, the CEVA-X1641 is also capable of operating at high frequencies above 700 MHz.

This high performance and easy to use DSP provides several different hardware and software partitions for mobile modem SoCs. Different baseband customers can use different implementations and partitions from single core to multiple cores in their modem design, and complete the modem function through different hardware accelerators. Choosing an extremely powerful market-standard DSP architecture (such as the CEVA-X1641) can protect software investments and define a blueprint for future product generations. A number of CEVA customers have chosen CEVA-X1641 as their preferred DSP to build 4G modems.

CEVA-XC is a communications processor designed and optimized for advanced wireless communications. CEVA-XC is based on CEVA-X and supports a variety of wireless interfaces through software, including the most demanding 4G mobile standards, LTE cat. 5 and WiMAX II (IEEE 802.16m), and 3G and 3.5G. This innovative processor can simultaneously support multiple wireless interfaces on the same architecture for a true software modem.

CEVA-XC uses a software programmable design architecture to perform all wireless processing with a single engine, eliminating the need for multiple baseband coprocessors, which eliminates the extra memory, data cache, and overall data traffic common to such distributed architectures. . Thereby reducing power consumption and reducing chip size.

The CEVA-XC architecture is based on 1, 2 or 4 Vector Communica Units integrated in the CEVA-X processor. Each vector unit is a 256-bit SIMD engine with three VLIWs and a large number of 16 MAC, algorithm, logic and displacement units. The CEVA-XC instruction set can meet the requirements of 4G wireless modems, including matrix processing, MIMO detectors, complex filtering, data exchange and bit stream processing.

As the industry moves toward 4G, the risks of traditional hard-wired design solutions increase due to development costs and multiple change standards. So, designing a flexible solution that quickly adapts to changing standards and can be reused across multiple generations is critical. A programmable design solution based on a hybrid solution or a fully software modem enables the reusability required and ensures a fast time to market.

Wireless baseband leaders have long recognized the trend toward turning to programmable solutions. Standardization around DSP architectures (such as the two CEVA DSP cores described in this article) is currently underway, and licensees can choose between their system architecture and level of flexibility. Moreover, the latest energy-saving and process geometry reduction technologies have made these programmable design options the preferred method of implementing 4G solutions in a variety of applications.

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