As 3G wireless technologies continue to make progress worldwide, developers at the Third Generation Partner-ship Project (3GPP) are looking beyond the current 3G wireless technologies, such as wideband CDMA (W-CDMA), which were developed for a mixture of voice and data communications over the same wireless network. Recent enhancements to W-CDMA, such as the high-speed packet access (HSPA), both on the uplink and downlink, are not enough. In fact, according to experts, significant further improvements are required if 3G must continue to dominate the global cellular market. Toward that goal, 3GPP has taken an initiative, resulting in long-term evolution (LTE) or 3G LTE. Some also refer to it as 4G. With competitive pressure building from mobile WiMAX and ultramobile broadband (UMB), a parallel effort of CDMA, developers are pushing the evolution in 3G LTE. As a result, initial standards are expected in the fourth quarter of this year. As per the timeline shown in Figure 1, prototype systems are expected in late 2007. Field trials may get under way toward the end of 2008 with real-world network deployments seen as early as 2010. According to a new study from ABI Research, network operators will invest almost $18 billion in LTE capital infrastructure over the period to 2014.

Aiming to achieve broadband-class data rates over the cellular network, LTE is intended to significantly increase the wireless network's capacity and data rates so that it can accommodate service enhancements and new multimedia applications such asinteractive video. Hence, by comparison, packet-based LTE is planning to significantly boost data rates by achieving speeds in the range of 100 Mbps for downlinks and 50 Mbps for uplinks. Presently, circuit switched 3G networks deliver speeds of 14.4 Mbps downlink and 5.76 Mbps uplink. According to the developers, one change that helps enable these increased speeds is the transformation of the network into an all-data network, which uses VoIP for voice traffic.

Signal processing

With such higher data rates, 3GPP LTE specifications will require complex signal-processing techniques such as multiple-input, multiple-output (MIMO) along with new radio modulation technologies like orthogonal frequency-division multiple access (OFDMA) and multicarrier code- division multiple access (MC-CDMA). Specifically, as per Texas Instruments' (TI) white paper, the uplink will use OFDMA and the downlink will use single-carrier frequency-division multiple access (SC-FDMA). Both frequency-division techniques employ fast Fourier transforms (FFTs) to segment the allocated bandwidth into smaller units that can be shared amongst the users. According to TI, SC-FDMA is used to reduce power consumption in the handset as the peak to average power ratio of SC-FDMA modulation is lower than that of OFDMA modulation. Also, from a computational standpoint, frequency division techniques scale more easily with bandwidth than code division systems, i.e., higher-bandwidth CDMA systems require much more computational power than OFDMA systems, as indicated in TI's white paper. In addition, the use of different-sized FFTs support implementation across multiple bandwidths allocations including 1.25 MHz, 1.6 MHz, 2.5 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. Plus, the ability to use either paired or unpaired spectrum allocations, has the additional benefit of allowing operators to be much more flexible in the rollout of LTE systems as they can deploy in different-sized bands depending on the available spectrum.

While higher spectral efficiencies can be achieved via MIMO or beamforming, it is also easier to implement MIMO in OFDMA systems than in CDMA systems, where noise is more uniformly spread. When combined with the other physical layer changes there should be a significant increase in the spectral efficiency of the system with the transition from CDMA to OFDMA, said John Smrstik, TI's worldwide marketing manager for communications infrastructure.

As an active participant in the development of LTE standards, Texas Instruments has created a development ecosystem (Figure 2), combining its wireless infrastructure optimized digital signal processors (DSP), software libraries and ATCA/AMC cards from leading systems developers like Mercury Computer Systems (MCS) and Silicon Turnkey Express (STx).

Utilizing the TMS320TCI6482 and the TMS320TCI6487 DSPs, TI has created a series of designs leveraging system-level benchmarks. These benchmarks illustrate various system architectures to support existing 3G standards, WiMAX and LTE. The software library capitalizes on TI's existing WiMAX Wave 2-compliant library, with a host of LTE-specific algorithms. Combined, this hardware/software package provides a starting point for LTE development and enables faster and easier development of prototype systems, noted Smrstik.

Since base station manufacturers are evaluating the new standard, TI haspartnered with system developers MCS and STx to offer ATCA/AMC-based development platforms that can dramatically reduce the OEMs time to market. These platforms allow developers to quickly assemble a hardware test platform, mirroring a typical system including DSPs, a general-purpose processor (GPP), and field-programmable gate arrays (FPGAs). Using an advanced mezzanine card (AMC) approach, separate cards with these devices can be easily connected and development can start before any final hardware decisions are made. Both the TCI6482 and TCI6487 baseband processors are available on AMC development cards, TI said.

While TMS320TCI6482 (TCI6482) is a 1 GHz very long instruction word (VLIW)-based programmable DSP for wireless base stations, TMS320TCI6487 is a 3-core, 3 GHz processor with the capacity to support a complete 10 MHz OFDMA sector on a single chip.

The TCI6482 has the latest TMS320C64x+ DSP core, which can perform up to eight 32-bit instructions during every cycle (cycle time is 1 ns). In addition to the 1 GHz performance of the basic architecture, the TCI6482 includes new extensions that facilitate code size reduction and processes complex arithmetic and bit manipulation functions across wireless standards.

Likewise, TMS320TCI6487 (TCI6487) device is a high-performance DSP designed specifically for wireless infrastructure baseband applications. With a high level of functional integration and a high-channel density supported on a single device, the TCI6487 DSP offers a modular and scalable design with a small footprint. Having rendered baseband solutions for UMTS, TDSCDMA, WiMAX and cdma2000 applications, the company is now leveraging the capabilities of TCI6487 DSP for LTE application. Because it offers a software-programmable solution and allows for the reuse of existing C64x and C64x+ DSP code, it is expected to accelerate the development of LTE baseband solutions. Advanced features such as MIMO, beam-forming and parallel interface cancellation (PIC) can be easily supported without the need for any hardware redesign.

“The broadband-class data rates of LTE will require tremendous performance in a wireless infrastructure framework to support the new customer applications enabled by these networks,” said Arnon Friedmann, software product manager for TI's communications infrastructure group. “The early development ecosystem, will help carriers and OEMs formulate the most cost-effective path toward LTE.”

One such ecosystem partner is Axis Network Technology, a U.K.-based wireless technology business focused on the development of multichannel OFDM digital radio and RF solutions for next-generation MIMO and AAS wireless systems. Axis is leveraging its current WiMAX platform for new LTE developments.

As an active participant in the development of the LTE standard, TI said it can quickly adapt and modify the software library to match any changes or new developments. In addition, since LTE is an OFDM-based system, TI is able to leverage more than 20 years of experience in both digital subscriber line (DSL) and previous OFDM-based wireless developments. According to TI, the first release of the LTE software package will be available at the end of the second quarter. The developer said that it will be updated throughout the year as the standard moves toward ratification.

Meanwhile, using TI DSPs, Mercury Computer Systems has readied a high-performance AMC card for next-generation base station development. Called Ensemble MTI-203 AMC, it combines three TCI6482 DSPs and a Xilinx FPGA node to support both LTE and WiMAX base station applications. According to Mercury Computer, combined DSP and FPGA enables optimal application partitioning. Furthermore, said the supplier, the card enables all the physical layer (PHY) baseband processing required for a 20 MHz, time-division duplex (TDD), multi-antenna solution with smart antenna MIMO enabled.

Modulation similarity

Due to the similarities between the OFDM/OFDMA modulation schemes found in the WiMAX/802.16 standards and those proposed by the 3GPP standards body for LTE/4G, a great deal of leverage and reuse is possible, said Michael Long, Analog Devices' product line manager for DSP & Systems division. Once the 4G or LTE air interface specifications have been finalized we see a certain move away from all-software, fully flexible baseband modem implementations, noted Long. This shift will, in turn, require further investment from semiconductor manufacturers to deliver cost/power consumption-compatible SoC solutions. However, he added, “In the immediate future (next 24 to 36 months), we foresee all LTE/4G developments continuing to reside on commercial off-the-shelf, board-level solutions such as the one built by BittWare, which use both DSP and FPGA processing elements.”

Consequently, for now, Analog Devices will leverage its 802.16 OFDMA software library, which provides a set of optimized software modules for the implementation of the OFDMA physical layer of an 802.16 base station on TigerSHARC DSP processor. “These code modules, coupled with the TS2012 “6Pac” board from our third-party partner BittWare form the core of our building blocks strategy for supporting the emergingLTE/4G standards,” stated Long.

While Freescale plans to leverage its existing multithread architecture for baseband processing, it will also develop new IPs to streamline its DSP engine, along with data converters, power amplifiers (Pas) and other RF front-end functions. However, to turn this complex strategy into a useful semiconductor solution, it will exploit the attributes of redistributed chip packaging (RCP) as shown in Figure 3.

In fact, packaging will play a crucial role in differentiating an LTE solution from WiMAX design, stated Kaivan Karimi, Freescale's director of global strategy for Networking and Computer Systems Group. While common OFDM base is important, LTE services will demand very small, low-power packaging, he added.

In essence, RCP will enable Freescale to pack three major functions — transceiver, broadband and power amplifier — in a single compact package with significantly lower parasitics. Some key benefits of RCP packaging include 30% size reduction, flexibility, ultralow k compatibility, halogen and lead-free packaging, and elimination of package substrate, wire bonds and flipchip bumps.

With standards rapidly changing and newer specs demanding rigorous computational capability, wireless infrastructure designers are seeking multiprotocol base stations. And hybrid FPGA/DSP-based platforms provide an effective design approach to comply with these ever-changing wireless standards, said Arun Iyengar, Altera's senior director for the communications business unit. Based on system throughput needs, such designs will provide intelligent partitioning between FPGA and DSP, noted Iyengar.

He added, “The partitioning strategy between FPGAs and DSPs depends on processing requirements, system bandwidth as well as system configuration, and the number of transmit and receive antennas. A typical distribution of signal-processing tasks between FPGA and DSP for baseband physical layer (PHY) functions in an OFDMA system, such as LTE, is shown in Figure 4. As shown in the diagram, the baseband PHY processing is partitioned into bit-level and symbol-level processing. For that, Altera is leveraging its high-performance Stratix III FPGA.

Test strategy

Test gear suppliers such as Aeroflex, Agilent, Anritsu, Keithley Instruments, Rohde & Schwarz and others have also outlined initial strategies for the development of test products to support the upcoming 3G LTE standard. According to Aeroflex, it is designing a pair of test products, the Aeroflex TM500 LTE and Aeroflex 6401 LTE, which will support the physical layer testing of networks and mobile devices, respectively. The complete visibility into the lowest layers of the radio modem will allow users to diagnose the actual cause of a synchronization problem rather than just knowing that synchronization has failed.

Aeroflex has developed a powerful graphical user interface to allow both the TM500 LTE and the 6401 LTE to be easily configured without the need to write software to execute the test. The GUI facilitates parameter configuration through dialogue boxes, which allows users to select their values using engineering terms and units. Furthermore, the TM500 and 6401 have been designed from the start to be integrated into an automated test environment.

The implementation of MIMO antennas to improve the signal strength received by mobile devices from the network is a key new feature of 3G LTE. The supplier said that it will develop test features especially for MIMO to ensure that both the network and mobile devices are able to get the signaling right and then transmit and receive in full synchronization with the signaling.

“Historically, new technology rollouts have been subjected to delays, often due to problems experienced when the new technology mobile devices are tested against the new technology networks,” said Phil Windred, business unit manager of the wireless group at Aeroflex Test Solutions. “Aeroflex will provide powerful test tools to support both network and mobile device testing at the earliest development stages of 3G LTE products. Experience shows that the earlier problems are detected, the lower the cost of correcting them. The TM500 and 6401 can play a major role in the mobile test industry in terms of providing diagnostic data geared specifically to the needs of 3G LTE that will not only ensure the early detection of implementation problems but also help accelerate the rate at which they are resolved.”

Likewise, Agilent is addressing LTE needs via its Advanced Design System (ADS) LTE design library, which can simulate uncoded EVM and features constellation and BER receiver measurements for the downlink with the latest subcarrier specifications. It also provides flexible, platform-based equipment that works in conjunction with ADS to generate and analyze real signals.

Throughout this year and next, the manufacturer will introduce several new measurement products and solutions specifically designed to address the emerging needs of LTE — including pattern generators, logic analyzers, signal generators and signal analyzers, as well as a one-box tester. These offerings will support early R&D in components, base station equipment and mobile devices with design automation tools and flexible instrumentation, said Agilent. Plus, it will also introduce solutions for manufacturing as LTE products near commercial launch.

Like others, Rohde & Schwarz also supports the enhancement of the standard with its test and measurement equipment. The company has already added the new technology to its product portfolio. The R&S SMx-K55 option turns the R&S SMU200A, R&S SMJ100A, and R&S SMATE200A into the generators for LTE signals. When equipped with the R&S FSQ-K100 option, the high-end R&S FSQ spectrum analyzer also lends itself to LTE.

Equipped with the SMx-K55 option, the SMU200A, SMJ100A, and SMATE200A signal generators generate high-quality downlink signals for performing tests on base station components as well as on mobile phone components and receivers. Since the option runs directly on the instruments, no external software is required, according to Rohde & Schwarz. The SMx-K55 option supports the OFDMA transmission method, channel bandwidths up to 20 MHz, as well as the QPSK, 16 QAM, and 64 QAM modulation modes. Plus, it offers predefined scenarios. Moreover, it is also possible to flexibly set the parameters of reference symbols and control and synchronization channels (BCH, SCH) as well as to configure the sub frames independently of each other. Modifications to the standard are covered via free updates, said the manufacturer.

For analyzing downlink LTE signals, the FSQ vector signal analyzer and high-end spectrum analyzer can be equipped with the FSQ-K100 option, which is a flexible PC software application for the development of base stations. This option also makes it possible to quickly respond to modifications to the standard. The FSQ-K100 option analyzes LTE signals in the spectral, time, and modulation domains with all channel bandwidths specified in the standard, stated Rohde & Schwarz. For measuring signals in the baseband — whether balanced or unbalanced — the optional baseband inputs of the FSQ are available, said the manufacturer.

As per the supplier, the two options are compatible with one another and can, therefore, be used together. Furthermore, according to R&S, their high flexibility accommodates the fact that the specifications are not yet fully defined in the standard. According to R&S, the uplink will be available for both solution in the near future — as a free-of-charge firmware update for the SMx-K55 option, and as the FSQ-K100 option for the FSQ.