Meet the challenges of LTE head on with RF CMOS

Although 3G networks are just gathering steam, many are already looking ahead to the next-generation, which in many cases is likely to be long-term evolution (LTE) networks. According to ABI Research, more than $3.3 billion will be spent on building LTE base stations in 2011, which will include approximately 142,000 base stations worldwide[i].

Next year in China, China Mobile will construct a pre-commercial LTE network, and, according to In-Stat, is expected to be the first operator to launch commercial LTE services[i]. In Spain Telefonica has conducted the first tests on a live LTE network installed in Madrid.  This consisted of VoIP calls and a videocall with data and image downloads at speeds in excess of 140 Mbps which is approximately ten times those possible with current 3G networks using HSPA technology.[ii] But moving to LTE is not as simple as switching out or adding more base stations. There are complex issues that must be resolved, particularly in terms of new operation bands, interference issues, and high data rates in an all Internet Protocol (IP) mobile system. To achieve this, system designers have to revisit the criteria for devices, and device manufacturers have to find ways to satisfy advanced performance demands.

 

As the number of wireless operators committed to LTE networks in 2010 continues to grow (26 at last count, according to GSA, the Global mobile Suppliers Association [iii]), technological challenges abound. For the most part, we can expect that it will be faster devices with low power, high linearity, and small form factors that enable LTE. RF CMOS technology's advantages of low power and ease of integration are well known. Building on that foundation, UltraCMOS™ technology is a CMOS process where a 50 to 100 nm silicon film is formed directly on a sapphire substrate. This provides for fully-depleted devices with little or no body charge under the gate. As a result, UltraCMOS processing delivers faster devices with reduced power loss, excellent linearity, and high isolation, making this process an excellent match for the unique demands of LTE mobile devices and base stations.

What makes LTE Unique?

The move to LTE is being driven by demand for broadband data by customers on the move. Already accustomed to exceptional user experiences in their desktops, consumers demand the same of their mobile devices, expecting fast upload and download rates and long battery life—all in a small form factor. To be successful, operators need to satisfy these demands and find ways to more efficiently use their available spectrum all while lowering capital and operating expenses to stay competitive.

 

Fortunately, LTE offers download data rates of 100Mb/s and upload rates of 50 Mb/s for every 20MHz of allocated spectrum; these are the types of data rates that will give consumers what they are looking for with the spectral efficiency that carriers need. Even higher data rates are possible, such as up to 326.3 Mb/s in the downlink, if multiple antennas are used. Since mobile consumers expect their service to work anywhere, it is important that, although it is optimized for speeds of 0-15 km/hr, LTE can support high mobility performance even when the device is travelling at 120-250 km/hr.

 

Another unique feature of LTE is that it employs two different access schemes: orthogonal frequency-division multiplexing (OFDM) on the downlink (base station to mobile) and single-carrier frequency-division multiple access (SC-FDMA) on the uplink (mobile to base station) with an adaptive modulation scheme that varies from QPSK to 64QAM. The use of different access schemes enhances power amplifier (PA) efficiency on the mobile side (longer battery life) and enhances the spectral efficiency on the base station side. It is also important to note that some LTE mobile devices will support both FDD and TDD duplexing schemes to allow the user to dynamically adapt to systems between countries. And, LTE operates across a scalable bandwidth of 1.4 to 20MHz in both uplink and downlink, supporting both new and existing mobile frequency bands.

 

Impact on System Design

In LTE's implementation of OFDM, several closely spaced orthogonal subcarriers make up a resource block (RB). Depending on a particular system's bandwidth, the number of RBs will vary. (Generally speaking, 1RB = 12 subcarriers of 15kHz each.) This approach makes receiver design very challenging in terms of adjacent channel selectivity, as the LTE specification notes a very large in-band interferer that is located only 1 RB away. As a result, system architecture choices (direct conversion or IF sampling) and signal processing schemes (analog vs. digital signal processing) greatly impact how well the LTE system requirements can be met. Overall though, devices with high linearity and isolation will help meet this challenge.

 

Initial LTE base stations will deploy 2x2 antenna technology and will likely move to 2x4 antenna technology quickly. This increases the market pressure for higher integration in order to control component count as well as minimize the bill of materials and design complexity. For instance, in a 2x2 configuration there will be a digital step attenuators (DSA) in each of the Tx and Rx paths and one in the digital pre-distortion feedback path, meaning there will be 15 DSAs per base station. Clearly, the need for smaller sizes and higher performance devices is also critical in system design.

 

Since LTE network support goes beyond current band deployments, broad bandwidth is also important. All of the devices in the LTE mobile devices and base stations, including switches, mixers, and DSAs will need to be extremely broadband to accommodate the additional frequencies. The switching task, in particular, must further achieve an unprecedented number of throws, up to 12 throws and higher in a single-pole (SP12T) device. Why so high? Additional states demand advanced serial interface to reduce I/Os and enhance functionality. As well, some deployments of LTE will incorporate TDD, which further drives up the requirement for switch throw count. The increased switching capacity needs to be implemented with fast settling time and in as small a form factor as possible with compact routing in order to accommodate the increased functionality content in the device.

 

Lastly, LTE requires very low loss in order to achieve the signal to noise ratios (SNRs) that are required to achieve high data rates. This is particularly challenging because of the additional bands of operation that LTE brings, which place heavy demands on the antenna. In addition to all of the other requirements, then, it is likely that active antenna tuning will be required to achieve the desired LTE device performance[i].

Meeting the Challenges

Fortunately, UltraCMOS processing is available in high volume to meet the fast speeds, low power consumption, high linearity, and greater integration requirements of the switches, mixers, and DSAs in the LTE signal chain.

 

Fast Speeds

A fast switching speed is imperative to protect the receive path from damage when strong blocking signals are present, and it is also key for gain control on the basestation. As throw counts increase for LTE, this specification becomes even more important. Faster switching speeds and shorter settling times lead to more reliable and more accurate performance, and UltraCMOS inherently offers these advantages. For instance, the UltraCMOS PE43204 is a DSA with a typical switching speed of 30ns (see Figure 1), while maintaining input third order intercept point (IIP3) of +61dBm (typical), insertion loss of 0.6dB, and electrostatic discharge (ESD) of 2kV. In comparison, a GaAs DSA demonstrates a typical switching speed of 130nS, which is more than 4x slower than the UltraCMOS DSA.

 Low Power Consumption

In the LTE specification, SC-FDMA was chosen on the uplink in order to reduce power consumption, but lower power devices throughout the mobile device will enable longer battery life, which is a key concern for consumers. An SP9T switch such as the UltraCMOS PE42692 is well suited for LTE applications because it demonstrates an Idd power supply current of 120µA (typ.).  

High Linearity

High linearity is not a new requirement for mobile devices. In fact, the RF front-end module has long been the most linear element in the mobile device. However, as the complexity increases in LTE systems, having a process that can integrate more functions and still achieve high linearity is challenging. Basically, even though data rates climb higher with LTE, the linearity requirements stay the same despite additional semiconductor content. As a result, the devices in an LTE system must offer better linearity than those used in previous mobile device generations.

 

Because of the insulating gate of CMOS technology and the ability to natively incorporate mixed-signal design techniques, UltraCMOS ICs can meet demanding linearity performance requirements in a monolithic solution. In fact, UltraCMOS is currently being used to design and manufacture devices that exhibit both high linearity and isolation. The PE42692 SP9T switch, for instance, demonstrates an input third-order intercept point (IIP3) of +71dBm (see Figure 2) with insertion loss (IL) of 0.6 dB and Tx-Rx isolation of 43 dB (900MHz). Specifications like these allow more data through the system and improve resistance to interference, all leading to better performance across the spectrum.

 

Linearity is closely tied to high isolation, which improves signal quality in the presence of interfering signals, and high isolation is needed to maintain stringent duplexer performance. Since LTE introduces new frequency bands of operation, isolation becomes even more important. In the basestation, digital pre-distortion (DPD) is a critical factor in improving PA efficiency. Basically, the system samples the transmit path, corrects the signal, and feeds it back to improve the PA's efficiency. Using components with high linearity/high isolation in the DPD feedback path is important to avoid adding distortion into the feedback path and thereby degrade the PA efficiency that you are trying to improve. Operating from DC up to 3000MHz, the UltraCMOS PE4257 SPDT switch, for instance, is well suited for use in the feedback loop design requirements for base stations with isolation of 64dB at 1000 MHz (see Figure 3) -- an isolation spec that is the result of highly insulating properties of the sapphire substrate in UltraCMOS.

 

Small Size/Higher Integration

Simply put, LTE services require devices that handle large volumes of data at high speeds across a broad bandwidth. This additional functionality will require more integration in order to maintain footprint and power expectations. Because it is a CMOS process, UltraCMOS supports high levels of integration. UltraCMOS switches, for instance, have an integrated decoder so they do not require the additional control signals that GaAs switches do. In addition, blocking capacitors are eliminated because the switches integrate a negative voltage generator to turn the FETs off. In an effort to further increase integration, engineers at Peregrine Semiconductor developed the MultiSwitch™ (see Figure 4), which incorporates four independent high-performance RF multi-throw switches on a monolithic flip-chip IC controlled by a single onboard CMOS controller providing for more than 85% size reduction over alternate solutions. For instance, the MultiSwitch measures 1.6 x 1.93mm, and the UltraCMOS SP9T with onboard decoder, voltage generator and ESD protection measures 1.36x1.28mm. In comparison, a GaAs SP9T implementation measures 3.0x3.5mm, requiring 29 wirebonds in a custom multi-chip package.

Unlike any other device available, the MultiSwitch RFIC delivers linearity of +71 dBm IIP3, and isolation of better than 70 dB at critical paths. The device integrates key elements that would be off-chip in GaAs, including three control lines that operate 12 independent paths.

 

By 2011, ABI Research expects that nearly 34 million users worldwide will subscribe to LTE[i], which is promising consumers speeds on their mobile devices that rival those available from cable or DSL. With DSAs, mixers, and SP9T switches in high volume today (and a roadmap to SP12T and higher), UltraCMOS is well suited to support emerging LTE mobile device and base station designs.