A high PAPR also lowers the efficiency of a transmitter’s power amplifier. Unlike single-carrier systems, OFDM is not a constant-envelope modulation scheme where information is conveyed by varying frequency or phase while amplitude remains constant. While within each symbol the amplitude and phase of each subcarrier is constant, over the duration of that symbol, there may be many large peaks. That means the power amplifier has to accommodate those peaks without saturating (e.g., clipping). In turn, that means using a larger amplifier and the resulting decrease in efficiency. For this reason, in particular, LTE uses a different method for UL.

Power consumption is a key design consideration for users’ handheld devices. Thus, SC-FDMA is used instead of OFDMA for UL. The basic transceiver architecture is practically identical for OFDMA and SC-FDMA, and both methodologies provide about the same degree of multipath adaptation. But because the UL waveform is essentially single-carrier, the PAPR is lower.
A Common FDD Frame Structure

In both DL and UL, LTE uses the same frame structure (for FDD). Transmissions are segmented into frames, each of which is 10 ms in duration (Fig. 2). The frames, in turn, are made up of 20 0.5-m slot periods. And, subframes (1.0 ms in duration) contain two slots.

The primary difference between WiMAX and LTE is the SC-FDMA UL. Both proposed standards accommodate channel bandwidths that vary from 1.25 to 20 MHz. Also, both have access methods that enable multiple users to share the same channel. But WiMAX uses OFDM for both UL and DL. Although WiMAX can be deployed for FDD, TDD, and half-duplex FDD, its most common deployment uses TDD.
In both cases, WiMAX and LTE, the basestation is responsible for access allocations. As was seen with LTE, using FDD, mobile devices are given PRB allocations. With WiMAX, mobile devices are given time-dependent access slots.

LTE Analysis Test Approach

Beginning with Wi-Fi, then WiMAX, the industry has moved toward a platform approach to testing these increasingly complex wireless modalities. For example, LitePoint introduced its IQview platform in 2004 for testing Wi-Fi devices. IQview consisted of a vector signal generator and vector signal analyzer plus controls in the hardware portion of the platform. It also featured a comprehensive GUI software portion of the platform to enable users to perform single capture with multiple analysis and EVM on Wi-Fi devices. The same approach was used in 2006 to support the development and production testing of WiMAX devices.

For LTE, Tektronix and LitePoint jointly developed a unique platform. The hardware is based on Tektronix’s real-time spectrum analysis hardware and application programming interface (API) complemented by a LitePoint GUI tailored to that hardware. This Tektronix-LitePoint solution provides the capture and analysis needed to examine and eliminate intermittent events.

Transients and Intermittent Events

First and foremost, designers need to verify signal compliance to the LTE standard, whether this is compliance to a spectral emissions mask or EVM on a specific channel. When LTE signals fail compliance, designers need to understand why. They need tools to identify and ultimately isolate where the problem exists, whether it’s an algorithm problem or signal interference (Fig. 3). These examples have one thing in common. When these transients occur, spectral leakage will occur, resulting in frequency “splatter.”

Best Test Approach to LTE Analysis

Designers first need a tool to evaluate compliance for the UL and DL portions of an LTE signal. The real power of a real-time spectrum analyzer (RTSA) with the LitePoint software stems from its ability to capture and analyze intermittent events (Fig. 4). By combining digital phosphor technology (DPX) for discovering all the transients for troubleshooting with the RTSA’s frequency mask trigger (FMT), designers can:

• Trigger on the occurrence of one LTE channel while ignoring the others
• Trigger on spectrum mask violations
• Trigger on signal interferers such as external transmissions in-band, digital hardware emissions, or transient power-amplifier problems