LTE Fundamentals

WiMAX and LTE share some common fundamentals, but there are many differences as well. It is helpful to examine the LTE fundamentals to better understand what needs to be tested and how best to do it. Primarily, LTE describes a standard for high-speed data and media transport plus high-capacity voice support. As such, it supports high-speed data plus multimedia unicast and broadcast services.

The PHY is meant to be an efficient means to convey data and control information between a basestation and users’ mobile systems. It is intended to support scalable bandwidths of 1.25, 2.5, 5.0, 10.0, and 20.0 MHz with peak data rates that scale with bandwidth. So for 20-MHz channels, the basestation peak rate is 100 Mbits/s, and the mobile device peak rate is 50 Mbits/s.

Like Wi-Fi and WiMAX, LTE has adopted multiple-input multiple-output (MIMO) technology to increase data rate or increase range. As such, basestation LTE systems may employ up to four transmit antennas and two receive antennas in configurations of 4x2, 2x2, 1x2, and 1x1. Handheld mobile devices may have one transmit antenna and up to two receive antennas in a 1x2 or 1x1 configuration. With regard to range, a basestation is intended to provide full performance at a distance up to 5 km, with degraded performance out to 30 km and the possibility of usable operation out to 100 km.

LTE uses different downlink (DL) and uplink (UL) technologies. For DL (e.g., transmissions from basestation to mobile system), the standard calls for orthogonal frequency-division multiple access (OFDMA). For UL (e.g., transmissions from mobile unit to basestation), it specifies single-carrier frequency-division multiple access (SC-FDMA).

OFDM, OFDMA, and SC-FDMA

For LTE DL, OFDM breaks the channel’s bandwidth into separate subcarriers. Each subcarrier transmits its assigned data in parallel with the data transmitted by the other subcarriers. As a result, it produces a parallel data stream within the channel. Therefore, it can achieve high data rates without having to increase the symbol rates. This, in turn, helps avoid escalating inter-symbol interference (ISI).

The sub-carriers are modulated using QPSK, 16QAM, or 64QAM. Each OFDM symbol is a combination of individual signals on each subcarrier in that channel at any instant. Each OFDM symbol is preceded by a cyclic prefix whose purpose is to mitigate or eliminate ISI. The subcarriers are very tightly grouped for very efficient spectral efficiency. But because their individual frequencies are determined so they produce no signal in adjacent subcarriers, the inter-carrier interference (ICI) is virtually eliminated (Fig. 1).

In OFDMA, unlike an 802.11a, b, g, or n network, the channel users are given a specific number of available subcarriers for a predetermined amount of time. In contrast with CDMA/CD, multiple users can be sharing the same channel via predetermined subcarrier allocations (called physical resource blocks, or PRBs). The LTE basestation performs the PRB allocation based on frame structures specific to frequency-division duplexing (FDD) and time-division duplexing (TDD). Depending upon the channel bandwidth (1.25 through 20 MHz), the number of PRBs available varies from six to 100 (e.g., six, 12, 25, 50, 75, and 100).

In addition to its positive attributes, OFDM has some disadvantages, particularly with regard to handheld, battery-powered devices. It is susceptible to carrier-frequency errors from local-oscillator (LO) offset and/or Doppler shifts while in motion. It also has a high peak-to-average power ratio (PAPR).

With regard to frequency errors, ideally the carrier signal and receiver LO are at the same frequency. Both systems will undergo some drift, though. Therefore, active processing is required to keep them in sync. With Doppler shift, speed and path can cause frequent differences between the carrier and LO. With OFDMA, then, signal frequencies must be diligently and continuously monitored to avoid relatively long frequency mismatches and excessive ICI with increased packet errors.

When it comes to PAPR, the instantaneous transmitted RF power can vary dramatically within a single OFDM symbol. Subcarrier voltage adding in phase at some points within a symbol will produce high instantaneous peak power that is considerably higher than average power. Hence, one ends up with a high PAPR, which directly affects the dynamic range requirements for analog-to-digital and digital-to-analog converters.