The development of Long-Term Evolution (LTE) and WiMAX has become, well, long term. Both technologies use advanced methods like orthogonal frequency-division multiple access (OFDMA) and multiple-input multiple-output (MIMO) (see “All Hail OFDM”). They’re also fully IP-based (Internet Protocol), offering high-speed data capability to deliver fast Internet access and advanced applications like video.
But do these standards really represent the fourth generation (4G) of wireless technology? The International Telecommunications Union (ITU) defines LTE and WiMAX as 3G. According to some experts, both are almost 4G, more like 3.9G. Perhaps 4G still awaits us in the future.
While these two systems may seem to compete, with one eventually winning at the expense of the other, that’s not always the case. Competition exists for data services in some geographic areas, but it’s not all-encompassing.
Both developed from different backgrounds and have now essentially found their separate places. LTE is the clear-cut cellphone successor to the UMTS/WCDMA/HSPA and cdma2000 3G technologies, while WiMAX is finding use in broadband wireless connectivity and back haul.
LONG-TERM-EVOLUTION
LTE is the Third Generation Partnership Project (3GPP) name for the worldwide 4G cell-phone standard. It’s the planned and agreed-upon successor to the current 3G technologies.
The technology evolved from the original GSM voice technology to GPRS and EDGE for data to the current UMTS WCDMA and HSPA advanced 3G methods. Most of the standard is complete at this point, but is not yet finalized. It’s currently working through the tedious steps of the ITU standardization process, and final completion is expected later this year.
Most cellular operators have agreed to adopt LTE as the 4G standard, getting almost everyone around the world on board. That includes carriers like Verizon and Sprint in the U.S., who use cdma2000, which differs from the UMTS WCDMA standard. China, though, wants an LTE variation based on time-division duplexing (TDD) rather than frequency-division duplexing (FDD). More than 30 operators have agreed to adopt LTE and plan to integrate it into their systems in the coming years.
Right now, the only operational LTE systems are trial test systems in various parts of the world. We will see some formal LTE in 2010, but most carriers suggest deployment in scale beginning in 2011 or 2012 and beyond.
The still new 3G systems are working well and are still rolling out. You can’t blame the operators, who want to get a little mileage and profit out of their most recent 3G investments before implementing a newer and even more complex and expensive technology.
LTE is the inevitable choice. Its greater subscriber capacity and higher data rates support current and forthcoming services such as video, as well as data-intensive services like Internet access on the handset.
It’s designed for mobile operations with downlink (DL) data rates as high as 100 Mbits/s and uplink (UL) rates of 50 Mbits/s. Compare that to the current maximum 14-Mbit/s DL and 5.7-Mbit/s UL for most HSPA 3G services. Evolved HSPA standards define maximum DL and UL rates of 84 Mbits/s and 22 Mbits/s, but such systems aren’t available. With LTE, these high rates enable fantastic access to video, Internet services, games, and other high-intensity applications.
Current WCDMA and HSPA channels use 5 MHz. LTE is designed to fit into different bandwidths, including 1.4, 3, 5, 10, 15, and 20 MHz. Each bandwidth uses 128, 256, 512, 1024, 1536, and 2048 fast Fourier transform (FFT) subchannels, respectively.
LTE can function on virtually any current cellular frequency assignment that has enough bandwidth. Not all frequencies can accommodate the wider bandwidth modes. In the U.S., look for LTE in the 2.1-GHz bands and in 700-MHz assignments.
LTE is strictly FDD, where the send and receive bands are separate from one another. The downlink and uplink frequency separation varies widely from about 12 to 18 MHz on the lowest bands and as much as 340 to 560 MHz on the higher frequencies. The highest data rates need wide bandwidths, and they use lots of spectrum space.
Quadrature phase-shift keying (QPSK), 16-phase quadrature amplitude modulation (16QAM), and 64-state QAM (64QAM) are the modulation methods, depending on bandwidth availability and data-rate needs. Different levels of MIMO are supported, including 4x4, 2x2, 2x1, and 1x1 for the downlink. Typical cell capacity is in the 200-subscriber range with a bandwidth of 5 MHz. Wider bandwidths can increase this to greater than 400 users per cell site. Spectral efficiency is in the 5-bit/Hz range, but that depends on the physical-layer (PHY) details.
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