Moving to LTE-Advanced… before LTE arrives!

Moving to LTE-Advanced… before LTE arrives!

True fourth-generation wireless service may be just around the corner; will it mean a significant change in instrumentation?

BY ANDREAS ROESSLER
Rohde & Schwarz, Columbia, MD
http://www.rohde-schwarz.com

As the Long Term Evolution (LTE) of UMTS networks begins its worldwide rollout based on 3rd Generation Partnership Project (3GPP) Release 8, and as new features are considered for Release 9, big improvements are already being discussed for the next enhancement — LTE-Advanced — which will form Release 10.

LTE-Advanced will add significant features to those of previous releases. Together, they will produce a true fourth-generation wireless technology with the potential for very high data rates that have so long been promised but haven’t yet appeared.

“What!” you say, “LTE is not a 4G standard?” If you’re talking to the marketing department, the answer is “Of course it is.” But if you ask the technical community, they’ll respond “No, it isn’t.” A little history explains why.

The concepts embodied in LTE-Advanced were formed in 2008 when the International Telecommunication Union (ITU) grouped them together within the term IMT (International Mobile Telecommunications) -Advanced. This separated IMT-Advanced from the capabilities of past digital wireless standards, which are included under the umbrella of the ITU’s original global wireless plan called IMT-2000. The latter includes UMTS/WCDMA, CDMA 20001xRTT and 1xEV-DO, TD-SCDMA, WiMAX, and LTE Release 8.

The data rate requirements of IMT-Advanced (100 Mbits/s in high-mobility scenarios and 1 Gbit/s in fixed- and low-mobility conditions) effectively rendered it impossible for LTE Release 8 to be considered a true 4G mobile communication system, even though some of its requirements were met (see Table 1 ). So LTE is not officially a 4G technology, but rather a major step in that direction. However, as Table 2 shows, LTE-Advanced takes a considerable step beyond LTE.

Table 1. Comparing LTE release 8, IMT-Advanced, and LTE-Advanced

Table 2. Potential, cost, and benefits of LTE-Advanced

Achieving 4G

LTE-Advanced will strive to improve LTE through higher peak and average data rates, greater spectrum efficiency, and reduced latency in the control and user planes. To achieve these goals, current LTE features have been improved and new ones defined.

Higher uplink and downlink peak data rates can be achieved by enhancing multiple input, multiple output (MIMO) capabilities and through carrier aggregation. MIMO in its simplest form means the use of multiple transmit or receive antennas or both (diversity) to achieve greater performance. For LTE-Advanced, single-user MIMO in the downlink can be up to 8 transmit and 8 receive (8 x 8 configuration) antennas and now also 4×4 in the uplink, which has not been defined with LTE as of Release 8.

Carrier aggregation is a method for combining the available spectrum in order to accommodate the 100-MHz maximum bandwidth defined with IMT-Advanced. However, as 100 MHz of continuous frequency spectrum is not available to any carrier worldwide, carrier aggregation (see Fig. 1 ) allows frequencies in different blocks to be combined to produce something reasonably close.

Fig. 1. The principle of contiguous and noncontiguous carrier aggregation in LTE-Advanced.

Up to five carriers, each with bandwidths up to 20 MHz, produce the transmission bandwidth of 100 MHz, and, to ensure backward compatibility, each carrier can be configured to be compliant with 3GPP Release 8, but need not be compatible with it.

Even if all of a carrier’s licensed frequency blocks are combined, the result will not be 100 MHz, and new spectrum allocations cannot be made until 2015, when the World Radio Conference (WRC) next convenes. Consequently, initial LTE-Advanced deployments will likely be limited to two or three carriers for a maximum downlink/uplink bandwidth of either 40 or 50 MHz, depending on the mode (FDD or TDD).

With LTE in Release 8, transmission of data and control information is decoupled and the terminal uses the physical uplink control channel (PUCCH) only to transmit information when it does not have to transmit any data on the physical uplink shared channel (PUSCH). This is no longer the case with LTE-Advanced, in which simultaneous transmission of PUCCH and PUSCH is possible, providing a significant increase in average throughput.

Release 8 uses a “localized” SC-FDMA mode (modulation symbols are assigned to adjacent subcarriers). This continuous mapping provides multiuser gain in the frequency domain. In Release 10, the uplink transmission scheme is extended to support clustered allocation of subcarrier (resource blocks). This enables the flexibility benefits of frequency-selective scheduling, but increases the peak-to-average power ratio, which makes more demands on transceiver and power amplifier designers to maintain linearity.

High speed on ‘the edge’

Peak spectrum efficiency can be achieved by using the highest possible level of MIMO and highest-order modulation scheme — which for the downlink means 8 x 8 MIMO and 64QAM modulation. Both enhancement techniques require a significant high signal-to-noise ratio, which is not likely to be present at the edge of the cell’s coverage area.

To improve performance in this area, LTE-Advanced improves spectral efficiency using Coordinated Multiple Point Transmission and Reception (CoMP). The CoMP concept (see Fig. 2 ) coordinates and combines signals from multiple base stations to maintain the high data rates necessary to allow LTE-Advanced to achieve its full potential, especially at or near the cell edge.

Fig. 2. Basic CoMP principles take signals from multiple base stations to maintain the high data rates.

A user at the edge of a cell’s coverage area may be able to receive signals from multiple cell sites, and the user’s transmitted signal may be receivable at multiple cell sites. Assuming this, by coordinating the signaling from the multiple cell sites, downlink performance can be significantly improved. Coordination can be as simple as focusing on interference avoidance. In the uplink, the signal can be received by multiple cell sites, and through coordination of different cell sites the network can use this multiple reception to improve link quality.

New challenges, new boxes?

It is reasonable to expect that the major LTE-Advanced enhancements will need new test equipment, but thanks to trends in test equipment design, this is not necessarily the case. For example, the R&S SMU200A vector signal generator and R&S AMU200A baseband signal generator/fading simulator combine two signal generators in one instrument so multiple component carriers can be generated. Using a single SMU200A configured with two baseband units, two component carriers up to 20 MHz in bandwidth can be generated and faded in real-time either with contiguous or non-contiguous placement. Generating and aggregating more component carriers is possible with additional signal generators.

If no real-time fading and individual power leveling are required, arbitrary multicarrier waveform signal generation is another option that simplifies the setup. With this approach, an instrument such as the R&S SMBV100A vector signal generator, which has a 120-MHz bandwidth, large waveform memory, and high clock rate, can generate complex modulated multicarrier waveforms for the proposed contiguously-deployed 100-MHz bandwidth of LTE-Advanced.

So LTE-Advanced promises performance gains in peak data rates, spectral efficiency, performance at the cell edge, overall coverage. and the delivery of what users of wireless-enabled devices have long wanted — truly high-speed data throughput equal to, or perhaps even better than, a wired solution at home. LTE-Advanced, which is at least four years away from deployment, will be received in direct proportion to how well its “3.9G” predecessor, LTE, is received in the coming years. However, based on its potential, it will be worth the wait. ■