A deluge of media coverage has created high consumer demand and high expectations for 4G mobile technology. 3GPP’s Long Term Evolution (LTE) has emerged as the leading 4G contender, and just as mobile operators begin the lengthy process of deploying and testing their LTE networks, 3GPP has published the next release of the LTE standard, known as LTE-Advanced. With performance enhancements such as peak data rates to 1 Gbps, LTE-Advanced will meet or exceed the requirements of the International Telecommunication Union (ITU) (www.itu.int) for its 4G radio-communication standard, IMT-Advanced.

In the feasibility study for LTE-Advanced, 3GPP determined that today’s LTE (Release 8/9) could meet most of the ITU’s 4G requirements apart from uplink spectral efficiency and peak data rates. These requirements are being addressed in Release 10 LTE-Advanced with the following features: wider bandwidths, enabled by carrier aggregation; and higher efficiency, enabled by enhanced uplink multiple access and enhanced multiple antenna transmission (advanced MIMO techniques).

Release 8 LTE itself is new and complex, introducing such features as multiple channel bandwidths, different transmission schemes for the downlink and uplink, 2 transmission modes (FDD and TDD), and the use of multiple antenna techniques (MIMO). LTE-Advanced raises the bar on performance expectations, and the new technologies will have to co-exist and interoperate with each other and with legacy 2G and 3G deployments for years to come. The challenges for the engineers who design, test, and ultimately deploy LTE-Advanced are many.

New Features for the Release
The higher requirements of IMT-Advanced are addressed in 3GPP Release 10 with the addition of the following LTE-Advanced features:

Carrier Aggregation: To achieve 1 Gbps, LTE-Advanced supports bandwidths up to 100 MHz formed by aggregating up to five 20-MHz component carriers. Contiguous and non-contiguous component carriers may be aggregated. A simple example is shown in Figure 1. Although not considered a problem for the base station, carrier aggregation will undoubtedly pose major difficulties for user equipment (smart phones and other wireless devices), which must handle multiple simultaneous transceivers. The use of simultaneous, non-contiguous transmitters creates a highly challenging radio environment in terms of spur management and self-blocking. Simultaneous transmit or receive with mandatory MIMO support will add significantly to the challenge of antenna design.

Figure 1. Aggregation of contiguous component carriers.

Enhanced Uplink Multiple Access: LTE’s uplink is based on single-carrier frequency division multiplexing (SC-FDMA), which allocates carriers across a contiguous block of spectrum, thus limiting scheduling flexibility. LTE-Advanced introduces clustered SC-FDMA in the uplink, which allows frequency-selective scheduling of component carriers for better link performance. The PUCCH and PUSCH can be scheduled together to reduce latency. However, clustered SC-FDMA increases peak-to-average power ratio, leading to transmitter linearity issues, and the presence of multi-carrier signals increases opportunity for in-channel and adjacent-channel spur generation.

Advanced MIMO: To improve single-user peak data rates and meet spectral efficiency requirements, LTE-Advanced specifies up to eight transmitters in the downlink (with the requisite eight receivers in the UE) enabling 8x8 spatial multiplexing in the downlink. The UE supports up to 4 transmitters allowing up to 4x4 transmission in the uplink when combined with 4 receivers in the base station. (See Figure 2.)

Figure 2. LTE-Advanced Release 10 maximum number of antenna ports and spatial layers.

MIMO increases the number of antennas in the system, and MIMO antennas have to be de-correlated. A major challenge will be designing multi-band MIMO antennas with good de-correlation to operate in the small space of an LTE-Advanced UE. New methods are required for predicting actual radiated performance of an advanced MIMO terminal in an operational network, so 3GPP is considering ways to extend MIMO over the air (OTA) testing for LTE-Advanced.

Other performance enhancements are under consideration for future 3GPP releases, even though they are not critical to meeting 4G requirements:

• Coordinated multipoint transmission and reception (CoMP)
• Relaying
• Support for heterogeneous networks
• LTE self-optimizing network (SON) enhancements
• Home enhanced-node-B (HeNB) mobility enhancements
• Fixed wireless customer premises equipment (CPE)

Deploying LTE-Advanced
Industry-supported field trials are already demonstrating the viability of many of the technical concepts in LTE-Advanced. Additionally, operators are showing considerable interest in the higher data rates and spectral efficiency improvements. However, the timing of LTE-Advanced deployment is difficult to predict and will be dependent on industry demand and the success of today’s Release 8 and 9 LTE rollouts. LTE-Advanced represents a big increase in system and device complexity, and it will take time for the industry to respond.

When the Release 10 features of LTE-Advanced are initially deployed in the field, it is expected that the major challenges (beyond those already posed by LTE) will center around interference problems and spurs. In the challenging RF environment, high-performance handheld tools such as Agilent’s FieldFox RF analyzer will be invaluable for monitoring the spectrum and identifying interference signals.

Jan Whitacre is LTE Program Lead for Agilent Technologies. She has more than 20 years of experience in wireless testing. For more information, please email jan_whitacre@agilent.com or visit www.agilent.com/find/.

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