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Examining LTE-A Release 12 transmitter (Part 2)

30 Mar 2015  | Damian Anzaldo

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Here is a two-part article series that reviews new developments in the Fourth Generation Long Term Evolution (4G-LTE) cellular standard. The articles explore LTE-Advanced (LTE-A) Release-12 (Rel-12) features and their impact on eNodeB radio frequency (RF) transmitters. The articles reveal how analogue integration can overcome design challenges arising from the latest 4G developments.

Part 1 was on Technology Evolution and examined market forces driving global adoption of the LTE standard and trends in fourth-generation (4G) radio access technology.

This article explores the analogue integration challenges in 4G base stations. Rel-12 features, such as wideband downlink CA, downlink multiple-input multiple-out (MIMO) spatial multiplexing, and AAS with embedded RF, present new design challenges in next-generation eNodeB radios. A bits-to-RF solution can help engineers shape alternative radio transmitter architectures (an example is given). The discussion focuses on novel RF digital-to-analogue converter (RF-DAC) technology that yields a single-chip, wideband RF transmitter. Topics include system-level applications of RF-DAC and the integration benefits that it delivers to eNodeB radio design.


Background
LTE is recognised as the fastest growing mobile broadband technology and becoming the most widely adopted cellular standard. The popularity of LTE is mainly due to its high spectral efficiency and high peak data rates, low-latency IP-based network, and evolutionary roadmap. But LTE is not "true 4G" service and is technically still considered 3.9G.

The "true 4G" radio communication standard, known as International Mobile Telecommunications-Advanced (IMT-Advanced), must meet the requirements set forth by the International Telecommunication Union Radio Sector (ITU-R). IMT-Advanced defines 4G as a service that delivers 100Mbps peak data rates for high-mobility users, and 1Gbps peak data rates for low-mobility clients. To comply with the IMT-Advanced vision, the 3GPP has developed many enhancements since the initial LTE Rel-8 standard published in 2008.

In Rel-10 the 3GPP introduced LTE-Advanced as "true 4G" service to meet or exceed the IMT-Advanced requirements. Presently, Rel-12 is close to introduction with a functional freeze date planned for March 2015. Figure 1 illustrates LTE development timelines, where it can be seen that theoretical peak downlink (DL) and uplink (UL) data rates have increased about 10x and 20x, respectively, from DL = 300Mbps/UL = 75Mbps in Rel-8 to DL = 3Gbps/UL = 1.5Gbps in Rel-10. The extraordinary increase in peak data rates is due, in part, to wideband CA complimented by multi-layer spatial multiplexing introduced in Rel-10 and now an important part of Rel-12 enhancements.


Figure 1: LTE release timeline showing evolutionary advancements in radio access technology.


Rel-12 enhancements will significantly impact how evolved NodeB (eNodeB) radios are designed. Some of the important Rel-12 items include new combinations of carrier aggregation, spatial multiplexing enhancements with downlink MIMO, and RF requirements needed in AAS. Figure 2 summarises some of the Rel-12 items with respective features and benefits. The Rel-12 feature enhancements bring many benefits to the LTE ecosystem, along with new radio design and radio architecture challenges. (For more detailed background information, Part 1).


Figure 2: The features and benefits of Release-12 work items.



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