Defining the 4G PHY architecture design challengesLTE-Advanced (LTE-A) is an emerging mobile communications standard that boasts a number of significant benefits, including the ability to take advantage of advanced topology networks. Specified as part of Release 10 of the 3GPP specifications, it is now approved for 4G IMT-Advanced. To a large extent, LTE-A builds on top of LTE parameters and even maintains some of its basic structures. However, it also incorporates new features consisting of enhancements to LTE Release 8/9, as well as newer emerging technologies captured in LTE Release 10 and beyond (Figure 1). As with most emerging technologies, design challenges for LTE-A abound, particularly when it comes to the physical layer (PHY) architecture development. Addressing these challenges remains critical to the successful development and deployment of LTE-A designs.
Figure 1. The new features of LTE-A include both enhancements to LTE Release 8/9 and newer emerging technologies. Proposed solutions for achieving the performance targets for the LTE-A radio interface are defined in 3GPP TR 36.814, “Further Advancements for E-UTRA Physical Layer Aspects.”
To better understand the challenges associated with designing LTE-A, it’s important to first have a clearer understanding of their root cause. In this case, the cause is quite obvious: the increased complexity brought on by the LTE-A enhancements themselves. Relative to LTE Release 8/9, LTE-A incorporates three key enhancements: carrier aggregation (CA), an enhanced multiple access scheme, and enhanced MIMO transmission.
Carrier aggregation is the mechanism by which LTE-A specifies spectrum allocations of up to 100 MHz. It allows the aggregation of contiguous and non-contiguous component carriers to provide the wider bandwidth (Figure 2). LTE-A also employs an enhanced multiple access scheme in the uplink (UL) known as clustered Single Carrier-Frequency Division Multiple Access (SC-FDMA). Similar to LTE’s SC-FDMA, this enhanced scheme has the advantage of allowing non-contiguous (clustered) groups of subcarriers to be allocated for transmission by a single piece of user equipment. This enables UL frequency-selective scheduling and better link performance.
Figure 2. LTE-A specifies spectrum allocations of up to 100 MHz using carrier aggregation. This image illustrates carrier aggregation with contiguous carrier components and with non-contiguous carrier components.
In addition to wider bandwidth, LTE-A is expected to provide higher data rates and improved system performance. Achieving these goals requires an extension of LTE’s support for multi-antenna transmission. For the LTE-A downlink (DL), up to eight layers can be transmitted using an 8x8 antenna configuration. This allows for a peak spectral efficiency exceeding the requirement of 30 bits/s/Hz and implies a possibility for data rates beyond 1 Gbit/s in a 40-MHz bandwidth and even higher data rates with wider bandwidth. LTE-A will also include spatial multiplexing of up to four layers for the UL, enabling a peak UL spectral efficiency that exceeds 15 bits/s/Hz.
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