Antenna Systems & Technology - Summer 2014 - (Page 8)

FEATURE ARTICLE Optimizing RF Signal Performance to Improve LTE Coverage and Capacity By Tim Keller, Senior Product Manager - Westell Technologies Over the last decade, the proliferation of advanced mobile devices such as smartphones, tablets and phablets has led to an astounding growth in mobile data traffic. In 2012 alone, there was more mobile data traffic than all previous years combined. Furthermore, with data-intensive applications such as video, cloud computing, social media apps and machine-to-machine (M2M) connections still gaining in popularity, explosive data traffic growth is expected for many years. In response to the phenomenal growth in mobile traffic, network operators are increasingly turning to 4G LTE (Long-Term Evolution) to provide the capacity their subscribers demand. Designed from the start as a datacentric network with a flat architecture and an air interface optimized for data, LTE is ideal for today's operators and, as such, has seen tremendous growth since its first live deployment in 2009. In the past 12 months, LTE deployments have grown by approximately 76 percent to more than 270 networks worldwide, and that number is expected to grow to more than 350 by the end of 2014. All LTE networks are not created equal, however, as evidenced by a recent report from Open Signal that compares download speeds and coverage areas of LTE networks worldwide. The report found that download speeds varied widely from the fastest of more than 24 Mbps to the slowest of just 5.3 Mbps. Coverage areas, as measured by percentage of time spent on an LTE network, also varied widely, with the best LTE networks available to subscribers 93 percent of the time and the worst only available 38 percent. In the US, LTE coverage is respectable with LTE networks available to subscribers 67 percent of the time. Conversely, download speeds are on the very low end of the scale and average only 6.5 Mbps. This poor download speed for LTE networks in the US is surprising because these networks are relatively mature and have seen continuous infrastructure improvements. This clearly demonstrates that optimization of LTE networks, including optimization of RF signal performance, is necessary to meet the needs of data-intensive applications. LTE Air Interface Characteristics To meet the aggressive performance goals of LTE, the 3rd Generation Partnership Project (3GPP) designed the air interface with a focus on high peak data rates, low latency and flexible bandwidth options. 3GPP also recognized the different requirements associated with the uplink signal (mobile device to eNodeB) compared to the downlink signal (eNodeB to mobile device) and thus specified separate modulation schemes. For the downlink, orthogonal frequency-division multiple access (OFDMA) was chosen, which offers peak download speeds of up to 300 Mbps. For the uplink, single-carrier frequency-division multiple access (SC-FDMA) was chosen to relieve the stress on the mobile power amplifier at the expense of peak upload speeds. This disparity between uplink and downlink speeds implies that the focus for improved performance should be on the uplink, which must be optimized to ensure that maximum possible speeds are maintained. Degradation of the uplink signal will have a noticeable effect to the mobile subscriber. In addition to different modulation schemes, the maximum power specified for each transmitter (eNodeB or mobile device) varies greatly and impacts the coverage area of a cell site. Assuming propagation losses are the same for both uplink and downlink, the maximum signal range will be different. The coverage area of a given cell site is determined by the range of the uplink signal. The link imbalance is especially relevant in rural areas with low user density where all of the transmit power of the eNodeB can be allocated to a single user. However, even in urban areas, coverage is uplink limited, and in both cases, rural and urban, the imbalance results in coverage gaps that can be filled by optimizing uplink signal performance. Like all wireless communications systems, and as originally proposed by Claude Shannon, the maximum errorfree data rate achievable for an LTE wireless network is a function of bandwidth and the signal-to-noise ratio (SNR) of the communication system. To support flexible deployments, 3GPP specified bandwidths from 1.4 MHz up to 20 MHz. In addition, link adaptation was adopted that allows more efficient spectrum usage, mapping the most efficient modulation scheme to a particular signal-to-noise ratio. 8 Antenna Systems & Technology Summer 2014 www.AntennasOnline.com http://www.AntennasOnline.com

Table of Contents for the Digital Edition of Antenna Systems & Technology - Summer 2014

Editor’s Choice
Optimizing RF Signal Performance to Improve LTE Coverage and Capacity
Managing the Tower Top for LTE Advanced
Connectivity for Internet of Things Success
Capitalizing on Network Architecture Trends
Antennas
Components/Subsystems
Short-Range Wireless
Software / System Design
Test & Measurement
Industry News
Marketplace
Insatiable Hunger Feeds DAS

Antenna Systems & Technology - Summer 2014