Introduction

Long Term Evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) was developed to ensure that the technology remains competitive for the foreseeable future. Requirements for the LTE Rel-8 system include improved system capacity and coverage, improved user experience through higher data rates and reduced latency, reduced deployment and operating costs, and seamless integration with existing systems. The requirements may be broken down into different categories – system performance, latency, coverage, deployment, and complexity. To achieve these goals, new designs for the radio access networks and system architectures are needed.

The next-generation wireless broadband technology is changing the way we work, live, learn, and communicate through effective use of state-of-the-art mobile broadband technology. The packet-data-based revolution started around 2000 with the introduction of 1x Evolved Data Only (1xEV-DO) and 1x Evolved Data Voice (1xEV-DV) in 3GPP2 and High Speed Downlink Packet Access (HSDPA) in 3GPP. The wireless broadband fourth-generation technology (4G) is an evolution of the packet-based 3G system and provides a comprehensive evolution of the Universal Mobile Telecommunications System specifications so as to remain competitive with other broadband systems such as 802.16e (WiMAX). Specification work was started in late 2004 on Long Term Evolution (LTE) of the UMTS Terrestrial Radio Access and Radio Access Network intended for commercial deployment in 2010. Two main components constitute the LTE system architecture — the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and the Evolved Packet Core (EPC).

Introduction

OFDM systems naturally benefit from the use of multi-antenna systems (MASs), which improves the capacity and coverage of the LTE system significantly. In the downlink, four different multi-antenna transmission techniques are supported – transmit diversity, closed-loop spatial multiplexing using precoding codebooks, open-loop spatial multiplexing, and user-specific reference-symbol-based beamforming [1]–[3]. Spatial multiplexing can be used to support single-user MIMO (SU-MIMO), whereby multiple data streams (or spatial layers in LTE terminology) are transmitted to the same user simultaneously in the same time–frequency resource, or multi-user MIMO (MU-MIMO), whereby multiple data streams (or layers) are transmitted to different users simultaneously using the same time–frequency resource. A significant gain in system capacity can be achieved with MIMO [4]. In the uplink, SU-MIMO is not possible since the UE can only transmit on one antenna. However, MU-MIMO can be supported in the uplink. In this chapter, a comprehensive description of various multi-antenna technologies for Rel-8 downlink and uplink is presented, together with details of their performance.

Introduction

Rel-8 LTE delivers improved system capacity and coverage, improved user experience through higher data rates, reduced-latency deployment, and reduced operating costs, and seamless integration with existing systems. Further enhanced requirements, however, were approved in 2008 to allow LTE to be approved as a radio technology for International Mobile Telecommunications-Advanced (IMT-Advanced). IMT-Advanced requirements are defined by the International Telecommunication Union, which is an organization that provides globally accepted standards for telecommunications. This further advancement for LTE is known as LTE-Advanced (LTE-A). The LTE-A requirements are shown in and focus mainly on improvements in system performance and latency reduction. From Table 6.1, it can be seen that the target cell and user spectral efficiencies have increased significantly. Peak data rates of 1 Gbps in the downlink and 500 Mbps in the uplink must be supported. Target latencies have been significantly reduced as well. In addition to advancements in system performance, deployment and operating-cost-related goals were also introduced. They include support for cost-efficient multi-vendor deployment, power efficiency, efficient backhaul, open interfaces, and minimized maintenance tasks. A comprehensive list of LTE-A requirements can be found in [1].

Introduction

LTE uplink provides an increase in capacity (both sector and cell-edge) by a factor of 2–3 compared with previous UMTS high-speed uplink packet access (HSUPA) systems at substantially less latency. This enables efficient support of high-rate data services such as FTP, HDTV broadcast, and HTTP as well as delay-sensitive services such as VoIP and video streaming. In LTE, several technological enhancements have been introduced in the uplink air interface to enable this improvement. They include orthogonal uplink transmission from intra-cell users, frequency-selective scheduling, shorter subframe size, support for 64-QAM modulation, multi-user spatial multiplexing, subframe bundling, semi-persistent scheduling, fractional power control, inter-cell interference control, and efficient control channels.

Introduction

LTE Rel-8 and WiMAX are the two main wireless broadband technologies based on OFDM which are currently being commercialized. Both of these technologies are being enhanced (LTE-Advanced and 802.16m) so as to support higher peak rates, higher throughput and coverage, and lower latencies, resulting in a better user experience. Further, both LTE-Advanced and 802.16m were approved by the ITU as IMT-Advanced technology. Also several operators are considering deploying both these technologies or migrating their existing WiMAX system to LTE or 802.16m. In this chapter, these two main broadband technologies are compared with respect to their features and system performance. Also, WiMAX and LTE co-existence and migration scenarios are briefly discussed.

Introduction

In this chapter the details of LTE downlink transmission are discussed. The LTE downlink air interface uses the OFDM multiple-access technique described in Chapter 2. The use of OFDM transmission technology provides significant advantages over other radio transmission techniques. They include high spectral efficiency, support for broadband data transmission, the absence of intra-cell interference (i.e. multiple users in the same cell can share the same subframe without interfering with each other), resistance to inter-symbol interference arising from multipath operation, natural support for MIMO schemes, a low-complexity receiver, and support for frequency-domain techniques such as frequency-selective scheduling, a single-frequency network, and soft fractional frequency reuse. In addition to OFDM, LTE also utilizes several other features to enhance system performance and user experience. They include short frame size to minimize latency, a single-frequency network to provide high-data-rate broadcast services, VoIP support to increase voice capacity, coverage for very large cells, and coverage for high-speed users (up to 350 km/h) [1]–[2].

System analysis and performance metrics

In this book, system-level performance results based on comprehensive system simulations of cellular networks are provided. An example of the cellular layout used for system simulation is shown in Figure A.1. This is a typical 19-site, 57-cell system using a hexagonal grid. In this case, a cell is viewed as a sector of the physical site. However, in LTE each cell is treated as an independent eNB. The spacing between each site and the next is dependent on the deployment scenario. For example, in urban micro-cell deployment, the inter-site distance is 200 m. Users are dropped randomly into the simulation space. For instance, in urban micro-cell deployment, 570 users are randomly dropped. After the users have been dropped, long-term radio characteristics such as pathloss and shadowing are calculated. Users are then assigned to the cell using the minimum pathloss as the cell-selection criterion. For the urban micro-cell example, on average, approximately 10 users will be associated with each cell.

Genesis of wireless technology

The digital cellular technology revolution started with the introduction of GSM (Groupe Special Mobile) in the late 1980s. The GSM technology was based on time-division multiple access (TDMA) and was capable of supporting data services of up to 9.6 kbps. In the early 1990s, IS-95, a standard based on code-division multiple-access (CDMA) technology was introduced. This offered data rates of up to 14.4 kbps and improved spectral efficiencies over a GSM system. Subsequently, both these technologies evolved over time, with each phase offering higher peak rates and improved sector/edge spectral efficiencies. Both GSM and IS-95 CDMA evolved in different phases. In 1997, the Generalized Packet Radio System (GPRS) based on packet data instead of circuit data was standardized, followed by Enhanced Data Rates for Global Evolution (EDGE). Also, at the end of 1998, the Third-Generation Partnership Project (3GPP) was started. This was responsible for defining a third-generation (3G) wideband CDMA (WCDMA) standard based on the evolved GSM core network. At the same time the GSM standardization work was moved from ETSI SMG2 to 3GPP, and was called GERAN. Similarly, in the United States the IS-95 standard evolved to cdma2000 under the umbrella of Third-Generation Partnership Project 2 (3GPP2).