Evolucion a largo termino

1 Introduction

The 3GPP Long Term Evolution (LTE) represents a major advance in cellular technology. LTE is designed to meet carrier needs for high-speed data and media transport as well as high-capacity voice support well into the next decade. It encompasses high-speed data, multimedia unicast and multimedia broadcast services.
The LTE PHY is a highly efficient means of conveying both dataand control information between an enhanced base station (eNodeB) and mobile user equipment (UE). The LTE PHY employs some advanced technologies that are new to cellular applications. These include Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) data transmission. In addition, the LTE PHY uses Orthogonal Frequency Division Multiple Access
(OFDMA) on thedownlink (DL) and Single Carrier – Frequency Division Multiple Access (SC-FDMA) on the uplink (UL).
These technologies will be described separately before delving into a description of the LTE PHY.
Although the LTE specifications describe both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD) to separate UL and DL traffic, market preferences dictate that the majority ofdeployed systems will be FDD.

1.1 LTE Design Goals

The LTE PHY is designed to meet the following goals:

1. Support scalable bandwidths of 1.25, 2.5, 5.0, 10.0 and 20.0 MHz

2. Peak data rate that scales with system bandwidth

a. Downlink (2 Ch MIMO) peak rate of 100 Mbps in 20 MHz channel
b. Uplink (single Ch Tx) peak rate of 50 Mbps in 20 MHz channel

3. Supported antennaconfigurations

a. Downlink: 4x2, 2x2, 1x2, 1x1
b. Uplink: 1x2, 1x1

4. Spectrum efficiency

a. Downlink: 3 to 4 x HSDPA Rel. 6
b. Uplink: 2 to 3 x HSUPA Rel. 6

5. Latency

a. C-plane: <50 – 100 msec to establish U-plane
b. U-plane: <10 msec from UE to server

6. Mobility

a. Optimized for low speeds (<15 km/hr)
b. High performance at speeds up to 120 km/hr
c. Maintain link atspeeds up to 350 km/hr

7. Coverage

a. Full performance up to 5 km
b. Slight degradation 5 km – 30 km
c. Operation up to 100 km should not be precluded by standard

2 LTE Basic Concepts

Before jumping into a detailed description of the LTE PHY, it’s worth taking a look at some of the basic technologies involved. Many methods employed in LTE are relatively new in cellular applications.These include OFDM, OFDMA, MIMO and Single Carrier Frequency Division Multiple Access (SC-FDMA).
LTE employs OFDM for downlink data transmission and SC-FDMA for uplink transmission. OFDM is a well-known modulation technique, but is rather novel in cellular applications.
When information is transmitted over a wireless channel, the signal can be distorted due to multipath. Typically (but notalways) there is a line-of-sight path between the transmitter and receiver. In addition, there are many other paths created by signal reflection on buildings, vehicles and other obstructions as shown in Figure 1. Signals traveling along these paths all reach the receiver, but are shifted in time by an amount corresponding to the differences in the distance traveled along each path.

2.1 Single CarrierModulation and Channel Equalization

To date, cellular systems have used single carrier modulation schemes almost exclusively. Although LTE uses OFDM rather than single carrier modulation, it’s instructive to discuss how single carrier systems deal with multipath-induced channel distortion. This will form a point of reference from which OFDM systems can be compared and contrasted.
The termdelay spread describes the amount of time delay at the receiver from a signal traveling from the transmitter along different paths. In cellular applications, delay spreads can be several microseconds. The delay induced by multipath can cause a symbol received along a delayed path to “bleed” into a subsequent symbol arriving at the receiver via a more direct path. This effect is depicted in Figure 2...