Long Term Evolution (LTE): a New Air Interface for Wireless Access
By Jan Whitacre, Agilent Technologies
While many owners use their mobile phones only for voice calls and short message services (texting), a large and growing number are using bandwidth-hungry applications such as web browsing, music downloads and streamed video. To match the way these kinds of applications are seen to work on a home PC with a broadband connection (either a DSL or cable modem), mobile network operators have been continuously investing in technology upgrades to remain competitive.
Third-generation wireless communication systems based on Wideband Code-Division Multiple Access (W-CDMA) have been deployed all over the world. To achieve higher downlink and uplink speeds, Universal Mobile Telecommunication System (UMTS) operators today are upgrading their 3G networks with High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), which are known collectively as HSPA and continue to evolve under the name HSPA+.
To meet the future demand for ever-higher data rates, Long Term Evolution (LTE) is the project name of a new air interface for wireless access being developed by the Third Generation Partnership Project (3GPP). It’s in the standards-setting and early development now, and is currently expected to be commercially introduced around 2010. LTE is the evolution of 3GPP’s UMTS towards an all-IP network, and the specifications provide a framework for increasing capacity, improving spectrum efficiency, improving cell-edge performance, and reducing latency. LTE offers a 100 Mbps download rate and 50 Mbps upload rate for every 20 MHz of spectrum. Support is intended for even higher rates, to 326.4 Mbps in the downlink, using multiple antenna configurations.
3GPP LTE is one of five major wireless standards sometimes referred to as “3.9G.” The others are HSPA+; 3GPP EDGE Evolution; 3GPP2 Ultra-Mobile Broadband (UMB), which is an evolution of CDMA2000 and 1xEV-DO, and Mobile WiMAX™, which is based on IEEE 802.16e. All have similar goals in terms of spectral efficiency, achieved primarily through the use of less robust, higher order modulation schemes and multi-antenna technology that ranges from basic transmit-and-receive diversity to Multiple Input/Multiple Output (MIMO). WiMAX is considered by many observers to be the major competitor. While LTE is gaining momentum and is a natural evolution of the established GSM-UMTS cellular legacy, WiMAX technology has the advantage of a head start in development, testing, and deployment. Regardless of which format ultimately dominates the market, LTE is expected to be a major force.
In parallel with its air interface development, LTE is linked closely with the concurrent System Architecture Evolution (SAE) project to define the LTE system architecture and Evolved Packet Core (EPC) network. Aimed at simplifying and speeding up network interaction with individual user equipment, SAE is critical to meeting many of the major speed and latency goals of LTE.
In LTE, rather than further developing modulation schemes based on Wideband Code Domain Multiple Access (W-CDMA), downlink and uplink transmissions are based on a new air interface: specifically, Orthogonal Frequency Division Multiple Access (OFDMA), a variant of Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Single-Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink.
Already used in non-cellular technologies as far back as 1998, OFDM was at that time under consideration by 3GPP as a transmission scheme for 3G UMTS. However, the technology was deemed inappropriate, in part because of the large amounts of baseband processing it required. Today, the cost of digital signal processing has been greatly reduced, such that it is now considered a commercially viable method of wireless transmission for the handset. Rather than transmit a high-rate stream of data with a single carrier, OFDM makes use of a large number of closely spaced orthogonal sub-carriers that are transmitted in parallel. Each sub-carrier is modulated with a conventional modulation scheme (such as QPSK, 16QAM, or 64QAM) at a low symbol rate. The combination of hundreds or thousands of sub-carriers enables data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
When compared to W-CDMA, OFDM offers a number of distinct advantages:
• Wide OFDM channels are more resistant to fading, and OFDM equalizers are much simpler to implement than CDMA equalizers.
• The long symbols, transmitted at low data rates separated by guard intervals that transmit the cyclic prefix, make OFDM almost completely resistant to multipath. This feature is particularly helpful for transmission in complex radio environments.
• Because OFDM can easily match transmission signals (sub-carriers) to the uncorrelated RF channels, the technology is well-suited to Multiple Input, Multiple Output (MIMO) implementations.
However, pure OFDM creates high peak-to-average ratio (PAR) signals, which would cause design issues that compromise the battery life of user equipment; that is why a modification of the technology called SC-FDMA is used in the uplink.
SC-FDMA was chosen because it combines the low PAR techniques of single-carrier transmission systems such as GSM and CDMA with the multipath resistance and flexible frequency allocation of OFDM/OFDMA. Another name for SC-FDMA is Discrete Fourier Transform Spread OFDM (DFT-SOFDM).
Figure 1 shows in frequency and time how OFDMA and SC-FDMA would each transmit a sequence of 8 QPSK symbols. In the OFDMA example, four symbols are taken in parallel, each of them modulating its own sub-carrier at the appropriate QPSK phase. Each data symbol occupies 15 kHz for one OFDMA symbol period. At the end of the symbol period, the guard interval containing the cyclic prefix (CP – a repeat of the first part of the symbol) is inserted before the next symbol period carrying the next four symbols arrives.
In the SC-FDMA case, data symbols are transmitted sequentially. Since this example involves four sub-carriers, four data symbols are transmitted sequentially in one SC-FDMA symbol period. The higher data rate symbols require four times the bandwidth, so each data symbol occupies 60 kHz of spectrum rather than 15 kHz. After the four data symbols have been transmitted, the CP is inserted. Note that the OFDMA symbol period and the SC-FDMA symbol period are the same.
As with the original W-CDMA and now HSPA, UE chipsets for LTE are being designed to have as long a life as possible so that manufacturers can recover their massive investment costs over a longer period. While the data rate supported by a chipset will be much greater than the rate actually available to the UE in a network, equipment suppliers must confirm correct operation up to its maximum specified rate.
Measurement products and solutions specifically designed to address the emerging needs of LTE must include support for new measurement methods to address mixed analog/digital radios, based on CPRI and OBSAI for base stations and DigRF and MIPI D-PHY for UEs, that remove or hide traditional test interfaces. Now people who previously dealt only in RF must learn new ways to characterize their devices. The tools required for these new measurements include system simulation, pattern generators, logic analyzers, signal generators, signal analyzers, and network emulation for protocol development.
As the world leader in test and measurement solutions, Agilent Technologies is at the forefront of emerging wireless and broadband markets. Agilent has up-to-date, reliable LTE design automation and test solutions that are available today, and is committed to providing the most complete measurement coverage – from RF to digital – throughout the entire product development cycle.
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