Air interface for wireless access
14 May 2008
While many use their mobile phones for voice calls and texting, a growing number of people are using bandwidth-hungry applications such as web browsing, music downloads and streamed video
To match the way these kind of applications are seen to work on a home PC with a broadband connection, mobile network operators have been continuously investing in technology upgrades to remain competitive.
Third-generation wireless communication systems based on W-CDMA (wideband codedivision multiple access) have been deployed all over the world. To achieve higher downlink and uplink speeds, UMTS (universal mobile telecommunication system) operators are upgrading their 3G networks with HSDPA (high-speed downlink packet access) and HSUPA (high-speed uplink packet access), which are known collectively as HSPA and continue to evolve under the name HSPA+.
To meet the future demand for ever-higher data rates, LTE (long term evolution) is the project name of a new air interface for wireless access being developed by the 3GPP (third generation partnership project). It is in the standards-setting and early development stage, but should be introduced commercially 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 100Mbps download rate and 50Mbps upload rate for every 20MHz of spectrum. Support is intended for even higher rates, to 326.4Mbps in the downlink, using multiple antenna configurations.
One of five major wireless standards is 3GPP LTE; sometimes referred to as 3.9G. The others are HSPA+, 3GPP EDGE Evolution, 3GPP2 UMB (ultra-mobile broadband), 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 MIMO (multiple input/multiple output). WiMAX is considered by many observers to be the major competitor and although LTE is gaining momentum as a natural evolution of the established GSMUMTS 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 SAE (system architecture evolution) project to define the LTE system architecture and EPC (evolved packet core) 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 W-CDMA (wideband code domain multiple access), downlink and uplink transmissions are based on a new air interfaces. Specifically, these are OFDMA (orthogonal frequency division multiple access), a variant of OFDM (orthogonal frequency division multiplexing) in the downlink, and SC-FDMA (single-carrier frequency division multiple access) in the uplink.
Already used in non-cellular technologies in 1998, OFDM was, at that time, under consideration by 3GPP as a transmission scheme for 3G UMTS. However, the technology was deemed inappropriate because of the large amounts of baseband processing that it required. Now, the cost of digital signal processing has been greatly reduced to the point that it is now considered to be 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. For example, wide OFDM channels are more resistant to fading, and OFDM equalisers are simpler to implement than CDMA equalisers. Also, 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. Furthermore, because OFDM can easily match transmission signals (sub-carriers) to the un-correlated RF channels, the technology is suited to MIMO implementations. However, pure OFDM creates high PAR (peak-to-average ratio) signals, which would cause design issues that compromise the battery life of user equipment, and 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 DFT-SOFDM (discrete fourier transform spread OFDM).
OFDMA and SC-FDMA
Figure 1 shows in frequency and time how OFDMA and SC-FDMA would each transmit a sequence of eight 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 15kHz for one OFDMA symbol period. At the end of the symbol period, the guard interval containing the CP (cyclic prefix), 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 60kHz of spectrum rather than 15kHz. After the four data symbols have been transmitted, the CP is inserted. 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 analogue/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. People who previously only dealt in RF must learn new ways to characterise their devices. The tools required for these new measurements include system simulation, pattern generators, logic analysers, signal generators, signal analysers, and network emulation for protocol development.
JAN WHITACRE, LTE programme manager, wireless business unit, Agilent Technologies
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