This application note describes the Long Term Evolution (LTE) of the universal mobile telecommunication system (UMTS), which is being developed by the 3rd Generation Partnership Project (3GPP). Particular attention is given to LTE's use of multiple antenna techniques and to a new modulation scheme called single carrier frequency division multiple access (SC-FDMA) used in the LTE uplink. Also, because the accelerated pace of LTE product development calls for measurement tools that parallel the standard's development, this application note introduces Agilent's expanding portfolio of LTE design, veriýcation, and test solutions.
Editor's Note:This is an excerpt from Agilent's application note. For full text, click here
LTE Concepts: Introduction
Third-generation UMTS, based on wideband code-division multiple access (WCDMA), has been deployed all over the world. To ensure that this system remains competitive in the future, in November 2004 3GPP began a project to define the long-term evolution of UMTS cellular technology. The specifications related to this effort are formally known as the evolved UMTS terrestrial radio access (E-UTRA) and evolved UMTS terrestrial radio access network (E-UTRAN), but are more commonly referred to by the project name LTE. The first version of LTE is documented in Release 8 of the 3GPP specifications.
3GPP's high-level requirements for LTE include reduced cost per bit, better service provisioning, flexible use of new and existing frequency bands, simplified network architecture with open interfaces, and an allowance for reasonable power consumption by terminals. These are detailed in the LTE feasibility study, 3GPP Technical Report (TR) 25.9121, and in the LTE requirements document, TR 25.913.2
Technical specifications for LTE are scheduled to be completed during the first half of 2008 with the UE conformance test specifications appearing towards the end of 2008. Commercial deployment is not expected before 2010, although there will be many field trials before then.
The timeline is acknowledged to be aggressive and although major progress has been made, many details are still to be finalized. The test specifications may enable user equipment (UE) certification by the first quarter of 2009, but actual UE certification will only be possible if commercial devices are available before this date to allow test system validation. In practice, test system validation and UE certification are likely to be later.
Summary of LTE Requirements
To meet the requirements for LTE outlined in TR 25.913, LTE aims to achieve the following:
- Increased downlink and uplink peak data rates, as shown in Table 1 in the complete application note. Note that the downlink is specified for single input single output (SISO) and multiple input multiple output (MIMO) antenna configurations at a fixed 64QAM modulation depth, whereas the uplink is specified only for SISO but at different modulation depths. These figures represent the physical limitation of the frequency division duplex (FDD) air interface in ideal radio conditions with allowance for signaling overheads. Lower rates will be specified for specific UE categories under non-ideal radio conditions.
- Scalable bandwidth from 1.4 to 20 MHz in both the uplink and the downlink
- Spectral efficiency, with improvements over Release 6 high speed packet access (HSPA) of three to four times in the downlink and two to three times in the uplink
- Sub-5 ms latency for small internet protocol (IP) packets
- Optimized performance for low mobile speeds from 0 to 15 km/h; supported with high performance from 15 to 120 km/h; functional from 120 to 350 km/h. Support for 350 to 500 km/h is under consideration
- Co-existence with legacy standards while evolving toward an all-IP network3
History of the UMTS Standard
LTE represents the future of the UMTS standard as it evolves from an architecture that supports both circuit-switched and packet-switched communications to an all-IP, packet-only system. To this end, development of the LTE air interface is linked closely with the concurrent 3GPP system architecture evolution (SAE) project to define the overall system architecture and evolved packet core (EPC) network.
Table 2 summarizes the history of the global system for mobile communication (GSM) and UMTS standards with the major features that have come to be associated with each release. To achieve higher downlink and uplink data rates, UMTS operators today are upgrading their 3G networks with high speed downlink packet access (HSDPA), which is specified in 3GPP Release 5, and high speed uplink packet access (HSUPA), which is specified in 3GPP Release 6. The formal name in the specifications for HSUPA is the enhanced dedicated channel (E-DCH). HSDPA and HSUPA are known collectively as HSPA and they continue to evolve in Release 7 and Release 8 under the name HSPA+.
Release 8 specifies LTE and SAE as well as further enhancements to the existing technologies HSPA+ and EDGE. In September 2007 the LTE physical layer specifications were released at version 8.0.0.
Finalization of the rest of the specifications should occur in the ýrst half of 2008, and the UE conformance test specifications will start to appear towards the end of 2008.
LTE in Context
3GPP LTE is one of five major wireless standards sometimes referred to as "3.9G." The other so-called 3.9G standards are: 3GPP HSPA+, 3GPP EDGE Evolution, 3GPP2 ultra-mobile broadband (UMB), and Mobile WiMAX (IEEE 802.16e), which encompasses the earlier WiBro developed by the Telecommunications Technology Association (TTA) in Korea.
All have similar goals in terms of improving spectral efficiency, with the widest bandwidth systems providing the highest single-user data rates. Spectral efficiencies are 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 the more advanced MIMO spatial diversity.
Of the 3.9G standards, EDGE evolution and HSPA+ are direct extensions of existing technologies. Mobile WiMAX is based on the existing IEEE 802.16d standard and has had limited implementation in WiBro. Both UMB and LTE are considered "new" standards.
3GPP LTE Specification Documents
Release 7 of the 3GPP specifications included the study phase of LTE. As a result of this study, requirements were published in TR 25.913 for LTE in terms of objectives, capability, system performance, deployment, E-UTRAN architecture and migration, radio resource management, complexity, cost, and service.
E-UTRA, E-UTRAN, and the EPC are defined in the 36-series of 3GPP Release 8:
- 36.100 series, covering radio specifications and evolved Node B (eNB) conformance testing
- 36.200 series, covering layer 1 (physical layer) specifications
- 36.300 series, covering layer 2 and 3 (air interface signaling) specifications
- 36.400 series, covering network signaling specifications
- 36.500 series, covering user equipment conformance testing
- 36.800 and 36.900 series, which are technical reports containing
background information
The work on the specifications is ongoing, and many of the technical documents are updated quarterly. The latest versions of the 36-series documents can be found at http://www.3gpp.org/ftp/specs/archive/36_series/.
LTE Air Interface Radio Aspects
The LTE radio transmission and reception specifications are documented in TS 36.1016 for the UE and TS 36.1047 for the eNB.
Radio access modes
The LTE air interface supports both FDD and time division duplex (TDD) modes, each of which has its own frame structure. Additional access modes may be defined, and half-duplex FDD is being considered. Half-duplex FDD allows the sharing of hardware between the uplink and downlink since the uplink and downlink are never used simultaneously. This technique has uses in some frequency bands and also offers a cost saving at the expense of a halving of potential data rates.
The LTE air interface also supports the multimedia broadcast and multicast service (MBMS), a relatively new technology for broadcasting content such as digital TV to UE using point-to-multi-point connections. The 3GPP specifications for MBMS first appeared for UMTS in Release 6. LTE will specify a more advanced evolved MBMS (eMBMS) service, which operates over a Multicast/ Broadcast over single-frequency network (MBSFN) using a time-synchronized common waveform that can be transmitted from multiple cells for a given duration. The MBSFN allows over-the-air combining of multi-cell transmissions in the UE, using the cyclic preýx (CP) to cover the difference in the propagation delays.
To the UE, the transmissions appear to come from a single large cell. This technique makes LTE highly efficient for MBMS transmission. The eMBMS service will be defined in Release 9 of the 3GPP specifications.
Transmission bandwidths
LTE must support the international wireless market and regional spectrum regulations and spectrum availability. To this end the specifications include variable channel bandwidths selectable from 1.4 to 20 MHz, with subcarrier spacing of 15 kHz. If the new LTE eMBMS is used, a subcarrier spacing of 7.5 kHz is also possible. Subcarrier spacing is constant regardless of the channel bandwidth. 3GPP has defined the LTE air interface to be "bandwidth agnostic," which allows the air interface to adapt to different channel bandwidths with minimal impact on system operation.
The smallest amount of resource that can be allocated in the uplink or downlink is called a resource block (RB). An RB is 180 kHz wide and lasts for one 0.5 ms timeslot. For standard LTE, an RB comprises 12 subcarriers at a 15 kHz spacing, and for eMBMS with the optional 7.5 kHz subcarrier spacing an RB comprises 24 subcarriers for 0.5 ms. The maximum number of RBs supported by each transmission bandwidth is given in Table 3.
Multiple access technology in the downlink: OFDM and OFDMA
Downlink and uplink transmission in LTE are based on the use of multiple access technologies: specifically, orthogonal frequency division multiple access (OFDMA) for the downlink, and single-carrier frequency division multiple access (SC-FDMA) for the uplink.
The downlink is considered first. OFDMA is a variant of orthogonal frequency division multiplexing (OFDM), a digital multi-carrier modulation scheme that is widely used in wireless systems but relatively new to cellular. Rather than transmit a high-rate stream of data with a single carrier, OFDM makes use of a large number of closely spaced orthogonal subcarriers that are transmitted in parallel. Each subcarrier 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 subcarriers enables data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
The diagram in Figure 5 taken from TS 36.8929 illustrates the key features of an OFDM signal in frequency and time. In the frequency domain, multiple adjacent tones or subcarriers are each independently modulated with data. Then in the time domain, guard intervals are inserted between each of the symbols to prevent inter-symbol interference at the receiver caused by multi-path delay spread in the radio channel.
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