Long Term Evolution (LTE) / System Architecture Evolution (SAE)

3GPP work on the Evolution of the 3G Mobile System started with the RAN Evolution Work Shop, 2 - 3 November 2004 in Toronto, Canada. The Work Shop was open to all interested organizations, members and non members of 3GPP. Operators, manufacturers and research institutes presented more than 40 contributions with views and proposals on the evolution of the Universal Terrestrial Radio Access Network (UTRAN).

It was also recommended that the Evolved UTRAN should bring significant improvements to justify the standardization effort and it should avoid unnecessary options. On certain aspects, the collaboration with 3GPP SA WGs was found to be essential: the new split between the Access Network and the Core Network, and the characteristics of the throughput that new services would require demanded close architectural coordination.

With the conclusions of this Work Shop and with broad support from 3GPP members, a feasibility study on the UTRA & UTRAN Long Term Evolution was started in December 2004. The objective was "to develop a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimized radio-access technology" The study focused on supporting services provided from the PS-domain, involving:

In adddition, the Next Generation Mobile Networks (NGMN) initiative, led by seven network operators (*) provided a set of recommendations for the creation of networks suitable for the competitive delivery of mobile broadband services. The NGMN goal is "to provide a coherent vision for technology evolution beyond 3G for the competitive delivery of broadband wireless services".

The NGMN long-term objective is to "establish clear performance targets, fundamental recommendations and deployment scenarios for a future wide area mobile broadband network". In a white paper (March 2006), they provided relative priorities of key system characteristics, System recommendations and detailed requirements.

Emphase was also on the IPR side, where the goal was "to adapt the existing IPR regime to provide a better predictability of the IPR licenses (...) to ensure Fair, Reasonable And Non-Discriminatory (FRAND) IPR costs" (NGMN White paper, March 2006).

All RAN WGs participated in the study, with collaboration from SA WG2 in the key area of the network architecture. The first part of the study resulted in agreement on the requirements for the Evolved UTRAN.

As a result, Technical Report (TR) 25.913 contains detailed requirements for the following criteria:

Study phase and requirements

The Study Item phase was concluded in September 2006 and the Work Item for 3G Long Term Evolution was created. As expected, in particular the E-UTRA system will provide significantly higher data rates than Release 6 WCDMA. The increase in data rate is achieved especially through higher transmission bandwidth and support for MIMO.

In particular, the study showed that simultaneous support for UTRA and E-UTRA UEs in the same spectrum allocation was possible.

Solutions chosen for the physical layer and layers 2/3 showed a convergence between paired spectrum and unpaired spectrum solutions for the Long Term Evolution (e.g. initial access, handover procedures, measurements, frame and slot structures).

Specification phase and achievements

The E-UTRAN overall architecture is described in Technical Specifications 36.300 and 36.401.

The E-UTRAN consist of eNBs which are interconnected with each other by the X2 interface. Each eNB is connected to the Evolved Packet Core (EPC) network by the S1 interface. On the User Plane the S1 interface terminates the Serving Gateway (S-GW), on the Signalling Plane the S1 interface terminates the Mobility Management Entity (MME).The eNBs are terminating points for Control- and User Plane towards the UEs in the Evolved UTRA.

The general principles guiding the definition of E-UTRAN Architecture as well as the E-UTRAN interfaces are the following:

Radio Interface

The figure below shows the protocol stack for the user-plane, where PDCP, RLC and MAC sublayers (terminated in eNB on the network side) perform header compression, ciphering, scheduling, ARQ and HARQ.

The figure below shows the protocol stack for the control-plane. The NAS control protocol is mentioned for information only and is part of UE -EPC comunication. The PDCP sublayer performs e.g. ciphering and integrity protection, RLC and MAC sublayers perform the same functions as for the user plane. The RRC performs broadcast, paging, RRC connection management, Radio Bearer control, Mobility functions, UE measurement reporting and control.

E-UTRA is designed to operate in the frequency bands defined in table 1. The requirements are defined for 1.4, 3, 5, 10, 15 and 20MHz bandwidth with a specif configuration in terms of number of resource blocks. (6, 15, 25, 50, 75 and 100 RB).

Figure 1 shows the relation between the total channel bandwidth, the transmission bandwidth configuration , i.e the number of resource blocks.

The channel raster is 100KHz (the center frequency must be a mulitple of 100KHz).

To support transmission in paired and unpaired spectrum, two duplex modes are supported: Frequency Division Duplex (FDD), supporting full duplex and half duplex operation, and Time Division Duplex (TDD).

E-UTRA Band	Uplink (UL) eNode B receive UE transmit			Downlink (DL) eNode B transmit UE receive			UL-DL Band separation	Duplex Mode
E-UTRA Band	F_{UL_low} – F_{UL_high}			F_{DL_low} – F_{DL_high}			F_{DL_low} - F_{UL_high}	Duplex Mode
1	1920 MHz	–	1980 MHz	2110 MHz	–	2170 MHz	130 MHz	FDD
2	1850 MHz	–	1910 MHz	1930 MHz	–	1990 MHz	20 MHz	FDD
3	1710 MHz	–	1785 MHz	1805 MHz	–	1880 MHz	20 MHz	FDD
4	1710 MHz	–	1755 MHz	2110 MHz	–	2155 MHz	355 MHz	FDD
5	824 MHz	–	849 MHz	869 MHz	–	894MHz	20 MHz	FDD
6	830 MHz	–	840 MHz	875 MHz	–	885 MHz	35 MHz	FDD
7	2500 MHz	–	2570 MHz	2620 MHz	–	2690 MHz	50 MHz	FDD
8	880 MHz	–	915 MHz	925 MHz	–	960 MHz	10 MHz	FDD
9	1749.9 MHz	–	1784.9 MHz	1844.9 MHz	–	1879.9 MHz	60 MHz	FDD
10	1710 MHz	–	1770 MHz	2110 MHz	–	2170 MHz	340 MHz	FDD
11	1427.9 MHz	–	1452.9 MHz	1475.9 MHz	–	1500.9 MHz	23 MHz	FDD
12	[TBD]	–	[TBD]	[TBD]	–	[TBD]	[TBD]	FDD
13	777 MHz	–	787 MHz	746 MHz	–	756 MHz	21	FDD
14	788 MHz	–	798 MHz	758 MHz	–	768 MHz	20	FDD
…
33	1900 MHz	–	1920 MHz	1900 MHz	–	1920 MHz	N/A	TDD
34	2010 MHz	–	2025 MHz	2010 MHz	–	2025 MHz	N/A	TDD
35	1850 MHz	–	1910 MHz	1850 MHz	–	1910 MHz	N/A	TDD
36	1930 MHz	–	1990 MHz	1930 MHz	–	1990 MHz	N/A	TDD
37	1910 MHz	–	1930 MHz	1910 MHz	–	1930 MHz	N/A	TDD
38	2570 MHz	–	2620 MHz	2570 MHz	–	2620 MHz	N/A	TDD
39	1880 MHz	-	1920 MHz	1880 MHz	-	1920 MHz	N/A	TDD
40	2300 MHz	-	2400 MHz	2300 MHz	-	2400 MHz	N/A	TDD

The multiple access scheme for the LTE physical layer is based on Orthogonal Frequency Division Multiple Access (OFDM) with a Cyclic Prefix (CP) in the downlink and a Single Carrier Frequency Division Multiple Access (SC-FDMA) with CP in the uplink. (These two multiple access schemes are sketched in figures 2 and 3.).

OFDMA technique is particularly suited for frequency selective channel and high data rate. It transforms a wideband frequency selective channel into a set of parallel flat fading narrowband channels, thanks to CP. This ideally, allows the receiver to perform a low complex equalization process in frequency domain, i.e 1 tap scalar equalization.

The baseband signal representing a downlink physical channel is defined in terms of the following steps, as shown in figure 2:

- scrambling of coded bits in each of the code words to be transmitted on a physical channel

- mapping of the complex-valued modulation symbols onto one or several transmission layers

- precoding of the complex-valued modulation symbols on each layer for transmission on the antenna ports

- mapping of complex-valued modulation symbols for each antenna port to resource elements

The baseband signal representing the physical uplink shared channel is defined in terms of the following steps, as shown in figure 3:

In 36.101 and 36.104 specifications different requirements can be found for the RF. In particular the following classification can be done

- Maximum Output Power (MOP), Maximum Power Reduction (MPR), Output power dynamics.

- Output RF spectrum emission: (Occupied bandwidth, spectrum emission mask. Out of band emission, Adjacent Carrier Leakage Ratio (ACLR), spurious emission)

- Unwanted emissions (occupied bandwidth, ACLR, Operating band unwanted emissions, transmitter spurious emissions )

- In-channel selectivity, Adjacent Channel Selectivity (ACS), blocking and narrow band blocking

In E-UTRA, the variable bandwidth of the system presents a special problem in the definition of the RF requirements, there may be the need to define some parameters as many times as there are bandwidth modes. Special attention has been given also to the coexistance issue, since LTE has to coexist with all the other alredy existing systems.

Specification 36.113 covers the assessment of E-UTRA base stations, repeaters and associated ancillary equipment in respect of Electromagnetic Compatibility.

Specification 36.141 specifies the Radio Frequency (RF) test methods and conformance requirements for E-UTRA Base Stations (BS) operating either in the FDD mode (used in paired bands) or the TDD mode (used in unpaired bands). These have been derived from, and are consistent with the E-UTRA Base Station (BS) specifications defined in 36.104.

The Layer 1 is defined in a bandwidth agnostic way based on resource blocks, allowing the LTE Layer 1 to adapt to various spectrum allocations. A resource block spans either 12 sub-carriers with a sub-carrier bandwidth of 15kHz or 24 sub-carriers with a sub-carrier bandwidth of 7.5kHz each over a slot duration of 0.5ms.

The radio frame structure type 1 is used for FDD (for both full duplex and half duplex operation) and has a duration of 10ms and consists of 20 slots with a slot duration of 0.5ms. Two adjacent slots form one sub-frame of length 1ms. The radio frame structure type 2 is used for TDD and consists of two half-frames with a duration of 5ms each and containing each 8 slots of length 0.5ms and three special fields (DwPTS, GP and UpPTS) which have configurable individual lengths and a total length of 1ms. A sub-frame consists of two adjacent slots, except for sub-frames 1 and 6, which consist of DwPTS, GP and UpPTS. Both 5ms and 10ms switch-point periodicity are supported.

To support a Multimedia Broadcast and Multicast Service (MBMS), LTE offers the possibility to transmit Multicast/Broadcast over a Single Frequency Network (MBSFN), where a time-synchronized common waveform is transmitted from multiple cells for a given duration. MBSFN transmission enables highly efficient MBMS, allowing for over-the-air combining of multi-cell transmissions in the UE, where the cyclic prefix is utilized to cover the difference in the propagation delays, which makes the MBSFN transmission appear to the UE as a transmission from a single large cell. Transmission on a dedicated carrier for MBSFN with the possibility to use a longer CP with a sub-carrier bandwidth of 7.5kHz is supported as well as transmission of MBSFN on a carrier with both MBMS transmissions and point-to-point transmissions using time division multiplexing.

Transmission with multiple input and multiple output antennas (MIMO) are supported with configurations in the downlink with two or four transmit antennas and two or four receive antennas, which allow for multi-layer transmissions with up to four streams.

Under Single-User MIMO, the base station allocates 1 or 2 streams to the selected user, in the case of Multi-User MIMO the allocation of different (1 oe 2) streams is done to different users. This is supported in both UL and DL.

- In addition, signals are defined as reference signals, primary and secondary synchronization signals.

The modulation schemes supported in the downlink and uplink are QPSK, 16QAM and 64QAM.

The channel coding scheme for transport blocks in LTE is Turbo Coding as for UTRA, with a coding rate of R=1/3, two 8-state constituent encoders and a contention-free quadratic permutation polynomial (QPP) turbo code internal interleaver. Trellis termination is used for the turbo coding. Before the turbo coding, transport blocks are segmented into byte aligned segments with a maximum information block size of 6144 bits. Error detection is supported by the use of 24 bit CRC..

There are several Physical layer procedures involved with LTE operation. Such procedures covered by the physical layer are;

Through the control of physical layer resources in the frequency domain as well as in the time and power domain, implicit support of interference coordination is provided in LTE.

Mobility and Radio Resource Management

Radio characteristics are measured by the UE and the eNode-B and reported to higher layers in the network. These include, e.g. measurements for intra- and inter-frequency handover, inter RAT handover, timing measurements and measurements for RRM.

Measurements for inter-RAT handover are defined in support of handover to GSM, UTRA FDD and UTRA TDD.

Measurements to be performed by a UE for mobility are classified in at least three measurement types:

In E-UTRAN RRC_CONNECTED state, network-controlled UE-assisted handovers are performed and various DRX cycles are supported.

E-UTRAN supports radio access network sharing based on support for multi-to-multi relationship between E-UTRAN nodes and EPC nodes (S1-flex).

Service-based redirection between GERAN / UTRAN and E-UTRAN is supported in both directions.

E-UTRAN mechanisms to support idle and active mode mobility between E-UTRAN and cdma2000 HRPD or 1XRTT.

Support for self configuration and optimisation

Self-configuration process is defined as the process where newly deployed nodes are configured by automatic installation procedures to get the necessary basic configuration for system operation.

This process works in pre-operational state. Pre-operational state is understood as the state from when the eNB is powered up and has backbone connectivity until the RF transmitter is switched on.

Self-optimization process is defined as the process where UE & eNB measurements and performance measurements are used to auto-tune the network.

This process works in operational state. Operational state is understood as the state where the RF interface is additionally switched on.

System Architecture Evolution

SA WG2 started its own Study for the System Architecture Evolution (SAE) whose objective is "to develop a framework for an evolution or migration of the 3GPP system to a higher-data-rate, lower-latency, packet-optimized system that supports, multiple RATs. The focus of this work [is] on the PS domain with the assumption that voice services are supported in this domain". SA2's SAE work is conducted under Work Item "3GPP system architectural evolution", approved in December 2004. It was initiated when it became clear that the future was clearly IP with everything (the "all-IP" network, AIPN - see TS 22.978), and that access to the 3GPP network would ultimately be not only via UTRAN or GERAN but by WiFi, WiMAX, or even wired technologies. Thus SEA has as its main objectives:

The figure below shows the evolved system architecture, possibly relying on different access technologies (extract of TR 23.882):