An Overview Of The Lte Standard

Published Date: 02 Nov 2017

After the 3G data networks (e.g. GSM to UMTS to HSPA to LTE or CDMA to LTE), the next step to step on is Long Term Evolution. LTE is based on standards developed by the 3rd Generation Partnership Project (3GPP).  It can be said that UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) transformed to the next stage, which is LTE. The standards for GSM/UMTS family were created by 3GPP. The standards for LTE are totally new, with some exceptions where it made sense. The key objectives for LTE are listed as follows.

Better downlink and uplink peak data rates.

Scalable bandwidth

Improved spectral efficiency

All IP network

A standard’s based interface, which is capable of supporting a wide multitude of user types.

The main goal or intension of LTE networks is to bridge the data transfer speed   gap   between   very high data rate   fixed wireless   Local   Area   Networks (LAN) and highly mobile cellular networks.

2.2.1 Overview of the LTE Standard

The original study on Long Term Evolution (LTE) of 3GPP Radio Access Technology was started with the goal to make sute that 3GPP RAT is more advanced in the future than that of the predecessors. The motive of the investigation was to enhance and improve the radio-access technology (UTRA) and optimization & simplification of radio access network (UTRAN). The main characteristics of LTE are:

Efficient spectrum utilization

Flexible spectrum allocation

Reduced cost for the operator

Improved system capacity and coverage

Higher data rate with reduced latency

2.2.1 Targets for LTE

Some of the key targets set for LTE are listed below (as per [3GPP TR 25.913])[17]

Increased peak data rate: 100Mbps for DL with 20MHz (2 Rx Antenna at UE), up to 50Mbps for UL with 20MHz

Improved spectrum efficiency: 5bps/Hz for downlink and 2.5bps/Hz for Uplink

Improved cell edge performance (in terms of data rate)

Relatively low latency.

2.2.3 Overall Network Architecture

The E-UTRAN makes use of simplified version single node architecture. This has the eNBs (E-UTRAN Node B). eNB and the Evolved Packet Core (EPC) communicates with each other through the S1 interface; specifically with the MME (Mobility Management Entity) and the UPE (User Plane Entity) identified as S-GW (Serving Gateway). This S-GW uses S1-C and S1-U for control plane and user plane respectively. MME and UPE are mostly implemented as separate network nodes so that independent scaling of the control and user plane can be implemented. This Is clearly illustrated in the figure 2.5.

https://sites.google.com/site/lteencyclopedia/_/rsrc/1293465421244/home/E-UTRAN.bmp

Figure 2.2: Overall Architecture [8] 

Multicast/Broadcast over a Single Frequency Network (MBSFN) is efficiently supported by LTE, where multiple cells transmit a common signal with appropriate time synchronization. Even though eNB is the only entity of the E-UTRAN, it supports all the functions in a typical radio network such as Radio Mobility management, Bearer control, scheduling and Admission control. The Access Stratum resides at the eNB.

https://sites.google.com/site/lteencyclopedia/_/rsrc/1293467072241/home/E-UTRAN2.bmp

Figure 2.3 Functional Split between E-UTRAN and EPC [8]

2.2.4 LTE Physical layer

To achieve the aim of high data rate and improved spectral efficiency, the LTE physical layer is built with Orthogonal Frequency Division Multiplexing scheme OFDM. Both time (aka slot) and frequency units (aka subcarrier) makes the spectral resources that are allocated/used as a combination. 2 or 4 Antennas are supported. UL and DL supports Multi-user MIMO. QPSK, 16QAM and 64QAM are the modulation schemes supported in the downlink and uplink spectrum.

2.2.4.1 Downlink (DL) Physical Channel

The downlink transmission uses the OFDM with cyclic prefix. OFDM is used due to the reasons that are given below:

Selective fading is countered by multiple carrier modulation (MCM) as the channel appears to have nearly flat frequency response for the narrow band subcarrier.

Flexible spectrum allocation is possible by changing or adapting to the channel condition the frequency range of the resource block and the number of resource blocks

Higher peak data rates are achievable with the help of multiple resource blocks and not by reducing the duration of the symbols or by using still higher order modulation.

Higher spectral efficiency is an advantage obtained by the multiple orthogonal subcarriers.

2.2.4.2 Uplink (UL) Physical Channel

The uplink transmission is built with the SC-FDMA (Single Carrrier FDMA) scheme. The SC-FDMA scheme is made of two stage process where the first stage converts the input signal to frequency domain (represented by DFT coefficients) and the second stage is where OFDM scheme is used to change these DFT coefficients to an OFDM signal. The SC-FDMA scheme is a scheme which is known as DFT-Spread OFDM because of this association with OFDM. The reasons for this choice are described below:

The two stage process facilitates to select appropriate frequency range for the subcarriers while mapping the set of DFT coefficients to the Resource Blocks. At any given time, Users get unique frequency. This avoids co-channel interference among the users of a cell.

The transformation is same as the shift in the centre frequency of the single carrier input signal. The subcarriers do not combine in random phases, which cause large variation in the modulated signal in terms of instantaneous power. This implies Peak to Average Power Ratio is low.

The Peak to Average Power Ratio (PAPR) of SC-FDMA is lesser than the PAPR of the conventional OFDMA.

2.3 Introduction to LTE Advanced

Observing the growth and success rate of the 3G technologies, it is obvious that the growth rate of cellular network should not slow down. The ideas to start up 4G technology started to flow in and the investigation started. In an initial investigation happened on 25th of December, 2006 which was released on 9th of February 2007, NTT DoCoMo briefed out the information about the their trial which succeeded in sending data to a mobile station moving at a rate of 10Km/h, with a speed up to 5 Gb/s. This was done with a 10Mhz bandwidth. The Technique which made this possible includes several technologies to achieve this, of which the significant technologies are variable spreading factor spread orthogonal frequency division multiplex, MIMO, multiple input multiple output, and maximum likelihood detection. Methods and procedures for these new 4G experiments were passed to 3GPP for their consideration

3GPP organized two workshops in 2008 on IMT Advanced. This is where the "Requirements for Further Advancements for E-UTRA" were outlined. Technical Report 36.913 was made out of this and then published in June 2008. the LTE-Advanced system was submitted to the ITU-R as their proposal for IMT-Advanced.

The evolution from the 3G services that were developed by making use of UMTS / W-CDMA Technologies is followed by the development of LTE Advanced / IMT Advanced technologies.

There exists another technology that competes with LTE. WiMAX is also there, offering very high data rates with high levels of mobility. However it now seems less likely that WiMAX will be adopted as the 4G technology, with LTE Advanced looks to have better probability.

2.3.1 LTE Advanced key features

With a large number of improvements are happening in LTE Advanced, a number of key requirements and key features are found. In spite of the specification not being fixed now, there are many high level targets for the new LTE Advanced specification. These specifications need to be verified and much work remains to be undertaken in the specifications before these are all fixed. Some of the main targets for LTE Advanced as of now are listed as follows:

Peak data rates: downlink - 1 Gbps; uplink - 500 Mbps.

Spectrum efficiency: 3 times greater than LTE.

Peak spectrum efficiency: downlink - 30 bps/Hz; uplink - 15 bps/Hz.

Spectrum use: the ability to support scalable bandwidth use and spectrum aggregation where non-contiguous spectrum needs to be used.

Latency: from Idle to Connected in less than 50 ms and then shorter than 5 ms one way for individual packet transmission.

Cell edge user throughput to be twice that of LTE.

Average user throughput to be 3 times that of LTE.

Mobility: Same as that in LTE

Compatibility: This is backwards compatible. LTE Advanced shall be capable of interworking with 3GPP legacy systems and LTE.

These are some of the key development targets for LTE Advanced. Their actual specifications and the actual implementation of them will need to be worked out during the implementation stage of the system.

2.3.2 LTE Advanced technologies

There are various key technologies that will enable LTE Advanced to achieve the high data throughput rates that are required. MIMO and OFDM are two of the base technologies that enable this amount of precision and efficiency. Not only these, there exists number of other techniques and technologies that will be made use of.

OFDM forms the foundation of the radio bearer. Apart from that, there is OFDMA (Orthogonal Frequency Division Multiple Access) along with SC-FDMA (Single Channel Orthogonal Frequency Division Multiple Access). These will be implemented in a hybrid format. Anyhow all these schemes works based on OFDM