Long term evolution

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Introduction

The mobile telephone providers have seen tremendous growth in wireless data usage as well as significant growth in the number of mobile subscribers, especially in the later years of the first decade in the 21st century. The International Telecommunication Union predicts that there will be over five billion mobile telephone subscribers worldwide in 2011. Data usage in that same year will approach twenty terabytes. Customers demand more and faster connectivity. Carriers are being pressured to accelerate their fourth generation (4G) deployment timelines. 4G mobile telephony consists of the Long Term Evolution (LTE) air interface structure and the Evolved Packet Core (EPC) for the transport infrastructure. When this change is made, voice services will now be carried using the Internet Protocol (IP) and all the phones will have an IP address. Many expect that the move to LTE with the EPC could be a major force in the deployment of IP version 6 (IPv6).

LTE Overview

Fourth generation (4G) mobile telephony standards are being developed by the Third Generation Partnership Project (3GPP). The 4G specification are based on the evolved Global System for Mobile Communications (GSM) specifications, which were the cornerstone of both 2G and 3G mobile systems worldwide.

The formal name for the 4G specifications is the Evolved Packet System, which relates to the transition of the transport infrastructure from circuit switched to packet switched using an all-IP network. The access (air interface) specification is called the Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN). The E-UTRAN specification is also referred to as Long Term Evolution (LTE). The IP-based core network is called the Evolved Packet Core (EPC) or the System Architecture Evolution (SAE).

The enabling technologies for LTE are many and varied. Orthogonal frequency division multiplexing (OFDM) is used on the downlink to maximize the data rates in the assigned spectrum. Frequency domain equalization is used to balance the actual data rates across the spectrum. Single carrier frequency division multiple access (SC-FDMA) is used on the uplink since the uplink data rate requirements are less than the downlink. It is interesting that the FDMA technologies used in the 1G systems live in the 4G systems while the code division multiple access (CDMA) of 3G systems has been replaced by the more spectral efficient OFDM access (OFDMA). To increase the bandwidth and data rates even more, 4G systems support multi-input multi-output (MIMO) antenna implementations. MIMO not only provides for better quality but also allows for virtual channels using multipath propagation delay to increase the data rate of the channel. To ensure optimal channel efficiency multicarrier channel-dependent resource scheduling is supported. Finally, fractional frequency reuse allows for a better distribution of spectrum to meet the data rate variations of mobile telephony applications.

Baseline LTE

In the creation of system specifications of any type the first step is to establish the baseline system requirements. For LTE the baseline peak date rate was set at 100 Mbps on the downlink and 50 Mbps on the uplink for a 20 MHz channel. To assist in backward compatibility of channel sizes, LTE can be set up with channels from 1.25 MHz to 20 MHz. From the capacity perspective, there should be up to 200 users in a cell with a 5 MHz channel size. Capacity requirements take into account the fact that numerous voice conversations (relatively low data rate requirements) will be needed to support the over 5 billion expected mobile subscriber in 2011. Because of the real time requirements of voice, the latency on the air interface must be less than five milliseconds. By keeping the air interface latency low, the actual total latency will approximate the latency of the IP core infrastructure.

As in the 3G specifications, mobility is specified based on the motion of the user. The highest speeds will be delivered to the technically stationary/pedestrian user where motion is from zero to fifteen kmh (zero to about nine mph). Vehicle-based subscribers can expect high performance in the fifteen to 120 kmh (about nine to 75 mph). Mobile subscriber support is available for speeds up to 350 kmh (about 210 mph). As specification evolves speeds of up to 600 kmh (about 360 mph) may be supported. The specification notes that as the speed increase the delivered data rate will decrease. To deal with service such as mobile television, the enhanced multimedia broadcast multicast service (E-MBMS) was defined. Because the core infrastructure is packet-based using IP, there is enhanced support for end-to-end quality of service (QoS) in the LTE requirements.

3GPP Release 8 Status

The Release 8 version of the LTE specification was locked in December 2008 by the 3GPP standards committee. The code was locked in March 2009. Speed tests for Release 8 showed download rates per 20 MHz spectrum of 326.4 Mbps using a four-by-four MIMO antenna configuration and 172.8 Mbps using a two-by-two MIMO antenna configuration. The upload rate per 20 MHz spectrum was 86.4 Mbps. Five different terminal classes that ranged from voice to high-speed data have been defined.

At least 200 active data clients per 5 MHz cell was achieved as was sub-5 ms latency for “small” packets. Spectrum “slices” from 1.25 MHz to 20 MHz we successfully tested. The optimal cell size is 5 km (about 3 miles), a 30 km (about 18 miles) got a “reasonable” service level and, a 100 km (about 60 miles) cell had “acceptable” service. Coexistence with legacy standards to promote intersystem roaming was successfully demonstrated. Finally, a multicast broadcast single frequency network for mobile TV was established and successfully tested.

Requirements for LTE

While the LTE specifications have their own requirements, the successful implementation of LTE by the carriers has its own set of requirements. The access network will require the allocation of additional spectrum for mobile services. In the U.S., the new spectrum in the 700 MHz range will probably be used by the carriers for LTE deployment. From both the user and carrier equipment perspective, there will need to be a radio update because of the new air interface and the new spectrum. There will also be a protocol update to handle voice over IP (VoIP) and the IP core with quality of service (QoS). The backhaul of traffic from the base station will migrate from TDM to a packet-mode backhaul like Ethernet with QoS. In the core network, there will need to be IPv6, QoS, and the session initiation protocol (SIP) for signaling.

Evolved Packet Core (EPC)

The Evolved Packet Core (EPC) will mark the end of circuit-switched voice as voice becomes another IP application. The plan calls for evolved wireless broadband to match or exceed the quality of experience (QoE) of wireline services. Mobility as part of the core network requires mobility management in the core. End-to-end QoS becomes essential as the low-latency, real-time media-based applications become available. Policy management and enforcement becomes a network responsibility and not a user equipment requirement. The network will definitely rule. New elements in the evolved packet core (EPC) include the serving gateway (SGW), packet data network gateway (PGW), mobility management entity (MME), and policy and charging rules function (PRCF). The challenge will be to keep the move to LTE as one of evolution and not revolution.

LTE Reality

As of this writing (August 2009), several things are known about the U.S. deployment of LTE. First, LTE deployment will take several years because equipment availability (e.g., antennas, chipsets) is still limited. That said Verizon Wireless has successfully tested LTE in the U.S. and announced commercial trials that will begin in 2010. AT&T Mobility expects to begin deploying LTE in 2010/2011 timeframe. The move to LTE will cost billions of dollars as infrastructure and handset changes are made. Markets rich with data users or at the limits of spectrum capacity will be first. Clearly, new revenues from data applications are needed to justify the investment.

LTE and CDMA/UMTS will coexist in the provider networks for many years to come as the move progresses. LTE requires new phones and the carriers will need to get subscribers to invest in new phones. In the long term, current CDMA/UMTS bandwidth will switch to LTE but not without huge planning/engineering challenges. One final note of interest is that LTE is based on the OFDM technology. WiMAX is also based on OFDM. In the U.S. Sprint/Clearwire has chosen WiMAX as its 4G solution. Could there ultimately be some form of convergence of wireless services because of the common technology. Note that Wi-Fi also uses OFDM.

PodSnacks

<mp3>http://hill-vt.com/podcast/tHAWT/tHAWT_082809.mp3%7Cdownload</mp3> | tHAWT Episode #169: Long Term Evolution

Knowledge Nuggets

Mobile Madness - The Quest for 4G


External references

3GPP Global initiative [1]