TD-LTE Air Interface - Physical Layer
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TD-LTE Introduction
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TD-LTE Air Interface - Physical Layer • TD-LTE Introduction
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TD-LTE Air Interface - Physical Layer • TD-LTE Introduction
Table of Contents:
1 2 3 4 5 6
LTE Architecture ................................................................................................ 4 TD-LTE Frequency Allocation ............................................................................ 5 LTE Physical Layer ............................................................................................ 6 Main Benefits of LTE (1) .................................................................................... 8 Main Benefits of LTE (2) .................................................................................... 9 TD-LTE Advantages ........................................................................................ 10
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LTE Architecture
Long Term Evolution (LTE) refers to a new high-performance radio access technology for mobile communications standardized by the 3rd Generation Partnership Project (3GPP). The LTE work in 3GPP is closely aligned to the 3GPP system architecture evolution (SAE) framework which is concerned with the evolved core network architecture. The LTE/SAE framework defines the flat, scalable, IP-based architecture of the Evolved Packet System (EPS) consisting of a radio access network part (Evolved UTRAN) and the Evolved Packet Core (EPC). Note that the Evolved Packet System is purely packet based. Consequently, voice transport is based on Voice over IP (VoIP) technology. In the case of circuit-switched services, the mobile equipment must be handed over to a 2G or 3G network. Move your mouse pointer over the items in the architecture figure for a short introduction to each item.
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TD-LTE Frequency Allocation
LTE supports both frequency division duplex (FDD) and time division duplex (TDD) modes of operation. In FDD, the uplink and downlink signals in a cell are carried in different frequency bands. In TDD, the uplink and downlink transmission takes place during different time intervals within the same spectral bandwidth. Since this course considers the TDD mode of operation, we shall not cover the paired spectrum allocation utilised in FDD systems. In practice, a frequency band available for TD-LTE is split up into different portions, depending on its geographical location, where each portion is allocated to a certain network operator. LTE offers the possibility to further split up the allocated portion into a number of channels with a variety of bandwidths between 1.4 and 20 MHz. The unpaired spectrum available for TD-LTE is shown in the table. The possible channel bandwidths for each LTE band, according to 3GPP, are also shown. As an example, bands 33, 34 and 38, with a total capacity of 85 MHz, are reserved for TD-LTE in Europe. In China, bands 34, 38, 39 and 40, with a total capacity of 205 MHz, are foreseen for TD-LTE. In LTE release RL25, the LTE bands 38 and 40 are supported. The band 41 is covered partially.
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LTE Physical Layer
Whereas GSM is based on Time Division Multiple Access (TDMA), and WCDMA and HSPA are based on Code Division Multiple Access, the LTE physical layer is based on Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink and Single-Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink direction. The second part of this course is entirely devoted to explaining the basic operation of these multiple access methods. For instance, the concept of subcarriers in the frequency domain should be familiar at this point. Obviously, the physical structure of the LTE interface contains more than just the multiple access method. The third part of the course addresses among others such issues as the frame structure, the basic idea of using resource blocks, the physical channels in downlink and uplink, and adaptive resource allocation. The course also briefly describes the protocol layers located above the physical layer. The LTE radio interface is standardised in the 36-series of 3GPP Release 8. The detailed physical layer structure is described in five physical layer specifications.
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Main Benefits of LTE (1)
The LTE radio technology offers the following benefits: ·
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TD-LTE with a downlink-to-uplink ratio of 2:2 offers peak data rates of up to 82 Mbit/s in downlink (assuming 2 x 2 MIMO and 20 MHz channel bandwidth) and up to 17 Mbit/s in uplink. Correspondingly, TD-LTE with a downlink-to-uplink ratio of 3:1 offers peak data rates of up to 112 Mbit/s in downlink and up to 9 Mbit/s in uplink. LTE enables round trip times (RTT) of less than 20 ms. The round trip time or user plane latency is the time it takes for information to travel from the mobile terminal to the destination in the network and back to the terminal. Also the control plane latency - the time needed to allocate transport resources - is important. The requirement for the control plane latency in LTE is less than 100 ms. Contrary to HSPA, LTE offers packet scheduling in the frequency domain in addition to packet scheduling in the time domain. This feature greatly increases the spectrum efficiency of LTE. The LTE capacity or spectrum efficiency is two to four times higher than that of a 3GPP Release 6 HSPA system.
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Main Benefits of LTE (2)
A major advantage of LTE over WCDMA or HSPA is the possibility of allocating spectrum bandwidths of varying size to the mobile users. LTE offers several channel bandwidth values between 1.4 and 20 MHz. By contrast, the channel bandwidth in WCDMA or HSPA is always fixed at 5 MHz. A small channel bandwidth allows easier spectrum refarming and is beneficial for mobile operators short on spectrum. On the other hand, a large channel bandwidth is required if large peak data rates are to be supported.
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TD-LTE Advantages
In the case of the TDD mode of operation, LTE offers the following additional benefits: 1. In TD-LTE, it is possible to allocate different transmission capacity for the uplink and downlink directions. This is not possible in FDD-based LTE. Voice communication is inherently symmetric in the uplink and downlink - in this case TDD does not offer any advantage over FDD. However, data traffic is asymmetric - more capacity is typically needed in the downlink than in the uplink. In this case TDD is more spectrally efficient compared to FDD. 2. The FDD LTE and TD-LTE versions of the 3GPP standard are very similar. As a result, devices can support both the FDD and TDD interfaces through a single chipset - in other words without any additional cost. This is a hugely important new development: TD-LTE will benefit from the wide availability of FDD LTE devices that will be able to support TD-LTE as well. It is also interesting to note that the TDD-specific solutions in TD-LTE build upon solutions that have already been defined for TD-SCDMA - a 3rd Generation TDDbased cellular mobile system developed in China. 3. The use of beamforming at the eNodeB is particularly interesting in TD-LTE because of the reciprocity between the downlink and the uplink channel. Beamforming permits an improvement in both the transmission capacity and in the receive signal quality.
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