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LONG TERM EVOLUTION (LTE) INTRODUCTION

1. With services such as Wi-MAX offering very high data speeds, work on developing the next generation of cellular technology has started. The UMTS cellular technology upgrade has been dubbed as LTE - Long Term Evolution. The idea is that 3G LTE will enable much higher speeds to be achieved along with much lower packet latency (a growing requirement for many services these days) and that LTE will enable cellular communications services to move forward to meet the needs for cellular technology to 2020 and well beyond. 2. HSPA (High Speed Packet Access), a combination of HSDPA and HSUPA and HSPA+ are now being deployed, the 3G LTE developments is being dubbed as 3.99G because it is not a full 4G standard, although in reality there are many similarities with the cellular technologies being touted for the use of 4G. However, regardless of the terminology, it is certain that 3G LTE will offer significant improvements in performance over the existing 3G standards.

Fig: A typical LTE connectivity 3. Many operators have not yet upgraded their basic 3G networks and LTE is seen as the next logical step for many operators, who will leapfrog straight from basic 3G straight to LTE as this will avoid providing several stages of upgrade. The use of LTE will also provide the data capabilities that will be required for many years and until the full launch of the full 4G standards known as LTE Advanced.

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3G LTE BEGINNINGS 4. 3GPP, the Third Generation Partnership Project that oversaw the development of the UMTS 3G system started the work on the evolution of the 3G cellular technology with a workshop held in Toronto, Canada in November 2004. The work on 3G LTE started with a feasibility study started in December 2004, which was finalised for inclusion on 3GPP release 7. LTE core specifications were then included thereafter. The workshop set down a number of high level requirements for 3G LTE given as under: (a)

Reduced cost per bit.

(b) Increased service provisioning - more services at lower cost with better user experience. (c)

Flexibility of use of existing and new frequency bands.

(d)

Simplified architecture, Open interfaces.

(e)

Allow for reasonable terminal power consumption.

5. In terms of actual figures, targets for LTE included download rates of 100Mbps and upload rates of 50Mbps for every 20MHz of spectrum. In addition to this LTE was required to support at least 200 active users in every 5MHz cell (i.e. 200 active phone calls). Targets have also been set for the latency in IP packet delivery. With the growing use of services including VoIP, gaming and many other applications where latency is of concern, figures need to be set for this. As a result, a figure of sub-10ms latency for small IP packets has been set. 3G LTE EVOLUTION 6. Although there are major step changes between LTE and its 3P predecessors, it is nevertheless looked upon as an evolution of the UMTS / 3GPP 3G standards. Although it uses as a different form of radio interface, using OFDMA / SC-FDMA instead of CDMA, there are many similarities with the earlier forms of 3G architecture and there is scope for much re-use. LTE can be seen to provide a further evolution of functionality, increased speeds and general improved performance. In addition to this, LTE is an all IP based network, supporting both IPv4 and IPv6. There is also no basic provision for voice, although this can be carried as VoIP. WCDMA (UMTS) Max downlink speed bps 384 k Max uplink speed bps 128 k Latency round trip time 150 ms approx Approx years of initial 2003/4 roll out Access methodology CDMA

HSPA HSDPA/ HSUPA 14 M 5.7 M 100 ms 2005 / 6 HSDPA 2007 / 8 HSUPA CDMA

HSPA+

LTE

28 M 11 M 50ms (max) 2008 / 9

100M 50 M ~10 ms

CDMA

2009 / 10 OFDMA/SCFDMA

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3G LTE TECHNOLOGIES 7. LTE has introduced a number of new technologies when compared to the previous cellular systems. They enable LTE to be able to operate more efficiently with respect to the use of spectrum and also to provide the much higher data rates that are being required. (a) OFDM (Orthogonal Frequency Division Multiplex). OFDM technology has been incorporated into LTE because it enables high data bandwidths to be transmitted efficiently while still providing a high degree of resilience to reflections and interference. The access schemes differ between the uplink and downlink: OFDMA (Orthogonal Frequency Division Multiple Access is used in the downlink; while SC-FDMA (Single Carrier - Frequency Division Multiple Access) is used in the uplink. SC-FDMA is used in view of the fact that its peak to average power ratio is small and the more constant power enables high RF power amplifier efficiency in the mobile handsets - an important factor for battery power equipment. (b) MIMO (Multiple Input Multiple Output). One of the main problems that previous telecommunications systems have encountered is that of multiple signals arising from the many reflections that are encountered. By using MIMO, these additional signal paths can be used to advantage and are able to be used to increase the throughput. When using MIMO, it is necessary to use multiple antennas to enable the different paths to be distinguished. Accordingly schemes using 2 x 2, 4 x 2, or 4 x 4 antenna matrices can be used. While it is relatively easy to add further antennas to a base station, the same is not true of mobile handsets, where the dimensions of the user equipment limit the number of antennas which should be place at least a half wavelength apart. (c) SAE (System Architecture Evolution). With the very high data rate and low latency requirements for 3G LTE, it is necessary to evolve the system architecture to enable the improved performance to be achieved. One change is that a number of the functions previously handled by the core network have been transferred out to the periphery. Essentially this provides a much "flatter" form of network architecture. In this way latency times can be reduced and data can be routed more directly to its destination. DUPLEX SCHEMES 8. It is essential that any cellular communications system must be able to transmit in both directions simultaneously. This enables conversations to be made, with either end being able to talk and listen as required. Additionally when exchanging data it is necessary to be able to undertake virtually simultaneous or completely simultaneous communications in both directions. 9. It is necessary to be able to specify the different direction of transmission so that it is possible to easily identify in which direction the transmission is being made. There are

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a variety of differences between the two links ranging from the amount of data carried to the transmission format and the channels implemented. The two links are defined as:(a) Uplink. The transmission from the User Equipment (UE) to the Node B or base station. (b) Downlink. The transmission from the Node B or base station to the User Equipment (UE).

Fig 2: Uplink and downlink transmission directions 10. In order to be able to be able to transmit in both directions, a user equipment or base station must have a duplex scheme. There are two forms of duplex that are commonly used, namely FDD, frequency division duplex and TDD time division duplex. TDD AND FDD DUPLEX SCHEMES 11. In order for radio communications systems to be able to communicate in both directions it is necessary to have what is termed a duplex scheme. A duplex scheme provides a way of organizing the transmitter and receiver so that they can transmit and receive. There are several methods that can be adopted. For applications including wireless and cellular telecommunications, where it is required that the transmitter and receiver are able to operate simultaneously, two schemes are in use. One known as FDD or frequency division duplex uses two channels, one for transmit and the other for receiver. Another scheme known as TDD, time division duplex uses one frequency, but allocates different time slots for transmission and reception. Both FDD and TDD have their own advantages and disadvantages. Accordingly they may be used for different applications or where the bias of the communications is different.

ADVANTAGES / DISADVANTAGES OF LTE TDD AND LTE FDD FOR CELLULAR COMMUNICATIONS 12. There are a number of the advantages and disadvantages of TDD and FDD that are of particular interest to mobile or cellular telecommunications operators. These are naturally reflected into LTE.

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Parameter Paired spectrum

LTE-TDD LTE-FDD Does not require paired spectrum Requires paired spectrum with as both transmit and receive sufficient frequency separation to occur on the same channel allow simultaneous transmission and reception Hardware cost Lower cost as no diplexer is Diplexer is needed and cost is needed to isolate the transmitter higher. and receiver. As cost of the UEs is of major importance because of the vast numbers that are produced, this is a key aspect. Channel Channel propagation is the same Channel characteristics different reciprocity in both directions which enables in both directions as a result of transmit and receive to use on set the use of different frequencies of parameters UL / DL It is possible to dynamically UL / DL capacity determined by asymmetry change the UL and DL capacity frequency allocation set out by ratio to match demand the regulatory authorities. It is therefore not possible to make dynamic changes to match capacity. Regulatory changes would normally be required and capacity is normally allocated so that it is the same in either direction. Guard period / Guard period required to ensure Guard band required to provide guard band uplink and downlink transmissions sufficient isolation between do not clash. Large guard period uplink and downlink. Large guard will limit capacity. Larger guard band does not impact capacity. period normally required if distances are increased to accommodate larger propagation times. Discontinuous Discontinuous transmission is Continuous transmission is transmission required to allow both uplink and required. downlink transmissions. This can degrade the performance of the RF power amplifier in the transmitter. Cross slot Base stations need to be Not applicable interference synchronised with respect to the uplink and downlink transmission times. If neighboring base stations use different uplink and downlink assignments and share the same channel, then interference may occur between cells.

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LTE TDD / TD-LTE AND TD-SCDMA 13. Apart from the technical reasons and advantages for using LTE TDD / TD-LTE, there are market drivers as well. With TD-SCDMA now well established in China, there needs to be a 3.9G and later a 4G successor to the technology. With unpaired spectrum allocated for TD-SCDMA as well as UMTS TDD, it is natural to see many operators wanting an upgrade path for their technologies to benefit from the vastly increased speeds and improved facilities of LTE. Accordingly there is a considerable interest in the development of LTE TDD, which is also known in China as TD-LTE. 14. With the considerable interest from the supporters of TD-SCDMA, a number of features to make the mode of operation of TD-LTE more of an upgrade path for TDSCDMA have been incorporated. One example of this is the sub frame structure that has been adopted within LTE TDD / TD-LTE. TYPES OF LTE FRAME STRUCTURE: 15.

There are two types of LTE frame structure. (a)

Type 1. Used for the LTE FDD mode systems.

(b)

Type 2. Used for the LTE TDD systems. TYPE 1 : LTE FRAME STRUCTURE

16. The basic type 1 LTE frame has an overall length of 10 ms. This is then divided into a total of 20 individual slots. LTE Sub frames then consist of two slots - in other words there are ten LTE sub frames within a frame.

Fig 3: Type 1 LTE Frame Structure TYPE 2 : LTE FRAME STRUCTURE 17. The frame structure for the type 2 frames used on LTE TDD is somewhat different. The 10 ms frame comprises two half frames, each 5 ms long. The LTE half-frames are further split into five sub frames, each 1ms long.

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Fig 4 : Type 2 LTE Frame Structure 18. The sub frames may be divided into standard sub frames of special sub frames. The special sub frames consist of three fields:(a)

DwPTS - Downlink Pilot Time Slot.

(b)

GP - Guard Period.

(c)

UpPTS - Uplink Pilot Time Slot.

These three fields are also used within TD-SCDMA and they have been carried over into LTE TDD (TD-LTE) and thereby help the upgrade path. The fields are individually configurable in terms of length, although the total length of all three together must be 1ms. LTE TDD / TD-LTE SUBFRAME ALLOCATIONS 19. One of the advantages of using LTE TDD is that it is possible to dynamically change the up and downlink balance and characteristics to meet the load conditions. In order that this can be achieved in an ordered fashion, a number of standard configurations have been set within the LTE standards. 20. A total of seven up / downlink configurations have been set and these use either 5 ms or 10 ms switch periodicities. In the case of the 5ms switch point periodicity; a special sub frame exists in both half frames. In the case of the 10 ms periodicity, the special sub frame exists in the first half frame only. It can be seen from the table below that the sub frames 0 and 5 as well as Dw PTS are always reserved for the downlink. It can also be seen that Up PTS and the sub frame immediately following the special sub frame are always reserved for the uplink transmission.

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Uplinkdownlink configuration 0 1 2 3 4 5 6 Where: (a)

Downlink to uplink switch periodicity

5 ms 5 ms 5 ms 10 ms 10 ms 10 ms 5 ms

Subframe Number

0 D D D D D D D

1 S S S S S S S

2 U U U U U U U

3 U U D U U D U

4 U D D U D D U

5 D D D D D D D

6 S S S D D D S

7 U U U D D D U

8 U U D D D D U

9 U D D D D D D

D is a subframe for downlink transmission.

(b)

S is a "special" subframe used for a guard time.

(c)

U is a subframe for uplink transmission. 3G LTE CHANNEL TYPES

21.

There are three categories into which the various data channels may be grouped. (a) Physical Channels. These are transmission channels that carry user data and control messages. (b) Transport Channels. The physical layer transport channels offer information transfer to Medium Access Control (MAC) and higher layers. (c) Logical Channels. Provide services for the Medium Access Control (MAC) layer within the LTE protocol structure. 3G LTE PHYSICAL CHANNELS

22. The LTE physical channels vary between the uplink and the downlink as each has different requirements and operates in a different manner. (a)

Downlink. (i) Physical Broadcast Channel (PBCH). This physical channel carries system information for UEs requiring to access the network. (ii) Physical Downlink Control Channel (PDCCH). The main purpose of this physical channel is to carry mainly scheduling information. (iii) Physical Hybrid ARQ Indicator Channel (PHICH). As the name implies, this channel is used to report the Hybrid ARQ status.

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(iv) Physical Downlink Shared Channel (PDSCH). used for unicast and paging functions.

This channel is

(v) Physical Multicast Channel (PMCH). This physical channel carries system information for multicast purposes. (vi) Physical Control Format Indicator Channel (PCFICH). provides information to enable the UEs to decode the PDSCH. (b)

This

Uplink. (i) Physical Uplink Control Channel (PUCCH). acknowledgement.

Sends Hybrid ARQ

(ii) Physical Uplink Shared Channel (PUSCH). This physical channel found on the LTE uplink is the Uplink counterpart of PDSCH. (iii) Physical Random Access Channel (PRACH). physical channel is used for random access functions.

This uplink

LTE TRANSPORT CHANNELS 23. The LTE transport channels vary between the uplink and the downlink as each has different requirements and operates in a different manner. Physical layer transport channels offer information transfer to medium access control (MAC) and higher layers. (a)

Downlink. (i) Broadcast Channel (BCH). Broadcast Control Channel (BCCH).

The LTE transport channel maps to

(ii) Downlink Shared Channel (DL-SCH). This transport channel is the main channel for downlink data transfer. It is used by many logical channels. (iii)

Paging Channel (PCH) To convey the PCCH.

(iv) Multicast Channel (MCH) This transport channel is used to transmit MCCH information to set up multicast transmissions. (b)

Uplink (i) Uplink Shared Channel (UL-SCH) This transport channel is the main channel for uplink data transfer. It is used by many logical channels. (ii) Random Access Channel (RACH). access requirements.

This is used for random

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LTE LOGICAL CHANNELS 24.

LTE logical channels are :(a)

Control channels. (i) Broadcast Control Channel (BCCH). This control channel provides system information to all mobile terminals connected to the eNodeB. (ii) Paging Control Channel (PCCH). This control channel is used for paging information when searching a unit on a network. (iii) Common Control Channel (CCCH). This channel is used for random access information, e.g. for actions including setting up a connection. (iv) Multicast Control Channel (MCCH). This control channel is used for Information needed for multicast reception. (v) Dedicated Control Channel (DCCH). This control channel is used for carrying user-specific control information, e.g. for controlling actions including power control, handover, etc.

(b)

Traffic channels. (i) Dedicated Traffic Channel (DTCH). This traffic channel is used for the transmission of user data. (ii) Multicast Traffic Channel (MTCH). transmission of multicast data.

This channel is used for the

(c) There are a growing number of LTE frequency bands that are being designated as possibilities for use with LTE. LTE FDD FREQUENCY BAND ALLOCATIONS 25. There are a large number of allocations or radio spectrum that has been reserved for FDD, frequency division duplex, LTE use. The FDD frequency bands are paired to allow simultaneous transmission on two frequencies. The bands also have a sufficient separation to enable the transmitted signals not to unduly impair the receiver performance. If the signals are too close then the receiver may be "blocked" and the sensitivity impaired. The separation must be sufficient to enable the roll-off of the antenna filtering to give sufficient attenuation of the transmitted signal within the received band.

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Band Number 1 2 3 4 5 6 7 8 9 10

Band Description / Uplink (MHz) Name IMT core 1920 - 1980 PCS 1900 1850 - 1910 GSM 1800 1710 - 1785 AWS (US) 1710 - 1755 850 (US) 824 - 849 850 (Japan) 830 - 840 IMT Extension 2500 - 2570 GSM 900 880 - 915 1700 (Japan) 1749.9 - 1784.9 3G Americas 1710 - 1770

Downlink (MHz) 2110 - 2170 1930 - 1990 1805 -1880 2110 - 2155 869 - 894 875 - 885 2620 - 2690 925 - 960 1844.9 - 1879.9 2110- 2170

LTE TDD FREQUENCY BAND ALLOCATIONS 26. With the interest in TDD LTE, there are several unpaired frequency allocations that are being prepared for LTR TDD use. The TDD LTE allocations are unpaired because the uplink and downlink share the same frequency, being time multiplexed. Band Designation A B C D

Band Name TDD 1900 TDD 2.0 PCS centre gap IMT extension centre gap

Allocation (MHz) 1900 - 1920 2010 - 2025 1910 - 1930 2570 - 2620

27. Along with 3G LTE - Long Term Evolution that applies more to the radio access technology of the cellular telecommunications system, there is also an evolution of the core network, known as SAE (System Architecture Evolution). This new architecture has been developed to provide a considerably higher level of performance that is in line with the requirements of LTE. As a result, it is anticipated that operators will commence introducing hardware conforming to the new System Architecture Evolution standards so that the anticipated data levels can be handled when 3G LTE is introduced. The new SAE, System Architecture Evolution has also been developed so that it is fully compatible with LTE Advanced, the new 4G technology. Therefore, when LTE Advanced is introduced, the network will be able to handle the further data increases with little change. CONCLUSION 28. The basic work on 3G LTE has now been completed by 3GPP, although the initial drafts were released in September 2007 and the parallel work on the infrastructure technology known as LTE System Architecture Evolution (SAE) followed shortly afterwards. In terms of the deployments of real systems some anticipate that the first deployments may be seen in 2010 although, one of the main problems will be the user equipment. Initially these are likely to consist of broadband "dongles" for use with laptops with other mobiles appearing later. One of the key elements of LTE is the use of OFDM (Orthogonal Frequency Division Multiplex) as the signal bearer and the associated access schemes, OFDMA (Orthogonal Frequency Division Multiplex) and SC-FDMA (Single Frequency Division Multiple Access).

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29. OFDM is used in a number of other of systems from WLAN, WiMAX to broadcast technologies including DVB and DAB. OFDM has many advantages including its robustness to multipath fading and interference. In addition to this, even though, it may appear to be a particularly complicated form of modulation, it lends itself to digital signal processing techniques. In view of its advantages, the use of ODFM and the associated access technologies, OFDMA and SC-FDMA are natural choices for the new LTE cellular standard.

Read More:1.

https:// site/lteencyclopedia/lte-network-infrastructure-and-elements.

2.

http://www.ericsson.com/res/docs/whitepapers/lte_overview.pdf

3.

http://www.motorolasolutions.com/web/Business/Solutions/LTE_Document