02 - LTEND - Alr Interface

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K Labs S.r.l. all right reserved

LTE Air Interface

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With FDD, downlink and uplink traffic is transmitted simultaneously in

separate frequency bands.With TDD the transmission in uplink and downlink is discontinuous within

the same frequency band.


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Time: time adjusted by the common SFN initialisation time, in units of

10ms to match the length of radio frame and accuracy accordingly;period(SFN): SFN period.

NOTE: When eNB is connected via TDM interfaces, these could be used to

synchronize frequency the eNB. The characteristics of these interfaces are

described in 25.411.

In case eNB is connected via TDM interface, it may be used to synchronize

frequency the eNB. The characteristics of the clock in the eNB shall be

designed taking into account that the jitter and wander performance

requirements on the interface are in accordance with network limits foroutput wander at traffic interfaces of either Reference [7], [8] or network

limits for the maximum output jitter and wander at any hierarchical

interface of Reference [9], whichever is applicable.

In case eNB is connected via Ethernet interface and the network supports

Synchronous Ethernet, the eNB may use this interface to get frequency

synchronization. In this case the design of the eNB clock should be done

considering the jitter and wander performance requirements on the

interface are as specified for output jitter and wander at EEC interfaces of

Reference [10], defined in section 9.2.1/G.8261. Further considerations on

Synchronous Ethernet recommendations and architectural aspects are

defined in clause 12.2.1 and Annex A of G.8261.


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The first LTE network infrastructure and terminal products will support

multiple frequency bands from day one. LTE will therefore be able to reachhigh economies of scale and global coverage quickly.

LTE is defined to support flexible carrier bandwidths from below 5MHz up

to 20MHz, in many spectrum bands and for both FDD and TDD

deployments. This means that an operator can introduce LTE in both new

and existing bands. The first may be bands where it, in general, is easiest to

deploy 10MHz or 20MHz carriers (for example, 2.6GHz (Band VII), AWS

(Band IV), or 700MHz bands), but eventually LTE will be deployed in all

cellular bands. In contrast to earlier cellular systems, LTE will rapidly be

deployed on multiple bands.


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The first LTE network infrastructure and terminal products will support

multiple frequency bands from day one. LTE will therefore be able to reachhigh economies of scale and global coverage quickly.

LTE is defined to support flexible carrier bandwidths from below 5MHz up

to 20MHz, in many spectrum bands and for both FDD and TDD

deployments. This means that an operator can introduce LTE in both new

and existing bands. The first may be bands where it, in general, is easiest to

deploy 10MHz or 20MHz carriers (for example, 2.6GHz (Band VII), AWS

(Band IV), or 700MHz bands), but eventually LTE will be deployed in all

cellular bands. In contrast to earlier cellular systems, LTE will rapidly be

deployed on multiple bands.


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The two best known modern wireless digital communication transmission

schemes are CDMA and OFDM. CDMA is used on the new 3G mobile phonesystem and is a wideband transmission scheme, which means that the

channel symbols (which are called chips for CDMA) are far shorter than the

maximum delay of the mobile channel. OFDM, as used on DAB and

Freeview actually uses narrowband channels (subcarriers), but there are

many of these narrowband channels transmitted in parallel, so the overall

spectrum is wide (but this doesn't mean that it uses wideband transmission

principles).


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Downlink and uplink transmission in LTE are based on the use of multiple accesstechnologies: specifi cally, orthogonal frequency division multiple access (OFDMA)

for the downlink, and single-carrier frequency division multiple access (SC-FDMA)for the uplink.OFDMA is a variant of orthogonal frequency division multiplexing (OFDM), a digitalmulti-carrier modulation scheme that is widely used in wireless systems butrelatively new to cellular. Rather than transmit a high-rate stream of data with asingle carrier, OFDM makes use of a large number of closely spaced orthogonalsubcarriers that are transmitted in parallel.Each subcarrier is modulated with a conventional modulation scheme (such asQPSK, 16QAM, or 64QAM) at a low symbol rate. The combination of hundreds orthousands of subcarriers enables data rates similar to conventional single-carriermodulation schemes in the same bandwidth.The diagram in Figure taken from TS 25.892 illustrates the key features of an OFDMsignal in frequency and time. In the frequency domain, multiple adjacent tones orsubcarriers are each independently modulated with data. Then in the time domain,guard intervals are inserted between each of the symbols to prevent inter-symbolinterference at the receiver caused by multi-path delay spread in the radio channel.

its use in mobile devices is more recent. The European TelecommunicationsStandards Institute (ETSI) fi rst looked at OFDM for GSM back in the late 1980s;however, the processing power required to perform the many FFT operations atthe heart of OFDM was at that time too expensive and demanding for a mobile

application. In 1998, 3GPP seriously considered OFDM for UMTS, but again chosean alternative technology based on code division multiple access (CDMA).Today the cost of digital signal processing has been greatly reduced and OFDM isnow considered a commercially viable method of wireless transmission for thehandset.


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Data symbols are synchronously and independently transmitted over a

high number of closely spaced orthogonal sub-carriers using linearmodulation (either PSK or QAM).


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In practice, the OFDM signal can be generated using IFFT digital signal

processing. The baseband representation of the OFDM signal generationusing an N-point IFFT is illustrated in Figure 3, where a(mN+n) refers to the

nth sub-channel modulated data symbol, during the time period mTu < t

(m+1)Tu.

The vector sm is defined as the useful OFDM symbol. Note that the vector

sm is in fact the time superposition of the N narrowband modulated sub-

carriers.


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It is therefore easy to realize that, from a parallel stream of N sources ofdata, each one modulated with QAM useful symbol period Tu, a waveform

composed ofN orthogonal sub-carriers is obtained, with each narrowbandsub-carrier having the shape of a frequency sinc function (see Figure 1).Figure 4 illustrates the mapping from a serial stream of QAM symbols to Nparallel streams, used as frequency domain bins for the IFFT. The N-pointtime domain blocks obtained from the IFFT are then serialized to create atime domain signal.


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A guard interval may be added prior to each useful OFDM symbol. This guard time

is introduced to minimize the inter-OFDM-symbol-interference power caused by

time-dispersive channels. The guard interval duration Tg (which corresponds to Np

prefix samples) must hence be sufficient to cover the most of the delay-spread

energy of a radio channel impulse response. In addition, such a guard time interval

can be used to allow soft-handover.

A prefix is generated using the last block of Np samples from the useful OFDM

symbol. The prefix insertion operation is illustrated in Figure 5. Note that since the

prefix is a cyclic extension to the OFDM symbol, it is often termed cyclic prefix.

Similarly, a cyclic postfix could be appended to the OFDM symbol.

The cyclic prefix should absorb most of the signal energy dispersed by themulti-path channel. The entire the inter-OFDM-symbol-interference energy

is contained within the prefix if the prefix length is greater than that of the

channel total delay spread.


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Il profilo spettrale del segnale OFDM a coseno rialzato e quindi

loccupazione di banda dipende dal fattore di roll-off scelto ed pari a(1+alfa)*1/(2*Ts) in banda base. Siccome non possibile utilizzare fattori di

roll-off troppo bassi, a causa delleccessiva complessit del filtro, i canali

OFDM attigui sono parzialmente sovrapposti. Per tale ragione, al fine cio

di evitare la cosiddetta InterCarrier Interference (ICI) le portanti laterali

vengono soppresse, cio non sono utilizzate per il trasporto dei bit.

Il valore di alfa oscilla solitamente da un minimo di 0.1 ad un massimo di

0.4.


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With standard OFDM, very narrow UE-specific transmissions can suffer

from narrowband fading and interference. That is why for the downlink3GPP chose OFDMA, which incorporates elements of time division multiple

access (TDMA).

OFDMA allows subsets of the subcarriers to be allocated dynamically

among the different users on the channel.


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As a reminder, IFFT (Inverse Fast Fourier Transform) is used in a transmitter

to create an OFDM waveform from modulated data streams, while FFT(Fast Fourier Transform) is used in a receiver to demodulate the data

streams. The FFT size equals the number of sub-carriers, e.g. in a

OFDM/OFDMA system with 256 sub-carriers, the FFT size is 256.

SOFDMA is the OFDMA mode used in Mobile WiMAX and LTE.

It supports channel bandwidths ranging from 1.25 MHz to 20 MHz.

With bandwidth scalability, Mobile WiMAX technology can comply with

various frequency regulations worldwide and flexibly address diverse

operator or ISP requirements, that's whether for providing only basic

Internet service or a broadband service bundle.


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The high peak-to-average ratio (PAR) associated with OFDM led 3GPP to

look for a different transmission scheme for the LTE uplink. SC-FDMA waschosen because it combines the low PAR techniques of single-carrier

transmission systems, such as GSM and CDMA, with the multi-path

resistance and fl exible frequency allocation of OFDMA.

A mathematical description of an SC-FDMA symbol in the time domain is

given in TS 36.211 sub-clause 5.6.

Note that OFDMA and SC-FDMA symbol lengths are the same at 66.7 s;

however, the SC-FDMA symbol contains M sub-symbols that represent

the modulating data. It is the parallel transmission of multiple symbols thatcreates the undesirable high PAR of OFDMA. By transmitting the M data

symbols in series at M times the rate, the SC-FDMA occupied bandwidth is

the same as multi-carrier OFDMA but, crucially, the PAR is the same as that

used for the original data symbols. Adding together many narrow-band

QPSK waveforms in OFDMA will always create higher peaks than would be

seen in the wider-bandwidth, single-carrier QPSK waveform of SC-FDMA.

As the number of subcarriers M increases, the PAR of OFDMA with random

modulating data approaches Gaussian noise statistics but, regardless of the

value of M, the SC-FDMA PAR remains the same as that used for the

original data symbols.


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As noted, SC-FDMA signal generation begins with a special pre-codingprocess. Figure shows the first steps, which create a time-domainwaveform of the QPSK data sub-symbols. Using the four color-coded QPSKdata symbols, the process creates one SC-FDMA symbol in the time domainby computing the trajectory traced by moving from one QPSK data symbolto the next. This is done at M times the rate of the SC-FDMA symbol suchthat one SC-FDMA symbol contains M consecutive QPSK data symbols.Time-domain filtering of the data symbol transitions occurs in any realimplementation, although it is not discussed here.Once an IQ representation of one SC-FDMA symbol has been created in thetime domain, the next step is to represent that symbol in the frequency

domain using a DFT. This is shown in Figure 10. The DFT samplingfrequency is chosen such that the time-domain waveform of one SC-FDMAsymbol is fully represented by M DFT bins spaced 15 kHz apart, with eachbin representing one subcarrier in which amplitude and phase are heldconstant for 66.7 s.A one-to-one correlation always exists between the number of datasymbols to be transmitted during one SC-FDMA symbol period and thenumber of DFT bins created. This in turn becomes the number of occupiedsubcarriers. When an increasing number of data symbols are transmittedduring one SC-FDMA period, the time-domain waveform changes faster,

generating a higher bandwidth and hence requiring more DFT bins to fullyrepresent the signal in the frequency domain. Note in Figure that there isno longer a direct relationship between the amplitude and phase of theindividual DFT bins and the original QPSK data symbols. This differs fromthe OFDMA example in which data symbols directly modulate thesubcarriers.


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The next step of the signal generation process is to shift the baseband DFT

representation of the time-domain SC-FDMA symbol to the desired part ofthe overall channel bandwidth. Because the signal is now represented as a

DFT, frequency-shifting is a simple process achieved by copying the M bins

into a larger DFT space of N bins. This larger space equals the size of the

system channel bandwidth, of which there are six to choose from in LTE

spanning 1.4 to 20 MHz. The signal can be positioned anywhere in the

channel bandwidth, thus executing the frequency-division multiple access

(FDMA) essential for effi ciently sharing the uplink between multiple users.


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SC-OFDMA combines:

Small variations in the instantaneous power of the transmittedsignal (single carrier property).

Possibility for low-complexity high-quality equalization in the

frequency domain.

Possibility for FDMA with flexible bandwidth assignment.

DFT/IDFT combination achieves multiplexing between users.

Padding with zeros in frequency domain reduces PAR

(remember the delta function example).Cyclic prefix simplifies the frequency domain equalization at

the receiver side.


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To complete SC-FDMA signal generation, the process follows the samesteps as for OFDMA. Performing an IDFT converts the frequency-shiftedsignal to the time domain and inserting the CP provides the fundamentalrobustness of OFDMA against multipath.

At this point, it is reasonable to ask how SC-FDMA can be resistant tomultipath when the data symbols are still short. In OFDMA, the modulatingdata symbols are constant over the 66.7 s OFDMA symbol period, but anSC-FDMA symbol is not constant over time since it contains M sub-symbolsof much shorter duration. The multipath resistance of the OFDMAdemodulation process seems to rely on the long data symbols that map

directly onto the subcarriers. Fortunately, it is the constant nature of eachsubcarriernot the data symbolsthat provides the resistance to delayspread. The DFT of the time-varying SC-FDMA symbol generated a set ofDFT bins constant in time during the SC-FDMA symbol period, even thoughthe modulating data symbols varied over the same period. It is inherent tothe DFT process that the time-varying SC-FDMA symbolmade of M serialdata symbolsis represented in the frequency domain by M time-invariantsubcarriers. Thus, even SC-FDMA with its short data symbols benefi ts frommultipath protection.It may seem counterintuitive that M time-invariant DFT bins can fully

represent a time-varying signal. However, the DFT principle is simplyillustrated by considering the sum of two fi xed sine waves at differentfrequencies. The result is a non-sinusoidal time-varying signalfullyrepresented by two fi xed sine waves.


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LTE uses OFDM for the downlink that is, from the base station to the terminal.OFDM meets the LTE requirement for spectrum flexibility and enables cost-efficient solutions

for very wide carriers with high peak rates. It is a well-established technology, for example instandards such as IEEE 802.11a/b/g,802.16,HIPERLAN- 2, DVB and DAB.OFDM uses a large number of narrow sub-carriers for multi-carrier transmission. The basicLTE downlink physical resource can be seen as a time-frequency grid, as illustrated in Figure.In the frequency domain, the spacing between the subcarriers, f, is 15kHz. In addition, theOFDM symbol duration time is 1/f + cyclic prefix. The cyclic prefix is used to maintainorthogonally between the sub-carriers even for a time-dispersive radio channel.One resource element carries QPSK, 16QAM or 64QAM. With 64QAM, each resource elementcarries six bits.The OFDM symbols are grouped into resource blocks. The resource blocks have a total size of180kHz in the frequency domain and 0.5ms in the time domain. Each 1ms Transmission TimeInterval (TTI) consists of two slots (Tslot).

Each user is allocated a number of so-called resource blocks in the timefrequency grid. Themore resource blocks a user gets, and the higher the modulation used in the resourceelements, the higher the bit-rate.Which resource blocks and how many the user gets at a given point in time depend onadvanced scheduling mechanisms in the frequency and time dimensions. The schedulingmechanisms in LTE are similar to those used in HSPA, and enable optimal performance fordifferent services in different radio environments.

In the uplink, LTE uses a pre-coded version of OFDM called Single Carrier Frequency DivisionMultiple Access (SC-FDMA). This is to compensate for a drawback with normal OFDM, whichhas a very high Peak to Average Power Ratio (PAPR). High PAPR requires expensive andinefficient power amplifiers with high requirements on linearity, which increases the cost of

the terminal and drains the battery faster.SC-FDMA solves this problem by grouping together the resource blocks in such a way thatreduces the need for linearity, and so power consumption, in the power amplifier. A lowPAPR also improves coverage and the cell-edge performance.


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The channel edges are defined as the lowest and highest frequencies of the

carrier separated by the channel bandwidth, i.e. at FC +/- BWChannel /2

Unlike the eNB, the UE does not normally transmit across the entire

channel bandwidth.


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The transmission bandwidth configuration shall be supported for each of

the specified channel bandwidths. The same (symmetrical) channelbandwidth is specified for both the TX and RX path.


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The channel spacing can be adjusted to optimize performance in a

particular deployment scenario.


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The carrier frequency in the uplink and downlink is designated by the E-

UTRA Absolute Radio Frequency Channel Number (EARFCN) in the range 0- 65535.


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02 - LTEND - Alr Interface