LTE Architecture and Air Interface

8/4/2019 LTE Architecture and Air Interface

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LTE Architecture and Air Interfac

Cap 2 - pag. 1K Labs S.r.l. all right reserved

LTE

RF Layer(Air Interface Radio Aspects)





11

Frequency Bands

• LTE is defined to support flexible BW from below 5MHz up to 20MHz for both FDD and TDD.

• An operator can introduce LTE in both new and existing bands.

• The first may be bands where it is easiest to deploy 10MHz or 20MHz carriers (i.e. 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.

TS 36.101

The first LTE network infrastructure and terminal products will support multiple frequency bands from

day one. LTE will therefore be able to reach high 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 deployedon multiple bands.





12

Multiple access technology in thedownlink: OFDM and OFDMA

Sub-carriersFFT

Time

Symbols

5 MHz Bandwidth

Guard Intervals

Frequency

The OFDM technique differs from traditional FDM in the following interrelated ways:1. multiple carriers (called sub-carriers) carry the information stream,

2. the sub-carriers are orthogonal to each other, and3. a guard time may be added to each symbol to combat the channel delay spread.

Downlink and uplink transmission in LTE are based on the use of multiple access technologies: specifi cally,orthogonal frequency division multiple access (OFDMA) for the downlink, and single-carrier frequency divisionmultiple access (SC-FDMA) for the uplink.OFDMA is a variant of orthogonal frequency division multiplexing (OFDM), a digital multi-carrier modulationscheme that is widely used in wireless systems but relatively new to cellular. Rather than transmit a high-ratestream of data with a single carrier, OFDM makes use of a large number of closely spaced orthogonal subcarriersthat are transmitted in parallel.Each subcarrier is modulated with a conventional modulation scheme (such as QPSK, 16QAM, or 64QAM) at a low

symbol rate. The combination of hundreds or thousands of subcarriers enables data rates similar to conventionalsingle-carrier modulation schemes in the same bandwidth.The diagram in Figure taken from TS 25.892 illustrates the key features of an OFDM signal in frequency and time. Inthe frequency domain, multiple adjacent tones or subcarriers are each independently modulated with data. Thenin the time domain, guard intervals are inserted between each of the symbols to prevent inter-symbol interferenceat the receiver caused by multi-path delay spread in the radio channel.

its use in mobile devices is more recent. The European Telecommunications Standards Institute (ETSI) fi rst lookedat OFDM for GSM back in the late 1980s; however, the processing power required to perform the many FFToperations at the 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 chose an alternative technology based on code divisionmultiple access (CDMA).Today the cost of digital signal processing has been greatly reduced and OFDM is now considered a commerciallyviable method of wireless transmission for the handset.





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SC-FDMA (DFTS-OFDM)

1. Data symbols in the time domain are converted to the frequency domain using a DiscreteFourier transform (DFT)

2. then in the frequency domain they are mapped to the desired location in the overall channelbandwidth before being converted back to the time domain using an inverse FFT (IFFT).

3. Finally, the CP is inserted. Because SC-FDMA uses this technique, it is sometimes calledDiscrete Fourier Transform Spread OFDM or (DFTS-OFDM).

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 was chosen 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 μs; however, the SC-FDMA symbol

contains M “sub-symbols” that represent the modulating data. It is the parallel transmission of multiple

symbols that creates 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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Simplified model of SC-FDMA andOFDMA signal generation and reception

To complete SC-FDMA signal generation, the process follows the same steps as for OFDMA. Performingan IDFT converts the frequency-shifted signal to the time domain and inserting the CP provides thefundamental robustness of OFDMA against multipath.

At this point, it is reasonable to ask how SC-FDMA can be resistant to multipath when the data symbolsare still short. In OFDMA, the modulating data symbols are constant over the 66.7 μs OFDMA symbolperiod, but an SC-FDMA symbol is not constant over time since it contains M sub-symbols of much

shorter duration. The multipath resistance of the OFDMA demodulation process seems to rely on thelong data symbols that map directly onto the subcarriers. Fortunately, it is the constant nature of eachsubcarrier—not the data symbols—that provides the resistance to delay spread. The DFT of the time-varying SC-FDMA symbol generated a set of DFT bins constant in time during the SC-FDMA symbolperiod, even though the modulating data symbols varied over the same period. It is inherent to the DFTprocess that the time-varying SC-FDMA symbol—made of M serial data symbols—is represented in thefrequency domain by M time-invariant subcarriers. Thus, even SC-FDMA with its short data symbolsbenefi ts from multipath protection.It may seem counterintuitive that M time-invariant DFT bins can fully represent a time-varying signal.However, the DFT principle is simply illustrated by considering the sum of two fi xed sine waves atdifferent frequencies. The result is a non-sinusoidal time-varying signal—fully represented by two fi xedsine waves.





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DL/UL Modulations

Uplink Modulation• QPSK• 16QAM

• 64QAM (optional in UE);

Downlink Modulation• QPSK• 16QAM

• 64QAM





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LTE channelization

The LTE Uplink and Downlink physical resource based on OFDM

• Real LTE signals are allocated in units of 12 adjacent subcarriers (15kHz x 12 = 180kHz ).• LTE scheduling/adaptation is on a 1 ms × 180 kHz basis (two Resource Blocks)

½ o f T T I

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 highpeak rates. It is a well-established technology, for example in standards such as IEEE 802.11a/b/g, 802.16, HIPERLAN- 2, DVB andDAB.OFDM uses a large number of narrow sub-carriers for multi-carrier transmission. The basic LTE downlink physical resource can beseen as a time-frequency grid, as illustrated in Figure. In the frequency domain, the spacing between the subcarriers, Δf, is 15kHz.In addition, the OFDM symbol duration time is 1/Δf + cyclic prefix. The cyclic prefix is used to maintain orthogonally between thesub-carriers even for a time-dispersive radio channel.One resource element carries QPSK, 16QAM or 64QAM. With 64QAM, each resource element carries six bits.

The OFDM symbols are grouped into resource blocks. The resource blocks have a total size of 180kHz in the frequency domainand 0.5ms in the time domain. Each 1ms Transmission Time Interval (TTI) consists of two slots (Tslot).Each user is allocated a number of so-called resource blocks in the time–frequency grid. The more resource blocks a user gets,and the higher the modulation used in the resource elements, the higher the bit-rate.Which resource blocks and how many the user gets at a given point in time depend on advanced scheduling mechanisms in thefrequency and time dimensions. The scheduling mechanisms in LTE are similar to those used in HSPA, and enable optimalperformance for different services in different radio environments.

In the uplink, LTE uses a pre-coded version of OFDM called Single Carrier Frequency Division Multiple Access (SC-FDMA). This is tocompensate for a drawback with normal OFDM, which has a very high Peak to Average Power Ratio (PAPR). High PAPR requiresexpensive and inefficient power amplifiers with high requirements on linearity, which increases the cost of the terminal anddrains the battery faster.SC-FDMA solves this problem by grouping together the resource blocks in such a way that reduces the need for linearity, and sopower consumption, in the power amplifier. A low PAPR also improves coverage and the cell-edge performance.





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LTE UL/DL Scheduling

• Uplink and Downlink Scheduling is in Time and Frequency• BW assignment can be equal or non-equal

Example of MaximumC/I scheduling





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Channel Bandwidth

100755025156Maximum number of RBs per Tslot supported N RB

201510531.4Channel bandwidth BWChannel

[MHz]

• The smallest amount of resource that can be allocated in the uplink or downlink is called aresource block (RB).

• An RB is 180 kHz wide and lasts for one 0.5 ms timeslot.

• For standard LTE, an RB comprises 12 subcarriers at a 15 kHz spacing• For eMBMS with the optional 7.5 kHz spacing an RB comprises 24 subcarriers for 0.5 ms.

Transmission

Center subcarrier (corresponds toDC in baseband) is not transmitted

in downlink

Active Resource Blocks

R e s o ur c e

b l o ck

Transmission Bandwidth

Bandwidth

Channel Bandwidth [MHz]

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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Channel arrangement

The spacing between carriers will depend on the deployment scenario, the size of the frequencyblock available and the channel bandwidths. The nominal channel spacing between two adjacent E-UTRA carriers is defined as following:

Nominal Channel spacing = (BW Channel(1) + BW Channel(2) )/2

where BWChannel(1) and BWChannel(2) are the channel bandwidths of the two respective E-UTRAcarriers.

The channel raster is 100 kHz for all bands, which means that the carrier centre frequency mustbe an integer multiple of 100 kHz.

Transmission

Center subcarrier (corresponds toDC in baseband) is not transmitted

in downlink

Active Resource Blocks

R e s o ur c e

b l o ck

Transmission Bandwidth

Bandwidth

Channel Bandwidth [MHz]

The channel spacing can be adjusted to optimize performance in a particular deployment scenario.





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Carrier frequency and E-UTRA Absolute RadioFrequency Channel Number (EARFCN)

The carrier frequency in the UL and DL is designated by the E-UTRA Absolute Radio FrequencyChannel Number (EARFCN).The relation between EARFCN N and the carrier F frequency in MHz for UL and DL is:

F DL/UL = F DL_low/UL_low + 0.1(N DL/UL – N Offs-DL/Offs-UL )

38650-3964938650230038650-3964928650230040

38250-3864938250188038250-3864928250188039

37750 – 3824937750257037750 – 3824927750257038

37550 – 3774937550191037550 – 3774927550191037

36950 – 3754936950193036950 – 3754926950193036

36350 – 3694936350185036350 – 3694926350185035

36200 – 3634936200201036200 – 3634926200201034

36000 – 3619936000190036000 – 3619926000190033

…

23730 - 23849237307045730 – 5849573073417

…

23280 – 23379232807885280 – 5379528075814

23180 – 23279231807775180 – 5279518074613

23000 - 23179230006985000 - 5179500072812

22750 – 22999227501427.94750 – 499947501475.911

22150 – 227492215017104150 – 47494150211010

21800 – 22149218001749.93800 – 414938001844.99

21450 – 21799214508803450 – 379934509258

20750 – 214492075025002750 – 3449275026207

20650 – 20749206508302650 – 274926508756

20400 – 20649204008242400 – 264924008695

19950 – 203991995017101950 – 2399195021104

19200 – 199491920017101200 – 1949120018053

18600 – 19199186001850600 − 119960019302

18000 – 185991800019200 – 599021101

Range of NULNOffs-ULFUL_low (MHz)Range of NDLNOffs-DLFDL_low (MHz)

UplinkDownlinkE-UTRAOperating

Band

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.

LTE Architecture and Air Interface

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