TEMA 5. Part 2. LTE Tx Blocks PDF

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SystemandCommunication TechnologiesforSmartCities FunctionalBlocksoftheLTETransmitter (Downlink) PereL.Gilabert [email protected]

Outline

Outline GeneralBlockDiagramoftheLTETransmitter CrestFactorReductionTechniques DigitalPredistortionLinearization

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Outline

Outline GeneralBlockDiagramoftheLTETransmitter CrestFactorReductionTechniques DigitalPredistortionLinearization

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General Block Diagram of the LTE Transmitter

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General Block Diagram of the LTE Transmitter

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General Block Diagram of the LTE Transmitter  The Orthogonal Frequency Division Multiplexing (OFDM) uses orthogonal carriers to allow overlapping among them and thus increase the spectrum efficiency.  The spacing between the subchannels in OFDM is such they can be perfectly separated at the receiver. This allows for a low‐complexity receiver implementation, which makes OFDM attractive for high‐rate mobile data transmission such as the LTE downlink. FDMneedsguardbands

OFDM allowsspectrumoverlapping Spectral Power Density

1/T

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Frequency

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General Block Diagram of the LTE Transmitter  InOFDM,thehigh‐ratestreamofdatasymbolsisfirstserial‐to‐parallelconverted formodulationontoM parallelsubcarriers.Thisincreasesthesymboldurationon eachsubcarrierbyafactorofapproximatelyM,suchthatitbecomessignificantly longerthanthechanneldelayspread.

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General Block Diagram of the LTE Transmitter  The signal to be transmitted is defined in the frequency domain. A Serial to Parallel (S/P) converter collects serial data symbols into a data block Sk of dimension M, where the subscript k is the index of an OFDM symbol (spanning the M sub‐carriers).  The M parallel data streams are independently modulated (e.g. M‐QAM) resulting in the complex vector of data symbols Xk.  Then, Xk passes through an IFFT resulting in a set of N complex time‐domain samples xk.  In a practical OFDM system, the number of processed subcarriers is greater than the number of modulated sub‐carriers (i.e. N ≥M), with the unmodulated sub‐carriers being padded with zeros.

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General Block Diagram of the LTE Transmitter  Therefore, an OFDM signal is built by adding N orthogonal subcarriers in a period T, that have been previously modulated independently.  The OFDM signal consists of blocks of T seconds (OFDM symbols), each block transporting N modulated symbols.

x (t ) 



N 1

  X (k ) e i

j 2 k  t  iT  Tu

rect  t  iT 

i  k  0

whereX(k)isthecomplexmodulationsymbolallocatedinthek‐th subcarrier(k= 0,1,…,N‐1)andassumingthatiistheOFDMinformationsymbolnumber.

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General Block Diagram of the LTE Transmitter

 The creation of a guard period at the beginning of each OFDM symbol is oriented to eliminate the remaining impact of ISI caused by multipath propagation.  The guard period is obtained by adding a Cyclic Prefix (CP) at the beginning of the symbol xk. The CP is generated by duplicating the last G samples of the IFFT output and appending them at the beginning of xk.  To avoid ISI, the CP length G must be chosen to be longer than the longest channel impulse response to be supported (delay spread).  The extended block is cyclic for any interval Tu inside the block.

T

T

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T -10-

General Block Diagram of the LTE Transmitter



Advantages of OFDM •

Inherent protection against multipath propagation (cyclic prefix)

•

Simplified equalisation.

•

High spectral efficiency. High protection against narrow band interferences and impulsive noise.

• • •



Computational efficiency (FFT) Easy filtering at reception: guard band just by disabling some border carriers.

Disadvantages of OFDM •

• •

High accuracy in synchronism required to avoid ICI due to oscillators tolerance and Doppler effect (orthogonality lose) Very high PAPR: very linear components required. Higher sensibility to time variations of the channel (long symbols), which generate ICI as well.

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General Block Diagram of the LTE Transmitter Downlink LTE signal (particularizing) • Duplexingmode:FDD • Maximumbandwidth:20MHz • Basebandsamplingfrequency(attheoutputof theIFFTandtheinputoftheFFT):30,72MSPS • Intercarrierseparation:f=15kHz • NumberofpointsofIFFT/FFT:2048 • Numberofactivecarriers:600+DCcarrier+600 • UsefulpartoftheOFDMsymbol:Tu =66,7s • ExtendedCP:¼oftheusefulpartoftheOFDM symbol

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General Block Diagram of the LTE Transmitter

 The position of the pilots (reference signals) are specified by the communications standard and thus known by the receiver. These pilots are used at the receiver for different applications such as, for example, channel estimation.  The resampling factor R (interpolation factor) is set according to the DAC clock in order to generate the OFDM signal at a sample rate of R x 30.72 MSPS. In order to relax the transition bandwidth of the of the (low‐pass or band‐pass) analog filter the resampling factor R has to be as high as possible.

I‐Q Mod.

IF Mod.

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Outline

Outline GeneralBlockDiagramoftheLTETransmitter CrestFactorReductionTechniques DigitalPredistortionLinearization

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Crest Factor Reduction Techniques

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Crest Factor Reduction Techniques The Peak to Average Power Ratio (PAPR) is a relationship between the maximum value of the peak power and the average power of the signal. out  Pmax PAPR  10 log  out P  avg



 max x[ n] 2  n  10 log   2  E x[n ]  



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

  [dB ]  

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Crest Factor Reduction Techniques The complementary cumulative distribution function (CCDF) of the PAPR is one of the most frequently used performance measures for PAPR reduction techniques. The CCDF of the PAPR denotes the probability that the PAPR of a data block exceeds a given threshold.

CCDF for an OFDM signal considering different clipping ratios. System and Communication Technologies for Smart Cities

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Crest Factor Reduction Techniques WhyisitimportanttoreducePAPR? PAPR can degrade completely a wireless transmission chain, since it can put most of those systems operating in a large‐signal nonlinear zone.

The D/A and A/D converters and power amplifier of the transmitter require large dynamic ranges to avoid amplitude clipping (and thus nonlinear distortion), which implies increasing both power consumption and component cost of the transceiver.

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Crest Factor Reduction Techniques TohavelinearamplificationatthePAoutput weneedtooperatewithaback‐off levelssimilartothePAPRoftheinputsignalwhichresultspowerinefficient… Psatout 22

P1out dB

 P out  PAPR  10 log  max  [dB ] out   Pavg  

Output Power (dBm)

20 18 16

in IBO  10 log  Psatin   10 log Pavg 

14 12

out OBO  10 log  Psatout  10 log  Pavg 

10 8 -2

0

2

10

P1indB

12

Psatin

14 12

 (%) 

PA E (%)

10 8 6

Pout PDC

PAE (%) 

4 2

Pout  Pin PDC

0 -2

0

2

4

6

8

10

12

Input Power (dBm)

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Crest Factor Reduction Techniques  In Digital‐to‐Analog Converters the main problem has to do with the available dynamic range that can be fulfilled without distortion noise addition. • The use of PAPR reduction techniques improves the DAC performance since the achieved SNR is increased. • To have the same SNR in classic OFDM and when a PAPR reduction technique is employed, the resolution N’ with respect to N is:

PAPR0  PAPRRED N'  N  6.02

N – required resolution of classic OFDM N’ – required resolution after using PAPR reduction PAPR0 – PAPR in classic OFDM PAPRRED – PAPR after using a reduction technique

• So, the required DAC resolution is reduced by one bit if the theoretical PAPR upper bound is decreased by 6 dB.

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Crest Factor Reduction Techniques

 In an Analog‐to‐Digital Converter, the increase of PAPR will for sure reduce its dynamic range.

ForinstancetheSNRinanADCcanbeapproximatedby:

SNR  6.02 N  K    10 log10 [2  OSR] N– number ofbits α– the PAPR, OSR– oversampling ratio K=1.76for asinusoidal(one tone)

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Crest Factor Reduction Techniques

 PAPR reduction techniques span from software to hardware solutions. Those include techniques based on: •

Coding

•

PartialTransmitSequence

• •

SelectedMapping Interleaving

• •

Tonereservation Toneinjection

•

Clipping/filtering

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Crest Factor Reduction Techniques

Coding •

A technique that can be used efficiently but at a cost of extra overheads. The idea is to select those codewords that minimize or reduce the PAPR for transmission.

•

For instance in a 3 bit word, the idea is to select the 4th bit that will create the OFDM signal with low PAPR.

•

However, this approach suffers from the need to perform an exhaustive search to find the best codes and to store large lookup tables for encoding and decoding, especially for a large number of subcarriers.

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Crest Factor Reduction Techniques

Partial Transmit Sequence (PTS)Techniques • In the PTS technique, an input data block of N symbols is partitioned into disjoint subblocks. • Then, the subcarriers in each subblock are weighted by a phase factor for that subblock. • The phase factors are selected such that the PAPR of the combined signal is minimized. • The phase information vector is further transmitted to the receiving part.

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Crest Factor Reduction Techniques

Selected Mapping Technique (SLM) •

This technique will use the transmit sequence and multiply it by a codeword (vector of phase‐shifts) that will change it in order to decrease the original PAPR.

•

In the SLM technique, the transmitter generates a set of sufficiently different candidate data blocks, all representing the same information as the original data block, and selects the most favorable for transmission (lowest PAPR).

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Crest Factor Reduction Techniques

InterleavingTechnique •

The interleaving technique for PAPR reduction is very similar to the SLM technique. In this approach, a set of interleavers is used to reduce the PAPR of the multicarrier signal instead of a set of phase sequences.

•

An interleaver is a device that operates on a block of N symbols and reorders or permutes them: •

•

Thus, data block X = [X0, X1, …, XN–1]T becomes X’ = [Xπ(0), Xπ(1), …, Xπ(N–1)]T where {n}↔ {π(n)} is a one‐to‐one mapping π(n)є{0, 1, …, N – 1} and for all n. To make K modified data blocks, interleavers are used to produce permuted data blocks from the same data block.

The PAPR of (K – 1) permuted data blocks and that of the original data block are computed using K IDFT operations; the data block with the lowest PAPR is then chosen for transmission.

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Crest Factor Reduction Techniques

Tone Reservation Technique • In this case the use of subcarriers, that are not used for transmission, are used to insert new information that is selected in order to reduce PAPR. • Tone reservation (TR) and tone interjection (TI), are two methods based on adding a data‐block‐dependent time domain signal to the original multicarrier signal to reduce its peaks. • This time domain signal can be easily computed at the transmitter and stripped off at the receiver.

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Crest Factor Reduction Techniques

Tone Injection (TI)andActiveConstellation Extension (ACE) •

These techniques use the fact that a different constellation diagram can be used to reduce the PAPR.

•

In the TI the use of a new tone imply that the constellation diagram is in fact altered in order to reduce PAPR. Since each symbol in a data block can be mapped into one of several equivalent constellation points, these extra degrees of freedom can be exploited for PAPR reduction.

•

This method is called tone injection because substituting a point in the basic constellation for a new point in the larger constellation is equivalent to injecting a tone of the appropriate frequency and phase in the multicarrier signal.

•

In the ACE the symbol to transmit is not necessarily the point in the constellation diagram, but a slight version of it.

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Crest Factor Reduction Techniques

AmplitudeClippingandFiltering •

Clipping is a technique that is used for abruptly reduce the peaks on the envelope signal. Nevertheless the peak cut‐off will generate nonlinear distortion spectral regrowth, which is somehow compensated by further band‐pass filtering.

•

This clipping + filtering is repeated several times until a proper compromise is obtained between distortion and peaks.

A  e yn   x n

jx

if xn  A if xn  A

 C  e jx if xn  U   C  D y n    xn  D    D if D  xn  U  U D   x if xn  D  n

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Crest Factor Reduction Techniques

AmplitudeClippingandFiltering ExampleofHardandSoftClipping+Filtering

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Crest Factor Reduction Techniques

•

Clipping, can in certain aspect minimize the PAPR, but at a cost of increasing the Symbol Error Rate (SER), since by filtering we can remove the adjacent channel distortion but not the co‐channel distortion, which is also manifested in the Error Vector Magnitud (EVM).

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Crest Factor Reduction Techniques

 There are some factors that have to be considered in order to properly select a specific crest factor reduction technique.  These factors include:  PAPR reduction capability (the amount of PAPR reduction achieved)  Introduction of in‐band distortion (e.g. Clipping + Filtering)  Power increase in transmit signal (e.g. Tone Reservation, Tone Injection)  BER increase at the receiver (e.g. Partial Transmit Sequence, Interleaving, Selected Mapping, Active Constellation Extension)  Loss in data rate (Coding, Partial Transmit Sequence, Selected Mapping, Interleaving)  Computational complexity increase (e.g. Partial Transmit Sequence, Interleaving, Selected Mapping)

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Crest Factor Reduction Techniques

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Outline

Outline GeneralBlockDiagramoftheLTETransmitter CrestFactorReductionTechniques DigitalPredistortionLinearization

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Digital Predistortion Linearization

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Digital Predistortion Linearization ThePAisa powerhungrydevice… AM‐AM

vin ( t)

PA

vout (t ) 

vout (t )   g k vink (t ) k 1

AM‐PM

Outputofatwo‐tonetest inaNonlinearPA In‐BandDistortion  Compression  Capture Out‐of‐BandDistortion  HarmonicDistortion(HD)  Intermodulation Distortion(IMD)

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Digital Predistortion Linearization …thatintroducesunwantednonlineardistortion… Out‐of‐bandDistortion

 P  f   df out

ACPR 

B

[dBr ]

 P  f   df   P  f  df out

LS

vin (t )

out

US

PA

vout ( t)

In‐bandDistortion

EVM 

1 N   I 2  Q 2  N 1 [%] 2 S max

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Digital Predistortion Linearization …andlineardistortion(memoryeffects)…  Description:

MainsourcesofmemoryeffectsinPAs

Memory effects can be described as a dependency of the gain of this nonlinear device on past events. Time responses are not instantaneous anymore but will be convolved by the impulse response of the system.  They appear as: • Asymmetries in the IMD products (frequency domain) • Dispersion in the decision points of the constellation (time domain)  Techniquestocancel orminimizememoryeffects: • Impedance optimization

Maintypes:

• Envelope injection

• Electrical

• Envelope filtering

• Electrothermal

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