4g LTE and LTE-A for mobile broadband-note

1. 1
4G LTE & LTE-A for Mobile
Broadband_Note
Jay Chang

2. 1 10 100
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Bandwidth-limited
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Ebn0ratio(dB)
Bandwidth utilization γ
power-limited
region
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Bandwidth-limited
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Bandwidth utilization γ
power-limited
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Def BW utilizat
log 1
log 1 log 1
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min
ion,
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b
b
b b
S
C BW
N
E RS
R C BW BW
N N BW
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R
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N
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γ
γ γ
γ
γ
 
= ⋅ + 
 
 ⋅ 
≤ = ⋅ + = ⋅ +  
⋅   
 
≤ + 
 
  −
⇒ ≥ =
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1. ߛ < 1, Eb/N0 = const, N0固定Eb也固定. S = Eb × R, S ↑, R ↑.
2. ߛ > 1, min required Eb/N0 increases rapidly with ߛ, if R of
same order or larger than BW, if without increase
corresponding BW, S ↑↑↑, R ↑.
2

3. High data rates in noise-limited scenarios
3

4. 4

5. Multi-carrier transmission
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7. 7

8. Basic principles of OFDM
OFDM
• Multi-carrier transmission.
• Use hundred subcarriers are transmitted.
a. HSPA multi-carrier use a 20 MHz overall transmission bandwidth consists of 4 subcarriers, each with a
bandwidth of the order of 5 MHz.
b. In comparison, OFDM transmission may imply several hundred subcarriers are transmitted over the same
radio link to the same receiver.
• Simple rectangular pulse shaping. This corresponds to a sinc-square-shaped per-subcarrier spectrum.
• Tight frequency-domain packing of the subcarriers with a subcarrier spacing ∆݂ = 1/Tu, where Tu is the per-
subcarrier modulation-symbol time.
(a) Per-subcarrier pulse shape and
(b) spectrum for basic OFDM transmission
8

9. 9
OFDM modulator
Basic principles of OFDM
Nc complex modulators
• In complex baseband notation, a basic OFDM signal x(t) during the time interval mTu ≤ t < (m + 1)Tu can thus
be expressed as
modulation symbols can be from any modulation alphabet, such as QPSK, 16QAM, or 64QAM.

10. 10
LTE Configurations and Parameters
• What subcarrier spacing to use depends on what types of environments the system is to operate in.
a. maximum expected radio-channel frequency selectivity (maximum expected time dispersion).
b. maximum expected rate of channel variations (maximum expected Doppler spread).
• Once the subcarrier spacing has been selected, the number of subcarriers can be decided based on the
assumed overall transmission BW.

11. 11
Basic principles of OFDM
• The “physical resource” in the case of OFDM transmission is often illustrated as a time–frequency grid
according to Figure, where each “column” corresponds to one OFDM symbol and each “row” corresponds to
one OFDM subcarrier.
OFDM time–frequency grid

12. 12
Basic principles of OFDM
OFDM demodulation
OFDM demodulation consisting of a bank of correlators, one for each subcarrier.
The orthogonality between subcarriers, in the ideal case, two OFDM subcarriers do not cause any interference
to each other after demodulation.
The avoidance of interference between OFDM subcarriers is not simply due to a subcarrier spectrum
separation (subcarrier spacing ∆݂ = 1/Tu), but also multi-carrier extension.
Any corruption of the frequency-domain structure of the OFDM subcarriers, for example due to a frequency-
selective radio channel, may lead to a loss of inter-subcarrier orthogonality and thus to interference between
subcarriers.
• To make an OFDM signal robust to radio-channel frequency selectivity, cyclic-prefix insertion is used.
Basic principle of OFDM demodulation

13. 13
Basic principles of OFDM
OFDM implementation using IFFT/FFT processing
Although a bank of modulators/correlators can be used to illustrate the basic principles of OFDM modulation
and demodulation respectively, these are not the most appropriate modulator/demodulator structures for
actual implementation.
OFDM allows for low-complexity implementation Fast Fourier Transform (FFT) processing.
OFDM modulation by means of IFFT processing
by selecting the IDFT size N equal to
2m for some integer m, the OFDM
modulation can be implemented by
means of implementation-efficient
radix-2 IFFT processing.

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for 3GPP LTE
• the number of subcarriers Nc is approximately 600 in the case of a 10 MHz spectrum allocation.
• The IFFT size can be selected as N = 1024.
• This corresponds sampling rate ݂‫ݏ‬ = ܰ∆݂ = 15.36 MHz, where ∆݂ = 15 kHz is the LTE subcarrier spacing.
Basic principles of OFDM
by selecting the IDFT size N equal to
2m for some integer m, the OFDM
modulation can be implemented by
means of implementation-efficient
radix-2 IFFT processing.
OFDM modulation by means of IFFT processing

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Basic principles of OFDM
OFDM demodulation by means of FFT processing
OFDM modulation by means of IFFT processing

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Basic principles of OFDM
Cyclic-prefix insertion
A modulated subcarrier xk(t) consists of an integer number of periods of complex exponentials during the
demodulator integration interval Tu = 1/∆݂, can be demodulated without any interference between subcarriers.
1. Time-dispersive channel the orthogonality between the subcarriers will be lost.
• The reason for this loss of subcarrier orthogonality in the case of a time-dispersive channel is that the
demodulator correlation interval for one path will overlap with the symbol boundary of a different path.
• In the case of a time-dispersive channel there will not only be inter-symbol interference within a
subcarrier, but also interference between subcarriers.
2. Frequency selectivity, the orthogonality between subcarriers will be lost with inter-subcarrier interference.

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Basic principles of OFDM
Cyclic-prefix insertion
1. Tx
• In practice, cyclic-prefix insertion is carried out on the time-discrete output of the transmitter IFFT.
• The last NCP samples of the IFFT output block of length N are copied and inserted at the beginning of the
block, increasing the block length from N to N + NCP.
2. Rx
• The corresponding samples are discarded before OFDM demodulation by means of DFT/FFT processing.
3. Drawback of cyclic-prefix insertion
• only a fraction Tu/(Tu + TCP) of the received signal power is actually utilized by the OFDM demodulator,
implying a corresponding power loss in the demodulation.
• Corresponding loss in terms of bandwidth as the OFDM symbol rate is reduced.
4. Reduce the relative overhead due to cyclic-prefix
• subcarrier spacing ∆݂ ↓, with a corresponding increase in the symbol time Tu as a consequence.
• Cause high Doppler spread and frequency errors.
• Trade-off between the power loss and the signal corruption (inter-symbol and inter-subcarrier
interference) due to residual time dispersion not covered by the cyclic prefix.

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Basic principles of OFDM
Without CP
With CP

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Basic principles of OFDM
Frequency-domain model of OFDM transmission
Assuming a sufficiently large cyclic prefix, the linear convolution of a time-dispersive radio channel will appear as a circular
convolution during the demodulator integration interval Tu.
The combination of OFDM modulation (IFFT processing), a time-dispersive radio channel, and OFDM demodulation (FFT
processing) can then be seen as a frequency-domain channel.
Where the frequency-domain channel taps H0, ..., HNc-1 can be directly derived from the channel impulse response.
Fig. Frequency-domain model of OFDM transmission/reception
• To properly recover the transmitted symbol for further
processing, for data demodulation and channel decoding.
Fig. Frequency-domain model of OFDM transmission/reception
with “one-tap equalization” at the receiver

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Basic principles of OFDM
Channel estimation and reference symbols
Scaling with the complex conjugate of the frequency-domain channel tap Hk should be applied after OFDM
demodulation (FFT processing) receiver needs an estimate of the frequency-domain channel taps H0, ..., HNc-1.
Frequency-domain channel taps求法:
• Indirectly: first estimating the channel impulse response and, calculating an estimate of Hk.
• Directly: by inserting known reference symbols or (pilot symbols), at regular intervals within the OFDM
time–frequency grid.
The receiver can estimate the frequency-domain channel around the location of the reference symbol.
Channel estimation algorithms:
• simple averaging in combination with linear interpolation.
• Minimum-Mean-Square-Error (MMSE) estimation.
Time-frequency grid with known reference symbols

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Basic principles of OFDM
Frequency diversity with OFDM: importance of channel coding
(a) WCDMA carrier, each modulation symbol is transmitted over the entire signal bandwidth.
• A single wideband carrier over a highly frequency-selective channel.
• Such transmission of information over multiple frequency bands with different instantaneous channel quality is also
referred to as frequency diversity.
(b) OFDM transmission each modulation symbol is mainly confined to a relatively narrow bandwidth.
• Certain modulation symbols may be fully confined to a frequency band with very low instantaneous signal strength.
• Transmission over a frequency-selective channel: OFDM BER is much worse than single wideband carrier.
Channel coding implies that each bit of information to be transmitted is spread over many code bits.
• 每個information bit會擴展到多個code bits each information bit will experience frequency diversity.
• code bits in the frequency domain is referred to as frequency interleaving.
In contrast to single wideband carrier, channel coding (combined with frequency interleaving) is an essential component in
order for OFDM transmission to be able to benefit from frequency diversity on a frequency-selective channel.
(a) Transmission of single wideband carrier
(b) OFDM transmission over a frequency-selective channel
Channel coding in combination with frequency-domain
interleaving to provide frequency diversity in OFDM transmission

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Basic principles of OFDM
Selection of basic OFDM parameters
In mobile-communication system, the following basic OFDM parameters need to be decided upon
1.
2.
3.

23. LTE Configurations and Parameters
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Summarized by
table
Reviewed

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Basic principles of OFDM
1. OFDM subcarrier spacing
1. ∆݂ = 1/Tu subcarrier spacing 越小越好 (Tu越大越好) minimize the relative cyclic-prefix overhead TCP/(Tu + TCP).
2. BUT too small ∆݂
• increases the sensitivity of the OFDM transmission to Doppler spread.
• different kinds of frequency inaccuracies.
3. OFDM subcarrier orthogonality to hold at the receiver side
4. The instantaneous channel does not vary noticeably during the demodulator correlation interval Tu.
Subcarrier interference as a function of the normalized Doppler spread

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Basic principles of OFDM
2. Number of subcarriers
The basic bandwidth of an OFDM signal equals ܰܿ ∙ ∆݂, # of subcarriers multiplied by the subcarrier spacing.
OFDM signal falls off very slowly outside the OFDM bandwidth and much slower than for a WCDMA signal.
The reason for the large out-of-band emission of OFDM signal is the use of rectangular pulse shaping leading to
per-subcarrier side lobes that fall off relatively slowly.
• time-domain windowing will be used to suppress a main part of the OFDM out-of-band emissions.
• In practice, typically of the order of 10% guard-band is needed for an OFDM signal.
Spectrum of a basic 5 MHz OFDM signal compared with WCDMA spectrum
rectangular pulse shaping

26. BWChannel and NRB
90%
P.S.
1RB = 12 subcarriers = 180 kHz
Reviewed

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Basic principles of OFDM
3. Cyclic-prefix length
Increasing the length of the cyclic prefix, without a corresponding reduction in the subcarrier spacing ∆݂,
implies an additional overhead in terms of power as well as bandwidth.
A longer CP may be needed is in the case of multi-cell transmission using SFN (Single-Frequency Network).

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Basic principles of OFDM
Variations in instantaneous transmission power
One of drawbacks of multi-carrier transmission is large variations in the instantaneous transmit power.
• Reduced power-amplifier efficiency.
• Higher mobile-terminal power consumption.
# of methods have been proposed to reduce the large power peaks (PAPR) of an OFDM signal:
1. tone reservation(載波預留), a subset of the OFDM subcarriers are not used for data transmission.
• these subcarriers are modulated in a way suppress the largest peaks of OFDM signal, allowing for a
reduced PA back-off.
• One drawback of tone reservation is the bandwidth loss.
2. prefiltering or precoding, linear processing is applied to the sequence of modulation symbols before OFDM
modulation.
• DFTS-spread OFDM (DFTS-OFDM) which is used for the LTE uplink, can be seen as one kind of prefiltering.
3. selective scrambling (選擇性加擾), the coded-bit sequence to be transmitted is scrambled with a number of
different scrambling codes.
• Tx: Each scrambled sequence is then OFDM modulated.
• Tx: The signal with the lowest peak power is selected for transmission.
• Rx: After OFDM demodulation at the Rx, descrambling and subsequent decoding is carried out for all the
possible scrambling sequences.
• Rx: Only the decoding carried out for the scrambling code actually used for the transmission will provide a
correct decoding result.(只有真正用於傳輸的擾碼才能被正確解碼)
• A drawback: increased receiver complexity, multiple decodings need in parallel.

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Basic principles of OFDM
OFDM as a user-multiplexing and multiple-access scheme
Consecutive: OFDM can be used as a user-multiplexing or multiple-access scheme, allowing for simultaneous
frequency-separated transmissions to/from multiple terminals.
Distributed: The benefit of distributed user multiplexing or distributed multiple access is a possibility for
additional frequency diversity as each transmission is spread over a wider bandwidth.
OFDM as a user-multiplexing/multiple-access scheme: (a) downlink and (b) uplink
Distributed user multiplexing
Consecutive
Distributed

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Basic principles of OFDM
OFDM as a user-multiplexing and multiple-access scheme
In UL OFDMA multiple-access scheme: transmissions from the different terminals arrive approximately time
aligned at the base station.
不同UE傳輸幾乎要同時達到BS = 不同UE到BS的timing misalignment < CP length 保持不同UE接收信號
subcarriers orthogonality to avoid inter-user interference.
Differences in the propagation time (which may far exceed the length of the cyclic prefix), it is necessary to
control the uplink transmission timing of each terminal.
Such transmission-timing control should adjust the transmit timing of each UE to ensure that uplink
transmissions arrive approximately time aligned at BS.
Uplink transmission-power control need to be applied in case of uplink OFDMA
• reducing the transmission power of UE close to the BS.
• ensuring that all received signals at BS will be of approximately the same power.
Uplink transmission-timing control

31. Multi-cell broadcast/multicast transmission and OFDM
The provision of broadcast/multicast services in a mobile-communication system implies that the same
information is to be simultaneously provided to multiple terminals, often dispersed over a large area
corresponding to a large number of cells.
When the same information is to be provided to multiple terminals within a cell it beneficial to use “broadcast”.
broadcast transmission:
• be dimensioned to reach the worst-case terminals, including terminals at the cell border.
• limited SNR that can be achieved at the cell edge, the achievable broadcast data rates may be relatively
limited.
Solutions: increase the broadcast data rates would be to reduce the cell size, thereby increasing the cell-edge
receive power.
Increase the # of cells to cover a certain area and is obviously negative from a cost-of-deployment point of view.
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Basic principles of OFDM
Broadcast scenario
Broadcast vs. unicast transmission
Good

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Basic principles of OFDM
One way to mitigate this and further improve the provision of broadcast/multicast services in a mobile-
communication network is to ensure that the broadcast transmissions from different cells are truly identical
and transmitted mutually time aligned. (保證不同小區發送的廣播信號是完全相同且發送時間互相同步).
The transmission of identical time-aligned signals from multiple cells, especially in the case of
broadcast/multicast services, is sometimes referred to as Single-Frequency Network (SFN) operation.
identical time-aligned transmissions from multiple cells, the “inter-cell interference” due to transmissions in
neighboring cells will, from UE point of view, be replaced by signal corruption due to time dispersion.
Equivalence between simulcast transmission and multi-path propagation

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Wider-Band Single-Carrier Transmission
Equalization against radio-channel frequency selectivity
Historically, the main method to handle signal corruption due to radio-channel frequency selectivity has been
to apply different forms of equalization at the receiver side.
The aim of equalization is to compensate for the channel frequency selectivity and thus, restore the original
signal shape.
Time-domain linear equalization
The most basic to equalization is the time-domain linear equalizer, consisting of a linear filter with an impulse
response ‫)߬(ݓ‬ applied to the received signal.
different filter impulse responses, different receiver/equalizer strategies.
1. In DS-CDMA-based systems a so-called RAKE receiver structure has historically often been used.
The RAKE receiver is simply the receiver structure where the filter impulse response has been selected to provide
channel-matched filtering ‫ݓ‬ ߬ = ℎ∗(−߬).
Filter response has been selected as the complex conjugate of the time-reversed channel impulse response. This is also
often referred to as a Maximum-Ratio Combining (MRC) filter setting.
Maximizes the post-filter signal-to-noise ratio.
缺點: MRC-based filtering does not provide any compensation for frequency selectivity, no equalization.
適用情境: when the received signal is mainly impaired by noise or interference from other transmissions.
General time-domain linear equalization
RAKE Receiver
1. channel-matched filtering, MRC (Maximum-Ratio Combining) filter ‫ݓ‬ ߬ = ℎ∗
(−߬).
2. linear convolution filtering, ZF (Zero-Forcing) equalization ℎ ߬ ⊗ ‫ݓ‬ ߬ = 1.
3. trade-off filtering, MMSE (Minimum Mean-Square Error) equalizer ߝ = ‫ܧ‬ ‫̂ݏ‬ ‫ݐ‬ − ‫)ݐ(ݏ‬ ଶ
.
Time-domain
linear equalization

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Wider-Band Single-Carrier Transmission
Time-domain linear equalization (conti.)
2. select the receiver filter to fully compensate for the radio-channel frequency selectivity.
By selecting the receiver-filter impulse response to fulfill the relation ℎ ߬ ⊗ ‫ݓ‬ ߬ = 1.
⊗ denotes linear convolution.
ℎ ߬ ⊗ ‫ݓ‬ ߬ = 1 filter setting, also known as Zero-Forcing (ZF) equalization, provides full compensation for any radio-
channel frequency selectivity (complete equalization) and thus full suppression of any related signal corruption.
缺點: Channel has large variations in its frequency response lead to a large increase in the noise level after equalization
and to an overall degradation in the link performance.
3. most cases, better alternative is to select a filter setting that provides a trade-off between signal corruption
due to radio-channel frequency selectivity and noise/interference.
Can be done by selecting the filter to minimize the mean-square error between the equalizer output and the transmitted
signal, to minimize ߝ = ‫ܧ‬ ‫̂ݏ‬ ‫ݐ‬ − ‫)ݐ(ݏ‬ ଶ
.
This also referred to as a MMSE (Minimum Mean-Square Error) equalizer setting.
General time-domain linear equalization
1. channel-matched filtering, MRC (Maximum-Ratio Combining) filter ‫ݓ‬ ߬ = ℎ∗(−߬).
2. linear convolution filtering, ZF (Zero-Forcing) equalization ℎ ߬ ⊗ ‫ݓ‬ ߬ = 1.
3. trade-off filtering, MMSE (Minimum Mean-Square Error) equalizer ߝ = ‫ܧ‬ ‫̂ݏ‬ ‫ݐ‬ − ‫)ݐ(ݏ‬ ଶ
.
Time-domain
linear equalization

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Wider-Band Single-Carrier Transmission
Linear equalization implemented as a time-discrete FIR filter
In practice, the linear equalizer has most often been implemented as a time-discrete FIR filter with L filter taps
applied to the sampled received signal.
Time-discrete equalizer complexity ∝ equalized bandwidth.
1. Wideband signal ↑, 受frequency selectivity影響 ↑, (equivalently,受time dispersion ↑).
• This implies that the equalizer needs to have a larger span.
• (Larger length L = more filter taps) to be able to compensate for the channel frequency selectivity.
2. Wideband signal ↑, sampling rate ↑ for the received signal.
• Thus, the receiver-filter processing needs higher rate.
filter有L個抽頭

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Wider-Band Single-Carrier Transmission

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Wider-Band Single-Carrier Transmission
Freq-domain linear equalization
carry out the equalization in the frequency domain reduce the complexity of linear equalization.
1. In frequency-domain linear equalization, the equalization is carried out block-wise with block size-N.
2. The sampled received signal is first transformed into the frequency domain by means of a size-N DFT.
3. Equalization is then carried out as frequency-domain filtering, with the frequency-domain filter taps W0, …,
WN-1, for example, being the DFT of the corresponding time-domain filter taps w0, ..., wL-1.
4. Finally, size-N inverse DFT, the equalized frequency-domain signal is transformed back to the time domain.
Note: The block size-N, N = 2n, for some integer n to allow for computational-efficient radix-2 FFT/IFFT
implementation of the DFT/IDFT processing.
1. 均衡逐塊實現, 每塊大小為N
2.
3.
Frequency-domain linear equalization Linear equalization implemented as a time-discrete FIR filter
filter有L個抽頭
4.
Freq-domain
less complexity!!
Win~

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Wider-Band Single-Carrier Transmission
1. 均衡逐塊實現, 每塊大小為N
2.
3.
Frequency-domain linear equalization
4.
Freq-domain
less complexity!!
Win~

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Wider-Band Single-Carrier Transmission

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To be continued…