20. 版本版本版本版本 IEEE 802.11a/g IEEE 802.11n
生成算法 複數 IFFT 複數 IFFT
階數 64 64
基波頻率 312.5 kHz 312.5 kHz
BW 20 MHz 20 MHz
Symbol時長 3.2 us 3.2 us
採樣點時長 50 ns 50 ns
子載波數量 52 56
GI 0.8 us 0.4/0.8 us
OFDM Symbol rate 250 ksps 277.8/250 ksps
OFDM - WLAN
21. OFDM - LTE
BW 10 MHz 15 MHz 20 MHz
IFFT階數 1024 1536 2048
基波頻率 15 kHz 15 kHz 15 kHz
Symbol時長 66.7 us 66.7 us 66.7 us
採樣點間格 65.1 ns 43.4 ns 32.5 ns
採樣頻率 15.36 MHz 23.04 MHz 30.72 MHz
子載波數量 600 900 1200
GI 4.76 us 4.76 us 4.76 us
OFDM Symbol rate 14 ksps 14 ksps 14 ksps
22. Major LTE Parameters
Parameter Downlink Uplink
Access scheme OFDMA SC-FDMA (DFTS-OFDM)
Subcarrier spacing 15 kHz
Bandwidth 1.4, 3, 5, 10, 15, or 20 MHz
Modulation QPSK, 16-QAM, 64-QAM
Cyclic prefix length 4.7 μs (short) or 16.7 μs (long)
OFDMA = orthogonal frequency division multiple access; DFTS = discrete Fourier transform spread
DFTS-OFDM (also called SC-FDMA = single-carrier frequency division multiple access) is a
transmission scheme that combines the desired properties for uplink :
1. Small variations in the instantaneous Tx signal power (single carrier’s property).
2. Possibility for low-complexity high-quality equalization in the frequency domain.
3. Possibility for FDMA with flexible bandwidth assignment.
Spectral efficiency is increased up to 4x compared with UTRA, and improvements in architecture
and signaling reduce round-trip latency.
MIMO antenna technology should enable 10x as many users per cell as 3GPP’s original WCDMA
radio access technology.
To suit many frequency band allocation arrangements, both paired (FDD) and unpaired (TDD) band
operation is supported. LTE can coexist with earlier 3GPP radio technologies.
23. Wireless Technology Evolution
LTE Technologies
Physical Layer
LTE Test Items
• Overview
• EPC
• E-UTRAN
• UE
Agenda
• OFDM
• MIMO
• Link Adaptation (AMC)
• HARQ
• Channel Scheduling
• Inter-Cell Interference Coordination (ICIC)
• Frequency Band
• Structure – frame, slots, resource blocks & elements
• Physical signals and channels
• Tx Characteristics
• Rx Characteristics
24. OFDM is a digital multi-carrier modulation scheme
Large number of closely-spaced orthogonal sub-carriers (e.g. 300/5 MHz BW).
Subcarriers modulated with a conventional modulation format (e.g. QPSK, 16/64QAM)
Low symbol rate similar to conventional single-carrier modulation schemes in the same bandwidth.
LTE symbol rate = 66.7µs, ∆f = 1/symbol rate = 15 kHz for each subcarrier.
In freq. domain 1 RE = 1 subcarrier, so 1 RB = 12 subcarriers = 180 kHz. In time domain 1 RB = 0.5 ms.
Orthogonal Frequency Division Multiplexing
OFDM
把高速的資料分成多個平行的低速資料, 把每個低速的資料分到N個子載波上, 在每個子載波上進行 FSK.
這些在N子載波上同時傳輸的資料符號, 構成一個OFDM符號(=SUM(subcarriers)).
25. Spectrum of single modulated OFDM subcarrier
The FFT of a rectangular pulse is a sinc or sin(x)/x with zeros at multiples of FP = 1/TP.
LTE symbol rate = 66.7µs, ∆f = 1/ symbol rate =
15 kHz for each subcarrier.
In freq. domain 1 RB = 12 subcarriers = 180 kHz.
In time domain 1 RB = 0.5 ms.
FFT
OFDM與傳統的多載波調製(MCM)相比, OFDM調製的各子載波間可相互重疊, 並且能夠保持各個子載波
之間的正交性.
選擇OFDM的一個主要原因在於該系統能夠很好地對抗頻率選擇性衰落或窄帶干擾.
26. Spectrum of multiple OFDM subcarriers
OFDM Operates as a Number of Orthogonal (Non-Interfering) Narrowband Systems
Closely spaced carriers overlap.
Nulls in each carrier’s spectrum land at the center of all other carriers for zero Inter-Carrier
Interference (ICI).
Carrier spacing creates orthogonality.
Phase noise, timing and frequency offsets decrease orthogonality.
Fig. Spectrum of multiple OFDM subcarriers of constant amplitude
28. OFDM PAPR ?
2
Crest factor peak
rms
x
C
x
PAPR C
= =
=
( )
/2 2
0
/2
0
1 1
sin 0.707
/ 2 2
1 2
sin 0.636
/ 2
rms peak peak peak
avg peak peak peak
V V d V V
V V d V V
π
π
θ θ
π
θ θ
π π
= = =
= = =
∫
∫
For sin wave:
29. OFDM general link level chains
Rx Channel estimation test signal get all freq. response use Equalizer lower BER.
32. OFDM Fundamentals – Frequency Domain Equalizer
Frequency domain equalizer outperforms with much less complexity !
Rx Channel estimation test signal get all freq. response use Equalizer lower BER.
33. OFDM advantages:
Multiple subcarriers allows.
– Scalable channel bandwidth.
– Frequency selective scheduling within the channel.
Wide channels are possible which support higher
data rates.
Resistance to multipath due to very long symbols.
OFDM Advantage and Disadvantage
OFDM disadvantages:
Sensitive to frequency errors and phase noise due to close
subcarrier spacing.
Sensitive to Doppler shift which creates interference
between subcarriers.
Pure OFDM creates high PAPR which is why SC-FDMA is
used on UL.
Guard Interval (GI) necessary (ISI&ICI), reduce data rate.
Table. Comparison of CDMA and OFDM
34. LTE uses OFDMA (Orthogonal Frequency Division Multiple Access)
more advanced form of OFDM where subcarriers are allocated to different users over time.
(Freq.)
(Freq.)
OFDM v.s. OFDMA
允許多個用戶在不同的時間(time slot), 來使用相同的頻率.
35. DL OFDMA
OFDMA provides flexible scheduling in time-frequency domain.
In case of multi-carrier transmission, OFDMA has larger PAPR than traditional single carrier
transmission. Fortunately this is less concerned with downlink.
Does OFDMA suits for uplink transmission ?
Uplink being sensitive to PAPR due to UE implementation requirements.
With wider bandwidth in operation, OFDMA in uplink will have lower power per pilot symbol
which in turn leads to deterioration of demodulation performance.
36. SC-FDMA-FDE general link level chains
LTE系統中上行鏈路採用SC-FDMA技術, 以降低PAPR, 提高效率, 通過DFT-S-OFDM技術來實現.
DFT-S-OFDM可以認為是SC-FDMA的頻域產生方式, 是OFDM在IFFT調製前進行了基於Fourier Transform的預編碼.
DFT-S-OFDM與OFDM的區別在於: OFDM是將1個符號資訊調製到1個正交的子載波上,而DFTS-OFDM是將M個輸入符
號的頻譜資訊調製到多個正交的子載波上去.
37. Multiple Access Technology in the Uplink: SC-FDMA
SC-FDMA is a hybrid transmission scheme:
low peak to average (PAPR) of single carrier schemes.
frequency allocation flexibility and multipath protection of OFDMA.
DFT “pre-coding” is performed on modulated data symbols to transform them into frequency domain.
IFFT and cyclic prefix (CP) insertion as in OFDM.
Each subcarrier carries a portion of superposed DFT spread data symbols, therefore SC-FDMA is also
referred to as DFT-spread-OFDM (DFT-s-OFDM).
DFT
Sub-carrier
Mapping
CP
insertion
Size-NTX Size-NFFT
Coded symbol rate= R
NTX symbols
IFFT
Frequency domain Time domainTime domain
Fig. Transmitter structure for SC-FDMA
Low
PAPR
Spreading
High
PAPR
Low
PAPR
Signal at each subcarrier is linear combination of all NTx symbols
41. Comparing OFDMA and SC-FDMA
QPSK example using M = 4 subcarriers
The following graphs show how a sequence of eight QPSK symbols is represented in frequency and time.
LTE symbol rate = 66.7µs, ∆f = 1/symbol rate = 15 kHz for each subcarrier.
In freq. domain 1 RE = 1 subcarrier, so 1 RB = 12 subcarriers = 180 kHz. In time domain 1 RB = 0.5 ms.
44. Comparing OFDMA and SC-FDMA
PAR and constellation analysis at different BW
Transmission scheme OFDMA SC-FDMA
Analysis bandwidth 15 kHz
Signal BW
(M x 15 kHz)
15 kHz
Signal BW
(M x 15 kHz)
Peak to average power
ratio (PAR)
Same as data
symbol
High PAR (Gaussian)
< data symbol (not
meaningful)
Same as data symbol
Observable IQ
constellation
Same as data symbol at
66.7 µs rate
Not meaningful
(Gaussian)
< data symbol (not
meaningful)
. Same as data symbol at M
X 66.7 µs rate
LTE symbol rate = 66.7µs, ∆f = 1/symbol rate = 15 kHz for each subcarrier.
In freq. domain 1 RE = 1 subcarrier
so 1 RB = 12 subcarriers = 180 kHz.
In time domain 1 RB = 0.5 ms.
45. Comparing OFDMA and SC-FDMA
Multipath protection with short data symbols
15 kHz
Frequency
fc
V
CP
OFDMA
Data symbols occupy 15 kHz for
one OFDMA symbol period
SC-FDMA
Data symbols occupy M*15 kHz for
1/M SC-FDMA symbol periods
fc
The subcarriers of each SC-FDMA symbol are not the same across frequency as shown in
earlier graphs but have their own fixed amplitude & phase for the SC-FDMA symbol duration.
The sum of M time-invariant subcarriers represents the M time-varying data symbols.
60 kHz Frequency
V
CP
It is the constant nature of the subcarriers throughout the SC-FDMA symbol
that means when the CP is inserted, multipath protection is achieved despite
the modulating data symbols being much shorter.
46. Similarities
Block-wise data processing and use of Cyclic Prefix.
Divides transmission bandwidth into smaller sub-carriers.
Channel inversion/equalization is done in frequency domain.
SC-FDMA is regarded as DFT-Precoded or DFT-Spread OFDMA.
Difference
Signal structure: In OFDMA each sub-carrier only carries information related to only one data symbol while in
SC-FDMA, each sub-carrier contains information of all data symbols. 一對一, 多對多.
Equalization: Equalization for OFDMA is done on per-subcarrier basis while for SC-FDMA, equalization is
done over the group of sub-carriers used by transmitter.
PAPR: SC-FDMA presents much lower PAPR than OFDMA does.
Sensitivity to freq. offset: yes for OFDMA but tolerable to SC-FDMA.
OFDMA v.s. SC-FDMA
Time domain:
OFDMA: symbol is a sum of all data symbols by IFFT.
SC-FDMA: symbol is repeated sequence of data “chips”.
Frequency domain:
OFDMA: modulates each subcarrier with one data
symbol.
SC-FDMA: “distributes” all data symbols on each
subcarrier.
OFDMA SC-FDMA
47. Wireless Technology Evolution
LTE Technologies
Physical Layer
LTE Test Items
• Overview
• EPC
• E-UTRAN
• UE
Agenda
• OFDM
• MIMO
• Link Adaptation (AMC)
• HARQ
• Channel Scheduling
• Inter-Cell Interference Coordination (ICIC)
• Frequency Band
• Structure – frame, slots, resource blocks & elements
• Physical signals and channels
• Tx Characteristics
• Rx Characteristics
49. MIMO (II)
MIMO = Multiple Input Multiple Output Antennas
WHY use Multiple Antennas ?
There are three main types of multiple antenna techniques.
1. Path diversity: one radiated path may be subject to fading loss and another may not.
2. Beamsteering (Beamforming): controlling the phase relationships of the electrical
signals radiated at the antennas to physically steer transmitted energy.
3. MIMO: employs spatial separation (the path differences introduced by separating
the antennas) through the use of spatial multiplexing.
優點
1. 信號穩定性提高
2. 信號強度提高
3. 頻譜利用率提高
c.f.
Beamforming is about shaping the beam, to some required angular range.
Beamsteering is about pointing the beam, in some desired direction.
50. A. Free-space path loss.
B. Reflection.
C. Diffraction.
D. Scattering.
E. Shadow fading.
F. Doppler effect.
Before Diversity
51. C = Max(A, B) C = A + B
優點
1. 信號穩定性提高
2. 信號強度提高
3. 頻譜利用率提高
MIMO - Diversity
Diversity技術分為: Rx Diversity, Tx Diversity
Diversity實施方式: space/time/frequency/polarization/path/angle diversity
Diversity信號合併
EGC (Equal Gain Combining)
SD (Selection Diversity)
MRC (Max Ratio Combining)對抗信號衰落效果最好
MRC = signal from each antenna is rotated and weighted according to the phase and
amplitude of the channel, such that the signals from all antennas are combined to yield
the maximal ratio between signal and noise terms.
52. Diversity – some thoughts (I)
( )
( )
( )
2
/ 2
2
log 1
log 1
log 1
SISO
Tx Rx
MIMO
C B SNR
C B M SNR
C M B SNR
= +
= + ×
= × +
53. Diversity – some thoughts (II)
( )
( )
( )
2
/ 2
2
log 1
log 1
log 1
SISO
Tx Rx
MIMO
C B SNR
C B M SNR
C M B SNR
= +
= + ×
= × +
55. No special encoding, and therefore easy to implement.
Different multipath, Rx can see different fading.
Rx can use two way to improve SNR.
1. Switched Diversity.
2. Max-Ratio Combining.
Maximum Ratio Combining depends on different fading
of the two received signals. In other words decorrelated
fading channels.
Rx Diversity (I)
C = Max(A, B) C = A + B
58. Tx Diversity (II) –
Space Time Coding
Fading on the air interface
The same signal is transmitted at different antennas.
Aim: increase of SNR increase of throughput.
Alamouti Coding = diversity gain approaches
Rx diversity gain with MRC (Maximal-Ratio Combining)
benefit for mobile communications.
MRC = signal from each antenna is
rotated and weighted according to the
phase and amplitude of the channel, such
that the signals from all antennas are
combined to yield the maximal ratio
between signal and noise terms.
performance of MISO
相同數據內容透過編碼由不同天線發射至UE
61. Spatial Multiplexing (I)
2
2
log det
bandwidth,
( ( )),
.
C B
B SNR
ρ
ρ
σ
= + ×
= = =
T
ss
I HH
R
PS.
nTx = # of Tx antennas
nRx = # of Rx antennas.
Consider nT
Consider nR
62. Spatial Multiplexing (II)
Channel capacity grows linearly with antennas.
Assumptions
Perfect channel knowledge.
Spatially uncorrelated fading.
Reality
Imperfect channel knowledge.
Correlation ≠ 0 and rather unknown.
Max Capacity ~ min(nTx, nRx)
PS.
nTx = # of Tx antennas
nRx = # of Rx antennas.
( )
( )
( )
2
/ 2
2
log 1
log 1
log 1
SISO
Tx Rx
MIMO
C B SNR
C B M SNR
C M B SNR
= +
= + ×
= × +
63. 優點
1. 信號穩定性提高
2. 信號強度提高
3. 頻譜利用率提高
MIMO – Space Division Multiplexing
單碼字傳輸: 一個資料流程進行通道編碼和調制之後再複用到多根天線上.
多碼字傳輸: 複用到多根天線上的資料流程可以獨立進行通道編碼和調制.
LTE支援最大的碼字數目為2. 為了降低回饋的量.
single codeword
multiple codeword
Space Division Multiplexing
頻譜利用率提高
單位帶寬能傳更多bit rate
throughput提升
64. MIMO (III)
Single input single output
Single input multiple output
Multiple input single output
Multiple input multiple output
SIMO = receive diversity.
This radio channel access mode is suited for low SNR
conditions in which a theoretical gain of 3 dB is
possible when two receivers are used.
There is no change in the data rate since only one data
stream is transmitted, but coverage at the cell edge is
improved due to the lowering of the usable SNR.
MISO = transmit diversity.
MISO increases the robustness of the signal to fading and can increase performance in low
SNR conditions.
MISO does not increase the data rates, but it supports the same data rates using less power.
MISO can be enhanced with closed loop feedback from the receiver to indicate to the
transmitter the optimum balance of phase and power used for each transmit antenna.
SIMO + MISO ≠ MIMO.
If N data streams are transmitted from < N antennas, the data cannot be fully descrambled by any number of
Rx since overlapping streams without the addition of spatial diversity creates interference.
So N data streams at least N Tx, N Rx will be able to fully reconstruct the original data streams provided the
path correlation and noise in the radio channel are low enough.
Transmissions from each antenna must be uniquely identifiable.
The spatial diversity of the radio channel means that MIMO has the potential to increase the data rate.
65. MIMO (IV)
2
2 2
2 1 2 2
log (1 ),
log (1 ( / ) ) log (1 ( / ) )
where / signal to noise ratio, a singular value of the channel matrix, .
C B SNR
C B N N
N H
σ ρ σ ρ
σ ρ
= +
= + + +
= =
For spatial
multiplexing system
Streams in a spatially multiplexed link:
ρ = 1, ideal but impractical case of no cross-coupling(double channel capacity).
ρ = 2, total in-phase coupling.
ρ = 0, capacity has dropped back to that of a SISO channel.
Channel capacity in 2x2 MIMO case ≤ twice SISO case and has substantial improvement in SNR at Rx if the
values of ρi << 1.
The matrix coefficients are known by Tx, outgoing signals can be modified (precoded) to equalize the
performance between the streams.
Precoding requires real-time feedback from Rx to Tx, so this is also known as closed-loop spatial multiplexing.
For effective precoding, the relative signal phase between Tx must be stable over the time interval of the
feedback process.
1 Tx, 1 Rx case
Fig. Orthogonal structure of downlink reference symbols for dual antenna.
95. Wireless Technology Evolution
LTE Technologies
Physical Layer
LTE Test Items
• Overview
• EPC
• E-UTRAN
• UE
Agenda
• OFDM
• MIMO
• Link Adaptation (AMC)
• HARQ
• Channel Scheduling
• Inter-Cell Interference Coordination (ICIC)
• Frequency Band
• Structure – frame, slots, resource blocks & elements
• Physical signals and channels
• Tx Characteristics
• Rx Characteristics
96. LTE FDD Frequency bands
P.S.
ARFCN = Absolute radio-frequency channel number
UARFCN = UMTS Absolute radio-frequency channel number
EARFCN = EUTRA Absolute radio-frequency channel number
A good website: http://niviuk.free.fr/lte_band.php
99. Table. Peak data rates for UE categories.
In order to scale the development of equipment, UE categories have been defined to limit
certain parameters.
The most significant parameter is the supported data rates:
100. Theoretical LTE Data Rate Calculation
Question: Assume 20 MHz bandwidth (100 RB) and normal CP calculate data rate = ?
Throughput symbols per second bits per second.
1 RB = 1 time domain(1 slot = 0.5 ms = 7 OFDM symbols) x 1 freq. domain(12 subcarriers)
= 7 x 12 x 2 = 168 symbols per ms
64 QAM = 26 QAM = 6 bits per symbol.
16800 symbols per ms = 16,800,000 symbols per sec = 16.8 Msps.
Throughput = data rate = 16.8 x 6 = 100.8 Mbps for single chain.
LTE 4x4 MIMO (4T4R) 100.8 x 4 = 403.2 Mbps for DL.
But there is 25% overhead use for controlling and signaling so 403.2 x 0.75 = 302.4 Mbps ~ 300 Mbps.
For UL we have only one transmit chain at UE end so after 25% 100.8 x 0.75 = 75.6 Mbps ~ 75 Mbps.
There is why we get the # of throughput 300 Mbps for DL and 75 Mbps for UL shown everywhere!!
101. Use 3GPP Spec. 36.213 for Throughput Calculation
Coding rate described the efficiency of the particular modulation scheme.
Example: 16 QAM with 0.5 coding rate means its can only carry 2 information bits.
The combination of the modulation and coding rate is called Modulation Coding Scheme (MCS).
Example: 100 RBs MCS Index = 28, the TBS = 75376, assume 4x4 MIMO so the peak data rate
= 75376 x 4 = 301.5 Mbps.
Table 7.1.7.2.1-1: Transport block size table (dimension 27××××110)
102. DL/UL Throughput calculation for LTE FDD
BW = 20 MHz
Multiplexing scheme = FDD
UE category = Cat 3
Modulation supported =
per Cat 3 TBS index 26 for DL (75376 for 100 RBs) and 21 for UL (51024 for 100 RBs)
Throughput = # of Chains x TB size.
DL throughput = 2 x 75376 = 150.752 Mbps.
UL throughput = 1 x 51024 = 51.024 Mbps.
Good website: http://niviuk.free.fr/ue_category.php
103. DL/UL Throughput calculation for LTE TDD
Table. LTE TDD frame configuration.
Table. Special subframe configuration.
BW = 20 MHz
Multiplexing scheme = TDD
UE category = Cat 3
Modulation supported = per Cat 3 TBS index 26 for DL (75376 for 100 RBs)
and 21 for UL (51024 for 100 RBs)
TDD frame configuration 2 (D-6, S-2 and U-2)
Special subframe configuration 7 (DwPTS-10, GP-2 and UpPTS-2)
DL Throughput = # of Chains x TB size x (DL Subframe + DwPTS in SSF)
UL Throughput = # of Chains x TB size x (UL Subframe + UpPTS in SSF)
DL Throughput = 2 x 75376 x (0.6 + 2(10/14)) = 112 Mbps.
UL Throughput = 1 x 51024 x (0.2 + 0.2(2/14)) = 12 Mbps.
104. Wireless Technology Evolution
LTE Technologies
Physical Layer
LTE Test Items
• Overview
• EPC
• E-UTRAN
• UE
Agenda
• OFDM
• MIMO
• Link Adaptation (AMC)
• HARQ
• Channel Scheduling
• Inter-Cell Interference Coordination (ICIC)
• Frequency Band
• Structure – frame, slots, resource blocks & elements
• Physical signals and channels
• Tx Characteristics
• Rx Characteristics
105. Frame Structure
FDD Frame Structure
TDD Frame Structure
1/ (15000 2048) 32.6 nssT = × =
Type 1 is defined for FDD mode.
Each radio frame is 10 ms long and
consists of 10 subframes. Each
subframe contains two slots.
In FDD, both uplink and downlink
have the same frame structure but use
different spectra.
Type 2 is defined for TDD mode.
Each radio frame is 10 ms long and
consists of two half frames. Each half
frame contains five subframes.
Subframe #1 and sometimes subframe
#6 consist of three special fields:
1. downlink pilot timeslot (DwPTS),
2. guard period (GP),
3. uplink pilot timeslot (UpPTS).
106. Frame Structure type 1 (FDD) FDD: Uplink and downlink are transmitted separately
#0 #2 #3 #18#1 ………. #19
One subframe = 1ms
One slot = 0.5 ms
One radio frame = 10 ms
Subframe 0 Subframe 1 Subframe 9
Frame Structure type 2 (TDD)
DwPTS, T(variable)
One radio frame, Tf = 307200 x Ts = 10 ms
One half-frame, 153600 x Ts = 5 ms
#0 #2 #3 #4 #5
One subframe, 30720 x Ts = 1 ms
Guard period, T(variable)
UpPTS, T(variable)
•5ms switch-point periodicity: Subframe 0, 5 and DwPTS for downlink,
Subframe 2, 5 and UpPTS for Uplink
•10ms switch-point periodicity: Subframe 0, 5,7-9 and DwPTS for downlink,
Subframe 2 and UpPTS for Uplink
One slot,
Tslot =15360 x Ts = 0.5 ms
#7 #8 #9
For 5ms switch-point periodicity
For 10ms switch-point periodicity
Frame Structure
107. Max FFT size generate OFDM symbols = 2048
Subcarrier frequency spacing = 15 kHz
Sampling rate = 15 kHz*2048 = 30.72 MHz.
Ts (sampling period)
= 1/sampling rate = 32.6 ns.
Sampling rate = 8*3.84 MHz = 30.72 MHz.
Frame Structure - Type 1 (FDD)
For 20 MHz BW:
There are 15360 samples in one time slot
(add all numbers in the red circle)
Ts (sampling period) = 0.5 ms/15360 = 32.6 ns.
108. OFDM symbols (= 7 OFDM symbols @ Normal CP)
The Cyclic Prefix is created by prepending each
symbol with a copy of the end of the symbol
160 2048 144 2048 144 2048 144 2048 144 2048 144 2048 144 2048 (x Ts)
1 frame
= 10 sub-frames
= 10 ms
1 sub-frame
= 2 slots
= 1 ms
1 slot
= 15360 Ts
= 0.5 ms
0 1 2 3 4 5 6
etc.
CP CP CP CP CPCPCP
P-SCH - Primary Synchronization Channel
S-SCH - Secondary Synchronization Channel
PBCH - Physical Broadcast Channel
PDCCH -Physical Downlink Control Channel
PDSCH - Physical Downlink Shared Channel
Reference Signal – (Pilot)
DL
symbN
#0 #1 #8#2 #3 #4 #5 #6 #7 #9 #10 #11 #12 #19#13 #14 #15 #16 #17 #18
Downlink Frame Structure - FDD
10 2 3 4 5 6 10 2 3 4 5 6
Table. Sample rates and FFT sizes for each LTE BW configuration.
Sample rate = 15 kHz*2048 = 30.72 MHz
110. Uplink Frame Structure & PUSCH Mapping
- FDD
10 2 3 4 5 6
#0 #1 #8#2 #3 #4 #5 #6 #7 #9 #10 #11 #12 #19#13 #14 #15 #16 #17 #18
1 frame
10 2 3 4 5 6
1 sub-frame
PUSCH - Physical Uplink Shared Channel
Demodulation Reference Signal for PUSCH
• • • • •
OFDM symbols (= 7 SC-FDMA symbols @ Normal CP)
The Cyclic Prefix is created by prepending each
symbol with a copy of the end of the symbol
160 2048 144 2048 144 2048 144 2048 144 2048 144 2048 144 2048 (x Ts)
1 slot
= 15360 Ts
= 0.5 ms
0 1 2 3 4 5 6
etc.
CP CP CP CP CPCPCP
UL
symbN
111. PUSCH
Zadoff-Chu
PUSCH ≥ 3RB
QPSK
PUSCH < 3RB
or PUCCH
Demodulation Reference Signal (for PUSCH)
PUCCH
Demodulation Reference Signal
for PUCCH format 1a/1b
64QAM QPSK BPSK(1a) QPSK(1b)16QAM
Uplink Mapping - FDD
112. Frame Structure - Type 2 (TDD)
Special subframes consist of the 3 fields
1. Downlink Pilot Timeslot (DwPTS),
2. Guard Period (GP), and
3. Uplink Pilot Timeslot (UpPTS).
Seven uplink-downlink configurations
with either 5 ms and 10 ms downlink-to-
uplink periodicity are support.
Table. Uplink-downlink configurations
“D” denotes a subframe reserved for downlink transmission,
“U” denotes a subframe reserved for uplink transmission, and
“S” denotes the special subframe.
115. LTE User Equipment Categories
There are five UE categories, the main differences are data rates and MIMO capabilities.
Parameters Cat 1 Cat 2 Cat 3 Cat 4 Cat 5
Peak data rate (Mbps) – downlink 10 50 100 150 300
Peak data rate (Mbps) – uplink 5 25 50 50 75
RF bandwidth (MHz) 20 20 20 20 20
Modulation – downlink QPSK
16-QAM
64-QAM
QPSK
16-QAM
64-QAM
QPSK
16-QAM
64-QAM
QPSK
16-QAM
64-QAM
QPSK
16-QAM
64-QAM
Modulation – uplink QPSK
16-QAM
QPSK
16-QAM
QPSK
16-QAM
QPSK
16-QAM
QPSK
16-QAM
64-QAM
Rx diversity
2x2 MIMO
4x4 MIMO
116. Slot Structure (I)
OFDM Symbol and Cyclic Prefix
Key advantage in OFDM systems is the ability to protect against multipath delay spread.
The long OFDM symbols allow the introduction of a guard period between each symbol to
eliminate inter-symbol interference (ISI) due to multipath delay spread.
If the guard period is longer than the delay spread in the radio channel, and if each OFDM
symbol is cyclically extended into the guard period (by copying the end of the symbol to the
start to create the cyclic prefix), then the ISI can be completely eliminated.
CP is created by prepending each symbol
with a copy of the end of the symbol.
Fig. OFDM symbol structure for normal cyclic prefix case (downlink).
Table. SC-FDMA CP length (uplink). Table. OFDM CP length (downlink).
5.2 s for first symbol
4.7 s for other symbols.
µ
µ
512 32.6 ns 16.7 s.µ× =
117. Resource Element and Resource Block
Slot Structure (II)
A resource element is the smallest unit in the physical layer and occupies one OFDM or
SC-FDMA symbol in the time domain and one subcarrier in the frequency domain.
A resource block (RB) is the smallest unit that can be scheduled for transmission. An RB
physically occupies 0.5 ms (= 1 slot) in the time domain and 180 kHz in the frequency domain.
Fig. Resource grid for uplink (a) and downlink (b).
Table. RB parameters for the uplink.
Table. RB parameters for the downlink.
• 7.5 kHz subcarrier spacing, which is used for multimedia
broadcast over single frequency network (MBSFN).
• Symbols are twice as long, which allows the use of a
longer CP to combat the higher delay spread in larger
MBSFN cells.
118. Configurable Channel Bandwidth
In CDMA systems, the transmission bandwidth is fixed and determined by the
inverse of the chip rate.
In OFDM systems, the subcarrier spacing is determined by the inverse of the FFT
integration time. So number of subcarriers and transmission bandwidth can be
determined independently. More flexibility.
Table. Transmission bandwidth configuration.
1 RB includes 12 subcarriers
LTE symbol rate = 66.7µs, ∆f = 1/ symbol rate = 15 kHz for each subcarrier.
In freq. domain 1 RE = 1 subcarrier, so 1 RB = 12 subcarriers = 180 kHz.