1. Technology training (Session 2)
Long Term Evolution
Technology training
(Session 2)

2. Outline Spectrum allocation for LTE LTE DL Physical Layer
Management in frequency domain
Management in time domain
Management in code domain

3. Spectrum allocation for LTE
Part 1
Spectrum allocation for LTE

4. LTE duplexing LTE may be: LTE supports different channel bandwidths
Time Division Duplexed (TDD)
Frequency Division Duplex (FDD)
Half Duplex Frequency Division Duplexing HD-FDD
LTE supports different channel bandwidths
1.4, 3, 5, 10, 15 or 20 MHz in a signal channel
Bandwidth flexibility – facilitates deployment across wide spectrum range
LTE supports frequencies from 450MHz to 3.8GHz
LTE-A supports carrier aggregation
Multiple channels may be aggregated in data delivery
Duplexing schemes in LTE

5. LTE standardized bands
3GPP standardizes bands allowed for LTE deployment
Band standardization – important for equipment availability
Not all bands are available in all geographical regions
Website reporting standardized LTE bands:
3GPP specification with LTE frequency bands
Spectrum report from niviuk website

6. Ex: FDD deployment – Band 3
Band 3 (1800+): standardized for FDD
Used throughout entire ITU region 1
Total of 2x75MHz of spectrum (75 for UL and 75 for DL)
Duplexing space: 95MHz

7. Ex: FDD deployment – Band 20
Band 20 (800 DD): standardized for FDD
Used throughout entire EMEA
Total of 2x30MHz of spectrum (30 for UL and 30 for DL)
Duplexing space: -41MHz (reverse duplex)

8. Ex: Interference on KSA-Qatar border
Note: the spectrum allocation results in severe interference from BS TX of one system to BS RX of the other one

9. Ex: Time division duplexing
STC deploys TDD LTE in 2300MHz (Band 40)
Band 40 – Chinese band!
TDD – same spectrum used for UL and DL
The transmission is time multiplexed
Typically – unequal times for uplink and downlink transmission

10. Frequency band and devices
Mobile devices support a set of bands
Supported bands determine where a device may be used
For a given device, manufacturers publish supported bands
Example: iPhone 6
KSA Provider
Supported LTE band
STC
3,40
Mobily
3, 38
Zain
3, 1, 38

11. Review questions Which LTE bands are currently used in KSA?
Which LTE bands are standardized for use in Middle East
Consider iPhone 6 Model: A1532 (GSM). Would this model work in KSA? How about Europe?
Find a model of Samsung phone that could work in 800 DD band.
Explain the difference between FDD and TDD.
Why is there difference between downlink and uplink time allocation in LTE TDD?

12. LTE Downlink physical layer frequency domain
Part 2
LTE Downlink physical layer frequency domain

13. Digital transmission of data
All modern cellular systems are digital
Majority of digital systems use I-Q based modulation technique
In I-Q modulator bits are used to modulate two carrier signals in quadrature
Block diagram of I/Q modulator
Bandwidth of the signal is proportional to symbol rate, i.e. Rb/n
Transmitted symbol is a mixture of sine and cosine. Symbols are represented as constellation points

14. Ex: Digital transmission - QPSK
Pi/4-QPSK
2 bits per symbol (Rs = Rb/2)
Example Rb = 100kb/s
Symbol = 2 bits
Symbol rate Rs = Rb/2 = 50ksym/s
Required bandwidth ~ 50kHz
Note: Bandwidth scales linearly with symbol rate
Pi/4 – QPSK constellation

15. Ex: Constellations used in LTE
QPSK – 2 bits/symbol
16 QAM – 4 bits /symbol
64 QAM – 6 bits per symbol
Higher modulation – more efficient
Higher modulation – requires better CINR (SINR)
Example: Rb = 100kbps
QPSK: Rs = 50ksps, BW~50KHz
16QAM: Rs = 25ksps, BW~25KHz
64QAM: Rs = 16.7ksps, BW~16KHz

16. Single carrier transmission
Data are used to modulate amplitude/phase (frequency) of a single carrier
Higher data rate results in wider bandwidth
Over larger bandwidths ( > 20KHz), wireless channel is frequency selective
As a result of frequency selectivity the received signal is severely distorted
Channel equalization needed
Complexity of equalizer increases rapidly with the signal bandwidth requirements
Transmission of single carrier in mobile terrestrial environment
Note: over small portion of the signal spectrum, fading may be seen as flat

17. Multi-carrier transmission
Channel fading over smaller frequency bands – flat (no need for equalizer)
Divide high rate input data stream into many low rate parallel streams
At the receiver – aggregate low data rate streams
Signal for each stream experiences flat fading

18. FDM versus OFDM
OFDMA minimizes separation between carriers
Carriers are selected so that they are orthogonal over symbol interval
Carrier orthogonality leads to frequency domain spacing Df=1/T, where T is the symbol time
In LTE carrier spacing is 15KHz and useful part of the symbol is 66.7 microsec
Note: orthogonality between carriers in time domain allows closer spacing in frequency domain.
FDM versus OFDM

19. LTE OFDM transmission
Schematic diagram of OFDM transmission during one symbol period
Notes:
Symbol rate is the same for all sub-carriers
Symbol period is the same for all subcarriers
Example: Consider 5MHz LTE DL transmission
# of carriers: 300 (300x15KHz = 4.5MHz; 500kHz used for guard bands and overhead)
QPSK on all carriers:
300 transmitted symbols; 2 bits per symbol; 600 transmitted bits
Symbol duration T = 66.7us; Total DL bit rate: 600bits/66.7us = 8.996Mbps
QAM on all carriers:
300 transmitted symbols; 6 bits per symbol; 1800 transmitted bits
Symbol duration T = 66.7us; Total DL bit rate; 1800bits/66.7us = Mbps

20. Typical OFDM spectrum Subcarriers at the end – not used (guard band)
Sub-carrier in the middle not used
Flat on average, but with large variations
Individual sub-carriers
Guard band sub-carriers
DC carrier – not used

21. Review questions What modulation scheme is used on LTE downlink?
What is the sub-carrier separation on LTE downlink?
What is the symbol duration on LTE downlink?
What modulation schemes are available for LT downlink?
How many bits per symbol are used in 16QAM?
If the modulation scheme is QPSK and bit rate is 120kbps, what is the symbol rate?
Consider an LTE deployment in 15MHz. The number of data sub-carriers is Determine total number of bits that may be transmitted over one symbol interval, if 16QAM is used on all subcarriers.

22. LTE Downlink physical layer Time domain
Part 3
LTE Downlink physical layer Time domain

23. OFDM transmission in time
Transmission one OFD symbol at the time
One OFDM symbol = N sub-carrier symbols
In between symbols – there is guard time (inserted CP)
Guard time prevents interference between symbols due to multipath propagation

24. Detailed time domain structure
Need for two different CP:
To accommodate environments with large channel dispersion
To accommodate MBSFN (Multi-Cast Broadcast Single Frequency Network) transmission
In case of MBSFN it may be beneficial to have mixture of sub-frames with normal CP and extended CP. Extended CP is used for MBSFN sub-frames
TCP: 160Ts (5.1us) for first symbol, 144Ts (4.7us) for other six symbols
TCP-e: 512 Ts (16.7 us) for all symbols
Basic timing unit: Ts = 1/(2048 x 15000) ~ ns

25. Physical resource block
PRB – smallest manageable data chunk transmitted by eNodeB. Allocated in pairs.
PRB – 12 carriers in frequency (180kHz) and 7/6 LTE symbols in time (0.5ms)
PRB contains
12x7 = 84 REs for short CP
12x6 = 72 REs for long CP
Each RE contains a symbol that carries n bits
QPSK: n=2
16QAM: n=4
64QAM: n=6
Example: 1PRB with short CP and 16QAM carries:
12x7x4 = 336 bits over 1 time slot
Time – frequency structure of PRB

26. OFDMA time-frequency scheduling
Minimum allocateable resource in LTE is Resource Block pair
Resource block pair is 12 carriers wide in frequency domain and lasts for two time slots (1ms)
Depending on the length of cyclic prefix RB pair may have 14 or 12 OFDM symbols
PHY channels consist of certain number of allocated RB pairs
Overhead channels are typically in a predetermined location in time frequency domain
Allocation of the radio block is done by scheduler at eNode B

27. Block diagram of full OFDM TX/RX
LTE supports numerous AMC schemes
AMC adds additional level of adaptation to the RF channel
Size of CP depends on the amount of dispersion in the channel
Two CP are used: normal (4.7 us) and extended (16.7 us)

28. LTE Downlink physical layer Modulation scheme
Part 4
LTE Downlink physical layer Modulation scheme

29. Need for different MCS MCS = modulation and coding scheme
LTE – data network
eNode B transmits at full power
No power control on DL
Depending on UE’s channel condition different MCS is assigned
Management of MCS = Data rate management
CINR = Carrier to Noise and Interference Ratio
High CINR -> High data rate
Low CINR -> Low data rate
MCS = modulation and coding scheme
Channel quality is reported by UE. Based on the reports, eNode B decides on MCS for a given user

30. Available MCS Table 7.2.3-1 from 3GPP TS 36.213
UE reports one of possible 16 CQI values
CQI value – UE’s estimate of the highest MCS that may be received with block error probability smaller than 10%
eNode B uses UE’s reports to configure DL MCS
Using higher MCS improves spectrum efficiency
Spectrum efficiency – data rate / used spectrum
Table from 3GPP TS
Example: CQI = 9; SE = If eNode B allocates 5 PRB to an UE, the data rate:
DR = 5 x 180KHz x = 2.165Mbps

31. Modulation scheme Radio resources are managed in three planes:
Frequency
Time
Code
The management of the resources – eNode B scheduler
Scheduler – the most sophisticated part of the radio interface
Scheduler task = assign the radio resources (PRB) in an optimum way

32. LTE OFDM Basic timing unit: Ts = 1/(2048 x 15000) ~ 23.552 ns
Parameter
Value
Bandwidth (MHz)
1.4 3 5 10 15 20
Frame /subframe duration
10/1 ms
Subcarrier spacing
15KHz
Useful symbol part
66.7us
FFT size
128
256
512
1024
1536
2048
Resource blocks 6 25 50 75
100
Number of used subcarriers 72
180
300
600
900
1200
Cyclic prefix length
Normal: 5.1us for first symbol in a slot and 4.7us for other symbols , Extended: 16.7us
OFDM symbols /slot
7 (normal CP), 6 (extended CP)
Error coding
1/3 convolutional (signaling); 1/3 turbo (data)
Basic timing unit: Ts = 1/(2048 x 15000) ~ ns

33. Exercise – OFDM data rate capability at the PHY
Case 1. Normal CP (no MIMO)
Resource block: 12 carriers x 14 OFDM symbols = 168 resource elements
Each resource element carries one modulation symbol
For 64 QAM: 1 symbol = 6 bits
Number of bits per subframe = 168 x 6 = 1008 bits/subframe
Raw PHY data rate = 1008/1ms = 1,008,000 bits/sec/resource block (180KHz)
For 20MHz, Raw PHY data rate = 100 RB x 1,008,000 bits/sec/RB = 100.8Mbps
Case 2. Extended CP (no MIMO)
Resource block: 12 carriers x 12 OFDM symbols = 144 resource elements
Number of bits per subframe = 144 x 6 = 864 bits/subframe
Raw PHY data rate = 864/1ms = 864,000 bits/sec/resource block (180KHz)
For 20MHz, Raw PHY data rate = 100 RB x 864,000 bits/sec/RB = 86.4Mbps
Note: with the use of MIMO the rates are increased

34. Review questions Define Resource Element (RE)?
How many bits may be carrier by a RE?
How many RE are in a Physical Resource Block (PRB)?
Can a base station allocated a single RE to a mobile?
Why is there a need for many modulation and coding schemes (MCSs)?
Who decides which MCS is used on the DL?
Ho long is an LTE sub-frame?
How long is an LTE frame?
Determine the number of bits allocated to a mobile that uses 4 PRB (2 pairs od 2) and 16QAM in a system with a short CP. What is the corresponding PHY data rate?

35. Part 6
EXTRA

36. OFDM transmitter/receiver
Practically OFDM TX/RX is implemented using IFFT/FFT
Use of the IFFT/FFT at the baseband means that there is no need for separate oscillators for each of the OFDM carriers
FFT (IFFT) hardware is readily available – TX/RX implementation is simple

37. Guard time OFDM symbols with guard time
Duration of the OFDM symbol is chosen to be much longer than the multi-path delay spread
Long symbols imply low rate on individual OFDM carriers
In multipath environment long symbol minimizes the effect of channel delay spread
To make sure that there is no ISI between OFDM symbols – guard time is inserted
OFDM symbols without guard time
OFDM symbols with guard time

38. Cyclic prefix Guard time eliminates ISI between OFDM symbols
Multipath propagation degrades orthogonality between carriers within an OFDMS symbol
To regain the orthogonality between subcarriers – cyclic prefix is used
Cyclic prefix fills in the guard time between the OFDM symbols