1. Δίκτυo Long Term Evolution (LTE)
Αν. Καθηγητής Γεώργιος Ευθύμογλου
Module Title

2. Introduction Long Term Evolution (LTE) OFDMA
LTE downlink physical parameters
Resource block (RB) and Subframe
LTE Layers Data Flow
VoIP Capacity in LTE
Module Title

3. Frequency selective channel
Multipath propagation results in frequency selective fading. OFDM solution to maintain subcarrier orthogonality is Cyclic Prefix

4. OFDM transmission
There are many advantages to using OFDM in a mobile access system, namely: 1- Long symbol time and guard interval increases robustness to multipath and limits intersymbol interference. 2- Eliminates the need for intra-cell interference cancellation. 3- Allows flexible utilization of frequency spectrum. 4- Increases spectral efficiency due to the orthogonality between sub-carriers. 5- Allows optimization of data rates for all users in a cell by transmitting on the best (i.e. non-faded) subcarriers for each user.

5. LTE OFDM transmission
Consider a time-discrete (sampled) OFDM signal where it is assumed that the sampling rate fs is a multiple of the subcarrier spacing Δf fs = 1/Ts = N • Δf As Nc • Δf can be seen as the nominal bandwidth of the OFDM signal, this implies that N should exceed Nc with a sufficient margin. N/Nc, is the over-sampling of the time-discrete OFDM signal.

6. OFDM transmission parameters
LTE OFDM transmission
As an example, for 3GPP LTE the number of subcarriers Nc is approximately 600 in the case of a 10 MHz spectrum allocation.
The IFFT size can then, for example, be selected as N = This corresponds to a sampling rate fs = N • Δf = MHz, where
Δf = 15 kHz is the LTE subcarrier spacing.
OFDM transmission parameters
The subcarrier spacing Δf.
The number of subcarriers Nc, which, together with the subcarrier spacing, determines the overall transmission bandwidth of the OFDM signal.
The cyclic-prefix length TCP. Together with the subcarrier spacing Δf = 1/Tu, the cyclic-prefix length determines the overall OFDM symbol time T = TCP + Tu or, equivalently, the OFDM symbol rate.

7. LTE OFDM demodulation
Recover the modulation symbols

8. Coded OFDM
Problem Solution: interleave consecutive bits coming out of channel encoder.

9. Frequency Interleaving
Channel coding implies that each bit of information to be transmitted is spread over several, often very many, code bits.
If these coded bits are then, via modulation symbols, mapped to a set of OFDM subcarriers that are well distributed over the overall transmission bandwidth of the OFDM signal each information bit will experience frequency diversity in the case of transmission over frequency selective channel
Distributing the coded bits in the frequency domain, is referred to as frequency interleaving.

10. OFDMAccess
Downlink: in each OFDM symbol interval, different subsets of the overall set of available subcarriers are used for transmission to different terminals. Uplink: in each OFDM symbol interval, different subsets of the overall set of subcarriers are used for data transmission from different terminals.

11. OFDM and peak-to-average power ratio (PARP)
OFDM has a large peak-to-average power ratio which means that the amplifiers have to be higher quality and are more expensive (and are also more power hungry). Picture from Ref. [4].

12. LTE Downlink: time domain
Time duration for one frame is 10 ms. This means that we have 100 radio frames per second.
Sampling frequency for 20MHz bandwidth is 15 KHz * 2048 (IFFT_size) = MHz = Fs
Sampling time Ts = 1/Fs = 1/(15 KHz * 2048) = 1/
30.72 MHz = 8 x 3.84 MHz (sampling frequency in UMTS)
Duration of time slot is 7 OFDM symbols + 7 CPs
Number of subframe in one frame is 10.
Number of slots in one subframe is 2. This means that we have 20 slots in one frame.
Each slot consists of a number of OFDM symbols which can be either 7 (normal cyclic prefix) or 6 (extended cyclic prefix)

13. LTE Downlink: time domain
Frame structure for LTE in FDD mode (Frame Structure Type 1).

14. LTE Downlink: time domain

15. LTE Downlink: time domain
The number of samples shown in this illustration is based on the case of MHz sampling rate. The slot length in time shown in this illustration does not vary with the Sampling Rate, but the number of samples in each symbol and CP varies with the sampling rate.

16. LTE Downlink: time domain
A few observations:
The first OFDM symbol within a slot is a little bit longer than the other OFDM symbols.
The number of samples shown in the previous illustration is based on the assumption that the sampling rate is Msamples/sec and 2048 bins/IFFT(N_ifft).
Real sampling rate and N_ifft may vary depending on system BW. We need to scale this number according to a specific BW.
Typical N_ifft for each system BW is given as folows:
System BW
1.4 3 5 10 15 20
N_ifft
128
256
512
1024
2048

17. LTE Downlink: time domain
The useful OFDM symbol time is Tu = 2048 × Ts ≈ 66.7 μs. For the normal mode, the first symbol has a cyclic prefix of length TCP = 160 × Ts ≈ 5.2 μs. The remaining six symbols have a cyclic prefix of length TCP = 144 × Ts ≈ 4.7 μs. The reason for different CP length of the first symbol is to make the overall slot length in terms of time units divisible by For the extended mode, the cyclic prefix is TCP-e = 512 × Ts ≈ 16.7 μs. The CP is longer than the typical channel delay spread of a few microseconds. The normal cyclic prefix is used in urban cells and high data rate applications while the extended cyclic prefix is used in special cases like multi-cell broadcast and in very large cells (e.g. rural areas, low data rate applications).

18. LTE Downlink: frequency domain
In the frequency domain, the number of sub-carriers N ranges from 128 to 2048, depending on channel bandwidth. N= 512 and 1024 correspond to 5 and 10 MHz, respectively, being most commonly used in practice. The sub-carrier spacing is Df = 1/Tu = 15 kHz. The sampling rate is fs = Df ・ N = N. This results in a sampling rate that is a multiple or sub-multiple of the WCDMA chip rate of 3.84 Mcps: LTE parameters have been chosen such that FFT lengths and sampling rates are easily obtained for all operation modes while at the same time ensuring the easy implementation of dual-mode devices with a common clock reference.

19. LTE Downlink: frequency domain
Frequency domain representation of resource block (RB) NRB determines number of subcarriers (12* NRB ) and depends on transmit bandwidth (given below). In the downlink, the DC subcarrier is counted (+1) but does not used to send data.

20. LTE Downlink: Resource Block
The transmission can be scheduled by Resource Blocks (RB) 1 RB = 12 consecutive sub-carriers, or 180 kHz, for the duration of one slot (0.5 ms), that is for (7 OFDM symbols, or 6 for extended CP) A Resource Element (RE) is the smallest defined unit which consists of one OFDM sub-carrier during one OFDM symbol interval. Each Resource Block consists of 12 ・ 7 = 84 Resource Elements (RE) in case of normal cyclic prefix (72 for extended CP). Each RE can “carry” number of bits depending on the modulation employed. For example, using for QPSK: a RB carries 84*2 bits per 0.5 msec.

21. LTE Downlink: Resource Block
Definition of Resource Blocks and Resource Elements. 1 RB = 12 consecutive sub-carriers, or 180 kHz, for the duration of one slot (0.5 ms), that is for (7 OFDM symbols, or 6 for 72 for extended CP)

22. LTE Downlink: Resource Block
In summary:
One frame is 10ms and it consists of 10 sub-frames.
One LTE subframe is 1ms and contains 2 slots.
One slot is 0.5ms in time domain and each 0.5ms assignment can contain N resource blocks [6 < N < 110] depending on the bandwidth allocation and resource availability.
One resource block is 0.5ms and contains 12 subcarriers for each OFDM symbol in frequency domain.
There are 7 symbols (normal cyclic prefix) per time slot in the time domain or 6 symbols in long cyclic prefix for LTE.
LTE Resource element is the smallest unit of resource assignment and its relationship to resource block is shown as below from both a timing and frequency perspective.

24. LTE Downlink: Resource Block
Red color for Δf = 15 KHz.

25. LTE: Physical resource blocks (PRBs) or subframes = 1msec
In OFDMA, users are allocated a specific number of subcarriers for a predetermined amount of time. Physical resource blocks (PRBs) or Subframe in the LTE specifications equal to 2 consecutive RBs, that is, 12 subcarriers for 14 OFDM symbols.

26. Channel bandwidth vs transmission bandwidth

27. Transmission bandwidth

28. Physical layer parameters for LTE in FDD mode

29. E-UTRA Bands, Channel Bandwidths, and Frequency Allocations

30. E-UTRA Bands for TDD

31. FDD and TDD

32. LTE Protocol Stack Layers
all the layers available in E-UTRAN Protocol Stack:

33. LTE Protocol Stack Layers
Physical Layer (Layer 1)
Physical Layer carries all information from the MAC transport channels over the air interface. Takes care of the link adaptation (AMC), power control, cell search (for initial synchronization and handover purposes) and other measurements (inside the LTE system and between systems) for the RRC layer.
Medium Access Layer (MAC)
MAC layer is responsible for Mapping between logical channels and transport channels, Multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be delivered to the physical layer on transport channels, and so on

34. LTE Protocol Stack Layers
Radio Link Control (RLC) operates in 3 modes of operation: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). RLC performs segmentation and reassembly and error correction functions using ARQ (in Acknowledged Mode). Radio Resource Control (RRC) The main services and functions of the RRC sublayer include broadcast of System Information related to the non-access stratum (NAS), and so on… Packet Data Convergence Control (PDCP) Layer is responsible for Header compression and decompression of IP data, Transfer of data (user plane or control plane), Maintenance of PDCP Sequence and so on… Non Access Stratum (NAS) Protocols The non-access stratum (NAS) protocols form the highest stratum of the control plane between the user equipment (UE) and MME. NAS protocols support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a PDN GW.

35. LTE Layers Data Flow
Packets received by a layer are called Service Data Unit (SDU) while the packet output of a layer is referred to by Protocol Data Unit (PDU).

36. LTE Layers Data Flow Let's see the flow of data from top to bottom:
IP Layer submits PDCP SDUs (IP Packets) to the PDCP layer.
PDCP layer does header compression and adds PDCP header to these PDCP SDUs.
PDCP Layer submits PDCP PDUs (RLC SDUs) to RLC layer.
RLC Segmentation: If an RLC SDU is large, or the available radio data rate is low (resulting in small transport blocks), the RLC SDU may be split among several RLC PDUs.
MAC layer adds header and does padding to fit this MAC SDU in TTI. MAC layer submits MAC PDU to physical layer for transmitting it onto physical channels.
Physical channel transmits this data into slots of sub frame.

37. LTE Layers Data Flow
PDCP Header Compression : PDCP removes IP header (Minimum 20 bytes) from PDU, and adds Token of 1-4 bytes. This provides a tremendous savings in the amount of header that would otherwise have to go over the air.

38. Schematic overview of the encoding process

39. From 3GPP TS V ( )
subframe = 1msec (2 slots) Scheduling is done every subframe!!! QPSK 1/3 72x2 (κανονικά (7*12) REs*2 slots =84*2, αλλά κάποια σύμβολα χρησιμοποιούνται για Reference Signals)

40. From 3GPP TS V ( )
subframe = 1msec (2 slots) 16QAM 3/4

41. VoIP capacity in LTE
LTE supports voice over IP (Internet Protocol) (VoIP) to provide voice services. 
We will carry out a simplified analysis below to estimate the VoIP capacity under a given set of assumptions. 
Assume that full-rate 12.2 kbps Adaptive Multi Rate (AMR) speech codec is used.
Every 20 ms, AMR speech codec generates (12.2 kbps * 20 ms= 244 bits) during the “speech on” interval (i.e., the user is indeed talking and not just listening during such interval). 
These bits are placed in an RTP/UDP/IP packet with about 3 bytes (=24 bits) of overhead. 
IP header compression is assumed to be active.

42. VoIP capacity in LTE
The VoIP packet entering the air interface protocol stack would contain about (244 speech bits + 24 IP-related header bits = 268) bits. 
The VoIP packet passes through these layers of the air interface protocol stack- PDCP, RLC, MAC, and PHY.  Let’s add 4 bytes (=32 bits) to account for
headers added by Packet Data Convergence Protocol (1 byte for short sequence number),
RLC (1 byte for Unacknowledged Mode operation with a 5-bit sequence number), and
MAC (2 bytes) layers, 
leading to the “target” payload of (268+32=300) bits entering the PHY layer from the MAC layer. 

43. VoIP capacity in LTE
Now, let’s calculate how many Physical Resource Blocks (PRBs) are needed to carry the target payload of 300 bits. According to Table A.3-1 of [3GPP, V8.7.0], 1 PRB can carry the payload of 104 bits when the modulation scheme is QPSK and the coding rate is (1/3). This payload is from the MAC layer to the PHY layer. When users are distributed across the cell, some would have good channel conditions and can support (16-QAM, coding rate=¾); others may have bad channels conditions and would require more robust (QPSK, coding rate=1/3). If 50% of users are able to use (16-QAM, coding rate=¾) and 50% of users need (QPSK, coding rate=1/3), the average number of PRBs consumed by a typical VoIP user in a cell would be (0.50*3 PRBs *1 PRB = 2 PRBs).

44. VoIP capacity in LTE
For BW=10MHz, in 1 ms subframe, there are 50 PRBs, allowing (50 PRBs/2 PRBs per user = 25 users). Since the AMR speech codec generates a new speech frame every 20 ms, during a span of 20 ms, we can have 20 subframes carrying VoIP packets for (20 subframes * 25 users per subframe= 500) users. These calculations assume that every single packet with a specific modulation scheme and certain amount of coding is received without any errors all the time. However, in practice, some packets would be lost, requiring HARQ retransmission. If we need one (additional) retransmission on average, PRBS would need to be allocated to a given VoIP user twice per 20 ms interval instead of just once per 20 ms interval. Since a VoIP users is now consuming twice as many PRBs during the 20 ms interval, the number of VoIP users would be reduced by half (i.e., 500/2= 250).

45. VoIP capacity in LTE
Comprehensive simulation-based analysis indicates that 123 VoIP users can be supported in 5 MHz bandwidth [3GPP, R , “Performance Evaluation Checkpoint: VoIP Summary.”], implying (123*2= 246) users can be supported in 10 MHz channel bandwidth. The VoIP capacity estimate calculated above can be adjusted by modifying assumptions and making suitable adjustments to the calculations. For example, instead of using just two combinations of modulation scheme and coding rate, multiple combinations can be used to estimate the number of PRBs required by an average user in a cell. The overall approach outlined above can still be used for an approximate VoIP capacity estimate. In summary, for the assumptions made here, the VoIP capacity in LTE is 250 in case of 10 MHz channel bandwidth.

46. References
4G LTE/LTE-Advanced for Mobile Broadband, by Erik Dahlman, Stefan Parkvall, and Johan Skold, 2011 Elsevier.
LTE in a Nutshell: The Physical Layer, White paper, 2010, Telesystem Innovations.
LTE Resource Guide at
LTE Physical Layer Overview:
LTE tutorial:
LTE E-UTRAN and its Access Side Protocols
TS V BS Radio Transmission and Reception