The document discusses 4G mobile communications technologies WiMAX and LTE. It provides an overview of the IEEE 802.22 standard for wireless regional area networks using vacant TV channels. It also discusses the history and development of 4G standards, requirements for IMT-Advanced 4G, and early commercial versions of Mobile WiMAX and LTE that provided speeds less than 1 Gbit/s. It compares key aspects of 3G and 4G mobile networks.
3. 3
IEEE 802.22, is a standard for wireless regional area network
(WRAN) using white spaces (vacant TV channels) in
the television (TV) frequency spectrum (in the VHF and UHF bands).
Operates in the range of frequencies between 54 MHz and 862 MHz.
Operates in lower population density areas.
The development of the IEEE 802.22 WRAN standard is aimed at
using cognitive radio (CR) techniques to allow sharing of
geographically unused spectrum allocated to the television broadcast
service.
Cognitive Radio The IEEE 802.22 standard
5. 5
In March 2008, the International Telecommunications Union-Radio
communications (ITU-R) specified a set of requirements for 4G standards, named
the International Mobile Telecommunications Advanced (IMT-Advanced)
specification, setting peak speed requirements for 4G service at 300 Mbit/s for high
mobility communication (such as from trains and cars) and 1Gbit/s for low mobility
communication (such as pedestrians and stationary users).
Since the first-release versions of Mobile WiMAX (first used in South Korea in
2007) and LTE (in Oslo, Norway and Stockholm, Sweden since 2009 )support much
less than 1 Gbit/s peak bit rate, they are not fully IMT-Advanced compliant, but are
often branded 4G by service providers.
On December 6, 2010, ITU-R recognized that these two technologies, as well as
other beyond-3G technologies that do not fulfill the IMT-Advanced requirements,
could nevertheless be considered "4G", provided they represent forerunners to IMT-
Advanced compliant versions .
History
6. 6
During the spring 2011 above two system provide their advanced
version as: Mobile WiMAX Release 2 (also known as WirelessMAN-
Advanced or IEEE 802.16m') and LTE Advanced (LTE-A, Based on
UMTS 3G technology) and promising speeds in the order of 1 Gbit/s
in 2013.
7. 7
3G 4G
Data Rates of 100 Kbps to 2 Mbps
Goal is 'to provide multimedia multirate
mobile communications anytime and
anywhere'.
Connection between the cellular world and
the wired Internet firmly established.
Mobile devices used mainly for Human-
to-Human and Human-to-Machine
communication
Data Rates up to 100 Mbps
Expansion on the 3G goal to provide a
wider range of new and improved
multimedia services.
Integration of broadcast, cellular, cordless,
Wireless LAN, short-range and fixed wire
systems to appear as a single seamless
network.
Not only the 3G modes of communication
but also characterized by a great deal of
Machine-to-Machine traffic
Comparison of 3G and 4G
8. 8
Some Key Challenges
• Coverage
– Transmit power limitations and higher frequencies limit the
achievable cell size
• Capacity
– Current air interfaces have limited peak data rate, capacity,
and packet data capability
• Spectrum
– Lower carrier frequencies (< 5 GHz) are best for wide-area
coverage and mobility
9. 9
WiMAX
• Wi-MAX : The Worldwide Interoperability for Microwave Access, is a
technology aimed at providing wireless data over long distances It is
based on the IEEE 802.16 standard.
Fig.1
10. 10
In 1998 IEEE802.16 protocol was developed to provide high speed
service of WMAN (Wireless Metropolitan Area Network). Next two
new version of above protocol were found as: IEEE 802.16d (in 2004)
was developed to support high speed wireless data service of fixed user
and its later version IEEE 802.16e (in 2005) supports both fixed and
mobile users.
With the advent of OFDMA based IEEE 802.16e, research is now
going on to implement VoIP service with adaptive modulation and
channel coding (MCS) scheme. To enhance the throughput of the
wireless system the modulation and coding scheme of the transmitter is
changed according to the fading condition of the channel.
Therefore the service becomes a variable bit rate service where the bit
rate depends on the fading condition of the wireless channel.
WiMAX
11. 11
Three common types of BW allocation algorithms are: Dedicated
Resource Allocation (Unsolicited Grand Service known as UGS
Algorithm) where fixed amount of BW is allocated to each user
hence possibility of waste of BW when a user needs to data send
data at low rate; Polling-Based Resource Allocation (Real-Time
Polling Service called rtPS Algorithm) where BS allocates the BW
dynamically therefore incurs some protocol overhead and delay;
Hybrid Resource Allocation Algorithm is the combination of above
two.
WiMAX also can be used as a complementary system to Wi-Fi.
Both of the two major 3G systems: CDMA2000 and UMTS,
compete with WiMAX.
WiMAX
12. 12
WiBro, Korean version of WiMAX has been deployed in Korea.
WiFi and WiMAX are the B3G (Beyond 3G) systems. WiMAX
may be an interim system of a 4G system.
13. 13
Some important features of WiMAX are given below:
OFDM in physical layer: The access technique used in physical
layer of WiMAX is OFDM; where the high speed serial data is
converted to low rate parallel streams and each stream is modulated
by separate carrier each one is known as subcarrier. Subcarriers are
mutually orthogonal and deals with low data rate hence can protect
multipath fading.
Very high peak data rates: The data rate of WMAX is 70Mbps
under the channel of bandwidth of 20 MHz. The rate can be further
increased using space division multiplexing i.e. incorporation of
multiple antennas.
14. 14
Adaptive Modulation and Coding: The IEEE 802.16e standard
changes modulation and channel coding scheme based on received
SNR. For example a SS close to the BS can use a high modulation
scheme (more bits per symbol) i.e. the system can get more capacity
but when the SS is at the cell boarder the system permits lower
modulation scheme (increased signal space on orthogonal basis
function coordinate system) to avoid huge symbol error rate. Therefore
the system can overcome the time selective fading (the channel
condition is better at some instant than other).
Error Correction Techniques: WiMAX incorporates two types of
strong error correction techniques: FEC (Forward Error Correction)
for multimedia traffic and ARQ (Automatic Repeat Request) for
data traffic to improve throughput.
15. 15
Support for TDD and FDD: Like mobile cellular communication it
supports both FDD (Frequency division duplexing) and TDD (Time
division duplexing), as well as a half-duplex FDD. Above features
provide the flexibility of using same or different carriers for up and
down link.
18. 18
We consider a single cell in a WiMAX network with a base station
and multiple subscriber stations (Fig.2). Each subscriber station
serves multiple connections. Admission control is used at each
subscriber station to limit the number of ongoing connections
through that subscriber station. At each subscriber station, traffic
from all users for uplink connections are aggregated into a single
queue.
OFDMA Based WiMAX Network
19. 19
The size of this queue is finite (i.e., X packets) in which some
packets will be dropped if the queue is full upon their arrivals. The
OFDMA transmitter at the subscriber station receives packets and
transmits them to the base station. The base station may allocate
different number of subchannels to different subscriber stations.
For example, a subscriber station with higher priority could be
allocated more number of subchannels.
Fig.2 System model
21. 21
Wireless Technology Evolution to 3.9G
CDMA
(IS-95A)
GSM
CDMA
(IS-95B)
cdma
2000
1xEV-DO
Rev 0/A/B
UMB
802.20
2G
2.5G
3G
3.5G
3.9G
GPRS
E-GPRS
EDGE
HSDPA
FDD/TDD
TDMA
IS-136
WCDMA
FDD/TDD
TD-
SCDMA
LCR-TDD
HSUPA
FDD/TDD
HSPA+
LTE
E-UTRA
IEEE
802.16
Fixed
WiMAX
802.16d
Mobile
WiMAX
802.16e
WiBRO
IEEE
802.11
802.11g
802.11a
802.11g
802.11n
CDMA GSM/UMTS IEEE Cellular IEEE LAN
1. What is 4G?
UMB (Ultra Mobile Broadband) was the brand name for a project within 3GPP2 to
improve the CDMA 2000 mobile phone standard for next generation applications and
requirements. No carrier had announced plans to adopt UMB, and most CDMA carriers in
Australia, USA, Canada, China, Japan and South Korea have already announced plans to
adopt either WiMAX or LTE as their 4G technology.
23. 23
In a hierarchical telecommunications network the backhaul portion of the network
comprises the intermediate links between the core network or backbone, of the network
and the small sub-networks at the "edge" of the entire hierarchical network.
27. The IEEE 802.16 data link layer layer is composed of three sub-layers
Service Specific Convergence Sub-layer (CS), MAC Common Part Sub-
layer (CPS) and the Security Sub-layer. Each sub-layer has a specific
function to perform.
The 802.16 Protocol Stack
The 802.16 protocol stack
28. 28
Upper layers
Service specific convergence layer
MAC sub-layer
Security sub-layer
Transmission convergence sub-layer
QPSK 16-QAM 64-QAM
Data link layer
Physical layer
Fig.5 The 802.16 protocol stack
Layers of WiMAX
29. 29
802.16 PHY
The IEEE 802.16e supports both time division duplexing (TDD)
and frequency division duplexing (FDD) modes. However, the
initial release of Mobile WiMAX profiles only considers the TDD
mode of operation for the following reasons:
Time Division Duplexing
Time division duplexing (TDD) refers to the interleaving of transmission and
reception of data on the same frequency. A common frequency is shared between
the upstream and downstream, the direction in transmission being switched in
time.
Frequency Division Duplexing
Frequency division duplexing (FDD) refers to the simultaneous transmission and
reception of data over separate frequencies, allowing for bidirectional full-duplex
communications.
30. 30
A single frequency channel in (downlink) DL and (uplink)UL can
provide more flexibility for spectrum allocation.
It enables dynamic allocation of downlink (DL) and uplink (UL)
radio resources to effectively support asymmetric DL/UL traffic that
is common in Internet applications.
It supports link adaptation, multi-input-multi-output (MIMO)
techniques, and closed loop advanced antenna technique such as
beam-forming.
31. 31
An SS (subscriber Station) close to the BS could use a high modulation scheme,
thereby giving the system more capacity. In contrast, a weak signal from a more
remote subscriber might only permit the use of a lower modulation scheme to
maintain the connection quality and link stability.
This feature enables the system to overcome time-selective fading.
The coding rate also change according received SNR of fading channel.
Adaptive Modulation and Coding
Modulation
QPSK, 16-QAM, 64-QAM
32. 32
Modulation Coding Schemes (MCSs)
PDU → Packet Data Unit
SDU → Service Data Unit
lm → the number of PDU allocated for a TDMA slot
33. 33
where the parameter lm is the size of the VoIP PDU, which is
modulated with the mth MCS level after encoding and xm is the
number of PDU at mth MCS level.
Uplink scheduling is feasible if the allocated uplink resources are
less than the total of available resources (number of PDU/slot) Nslot,u.
Hence, we have
34. 34
For example, we consider Nslot,u = 50 and M = 4. Then, the MCS-level
distributions of packets are denoted as,
X =(x1, x2, x3, x4).
If the MCS-level distributions of six packets in the uplink queue are (0, 0, 0, 6)
or (0, 0, 1, 5) and the MCS level of the seventh packet in the queue is not four,
the BS schedules six packets according to the uplink feasibility condition.
= 0 + 0 + 0 + 6*6 = 36 <50
X = (0, 0, 0, 6)
X = (0, 0, 1, 5)
= 0 + 0 + 1*12 + 5*6 = 42 < 50
35. 35
However, if the MCS level of the seventh packet is four, the BS
can schedule more packets than six because the MCS-level
distribution of seven packets becomes (0, 0, 0, 7) or (0, 0, 1, 6),
which satisfies feasibility condition.
= 0 + 0 + 0 + 6*7= 42 <50
X = (0, 0, 0, 7)
X = (0, 0, 1, 6)
= 0 + 0 + 1*12 + 6*6= 48 <50
36. Data link layer
Upper layers
Service specific convergence layer
MAC sub-layer
Security sub-layer
Transmission convergence sub-layer
QPSK 16-QAM 64-QAM
Fig.5 The 802.16 protocol stack
From the reference model as illustrated in Figure 5, there are three
sub-layers in the data link layer composed of i) a security sublayer, ii)
a MAC common part sublayer, and iii) a convergence sublayer. It
provides only connection oriented service
37. 37
The CS, which is the interface between the MAC layer and layer 3 of the
network, receives data packets from the higher layer. These higher layer packets
are known as service data unit (SDU).
The CS is responsible for performing all operations that are dependent on the
nature of higher-layer protocol, such a header compression and address mapping.
The CS can be viewed as an adaptation layer that masks the higher-layer protocol.
Packet header suppression (PHS): At the transmitter it involves removing the
repetitive part of the header of each SDU. For example, if the SDUs delivered to
the CS are IP packets, the source and destination addresses contained in the header
of each IP packet do not change from one packet to the next and thus can be
removed before being transmitted over the air. Similarly at the receiver: the
repetitive part of the header can be reinserted into the SDU before being delivered
to the higher layer.
Service Specific Convergence Sub-layer (CS):
38. 38
CS is also responsible for the mapping the higher layer address,
such as IP address, of the SDUs into the identity of the PHY and
MAC connections to be used for its transmission. The WiMAX
MAC layer is connection oriented and identifies a logical connation
between the BS and the MS by a unidirectional connection
identifier (CID). The CID for UP and DL connections are different.
40. 40
The MAC layer takes packets from the upper layer (CS) and these
packets are called MAC service data units (MSDUs) and organize
them into MAC protocol data units (MPDUs) for transmission over
the air.
The WiMAX MAC uses a variable length MPDU and offer a lot of
flexibility to allow for their efficient transmission. For example
multiple MPDUs of same or different lengths may be arranged into a
single burst when they are destined to the same receiver.
MAC Common Part Sublayer
41. 41
Similarly , multiple MSDUs from the same higher-layer service
may be concatenated into a single MPDU to save MAC header
overhead.
Large MSDUs may be fragmented into smaller MPDUs and send
across multiple frames. When an SDU is fragmented, the position of
each fragment within the SDU is tagged by a sequence number. The
sequence number enables the MAC layer at the receiver to assemble
the SDU from its fragments in the correct order.
WiMAX has two types of PDUs, each with a very different header structure.
1. The generic MAC PDU is used for carrying data and MAC-layer signaling messages.
2. The bandwidth request PDU is used by the MS to indicate to the BS that more BW is
required in UL, due to pending data transmission. A bandwidth request PDU consists
only of a bandwidth-request header, with no payload or CRC.
42. 42
Packed fixed
size MSDU
GMH
Other
SH
Packed fixed
size MSDU
CRC…….
Fig.6 MAC PDU frame carrying several-fixed length MSDUs packed together
GMH → Generic MAC Header (used for carrying data and MAC-layer signaling messages)
SH → Sub-header
Each MAC frame is prefixed is prefixed with GMH (generic MAC
header).
Field (in SH) to indicate whether the payload is encrypted or not. If the
payload is encrypted then the encryption key is also given.
Header CRC field is a checksum over the header only using the
generator polynomial x8+x2+x+1. The length of this field is 8bits.
43. 43
Field Length description
HT 1 Header type (set 0 for such header)
EC 1 Encryption control (0 = payload not encrypted; 1 = payload encrypted
Type 6 type
ESF 1 (1 = ES present; 0 = ES not present)
CI 1 CRC indicator (1=CRC included; 0=CRC not included)
EKS 2 Encryption key sequence (index of the traffic encryption key and the
initialization vector used to encrypt the payload)
Rsv 1 Reserved
LEN 11 Length of MAC PDU in bytes, including the header)
CID 16 Connection identifier on which the payload is to be sent
HCS 8 Header check sequence; generation polynomial x8+x2+x+1
Generic MAC Header Fields
LEN
msb
(3)
H
T
CID msb (8)LEN lsb (8)
E
C
Type (6 bits)
rs
v
C
I
EKS
(2)
rs
v
HCS (8)CID lsb (8)
44. 44
Field Length Description
HT 1 Header type (set 1 for such header)
EC 1 Encryption control (set 0 for such header)
Type 3 type
BR 19 BW request ( the number of bytes of UL BW
requested by the SS for the given CID)
LEN 11 Length of MAC PDU in bytes, including the
header
CID 16 Connection identifier
HCS 8 Header check sequence
Bandwidth Request MAC Header Fields
BW Req.
msb (11)
H
T
CID msb (8)BWS Req. lsb (8)
E
C
Type (3 bits)
HCS (8)CID lsb (8)
45. 45
GMH
Other
SH
CRCFSH MSDU Fragment
Fig.7 MAC PDU frame carrying a single fragmented MSDU
FSH → Fragmentation Sub-header
PSH → Packing Sub-header
GMH
Other
SH
CRCPSH
Variable size MSDU or
Fragment
PSH
Variable size MSDU or
Fragment
…
Fig.8 MAC PDU frame carrying several variable length MSDUs packed together
The type of payload is identified by the sub-header immediately
precedes it. For example FSH or PSH of above figure.
46. 46
CRC
(optional)MAC PDU payload (optional)
Generic MAC
Header
(6 bytes)
msb lsb
LEN
msb
(3)
H
T
CID msb (8)LEN lsb (8)
Generic MAC Header Format
(Header Type (HT) = 0)
E
C
Type (6 bits)
rs
v
C
I
EKS
(2)
rs
v
HCS (8)CID lsb (8)
BW Req. Header Format
(Header Type (HT) =1)
BW Req.
msb (8)
H
T
CID msb (8)BWS Req. lsb (8)
E
C
Type (6 bits)
HCS (8)CID lsb (8)
48. Long Term Evolution (LTE)
48
Long-term evolution (LTE) standard is one of the newly developed
fourth generation standards for mobile communications. In the
standard, either frequency division duplexing (FDD) or time division
duplexing (TDD) schemes can be used to achieve two-way
communications.
51. 51
Features of LTE
The LTE- Advanced (Long Term Evolution- Advanced) is 4G wireless service
proposed by 3GPP (Third generation Partnership Project). In 2009 4G LTE started its
commercial service in Scandinavia. Three important features of LTE are: femtocell
deployment , OFDMA-based physical layer access and MIMO.
The FBS (Femto BS) is named as Home evolved Node-B (HeNB) in LTE-A placed
in public places to provide higher data rate and improve resource usage to a number of
users. Femtocells are different in the sense that they are installed by customers in an
ad hoc fashion without any RF planning. Objective of eNodeB Femtocells lies in off-
loading of traffic.
53. 53
LTE provides OFDMA-based physical layer
access where:
• OFDMA minimizes separation between
carriers
• Carriers are selected so that they are
orthogonal over symbol interval
• Carrier orthogonality leads to frequency
domain spacing ∆f =1/T, where T is the
symbol time
• In LTE carrier spacing is 15KHz and useful
part of the symbol is 66.7 microsec
54. 54
MIMO communication takes place between eNB and UE. LTE
standard requires support for:
eNodeB can have maximum 4 antennas
UE can have maximum 2 antennas
56. 56
The architecture of LTE consists of two major parts: the E-UTRAN
(Evolved Universal Terrestrial Radio Access Network) and the EPC
(Evolved Packet Core). The first part provides air interface between
MS or UE to BS and the second part is interconnected switching
network called backbone or core network.
57. 57
User Equipment (UE)
As the name suggests, a UE is the actual device that the LTE
customers use to connect to the LTE network and establish their
connectivity. The UE may take several forms; it can be a mobile
phone, a tablet, or a data card used by a computer/notebook.
Similar to all other 3GPP systems, the UE consists of two main
entities: a SIM-card or what is also known as User Service Identity
Module (USIM), and the actual equipment known as Terminal
Equipment (TE).
SIM-card carries the necessary information provided by the
operator for user identification and authentication procedures. The
terminal equipment on the other hand provides the users with the
necessary hardware (e.g., processing, storage, operating system) to
run their applications and utilize the LTE system services.
58. 58
Evolved UTRAN (E-UTRAN)
The E-UTRAN in LTE consists of directly interconnected eNodeBs which are
connected to each other through the X2 interface and to the core network through the
S1 interface.
This eliminates one of the biggest drawbacks of the former 3GPP systems (UMTS):
the need to connect and control the NodeBs through the Radio Network Controller
(RNC), which make the system vulnerable to RNC failures.
The LTE E-UTRAN architecture can be seen in Figure below.
59. 59
Evolved Packet Core (EPC)
The EPC (also known as the LTE core network) consists of three
main entities: Mobility Management Entity (MME), Serving Gateway
(S-GW) and the Packet Data Network Gateway (PDN-GW). In
addition, there are some other logical entities like the Home
Subscriber Server (HSS) and Policy and Charging Rules Function
(PCRF).
60. 60
The EPC consists of six nodes:
Home Subscriber Server (HSS) is like the combination of HLR and AUC of UMTS or
GSM
The Packet Data Network (PDN) Gateway (P-GW) provides connectivity between
UE and external packer switching network. It works like a gateway SGSN (Serving
GPRS Support Node) of UMTS.
The serving gateway (S-GW) works as a router whose function is to forward data
between the BS and the PDN gateway(P-GW). It also works as the mobility anchor of
eNB handovers and do the similar job between LTE and other 3GPP technologies.
61. 61
The MME (for Mobility Management Entity) deals with the signaling
(between UE and CR) related to mobility of users, paging of UE in idle-
mode and security for E-UTRAN access.
The Policy Control and Charging Rules Function (PCRF): This
module works like: Packet filtering and billing on flow basis.
ePDG (Evolved Packet Data Gateway) provides secured data
transmission between UE to un-trusted non-3GPP access through EPC.
62. 62
S-GW and MME HSS
P-GW
E-UTRAN
Macro
Femto
Radio tower
Radio tower
Radio tower Trusted non
3GPP
access
ePDG
Untrusted
non 3GPP
access
Fig.1 Architecture of LTE
63. 63
Interfaces
S1 (eNode B to SGSN)
S1 (eNode B to MME)
X2 between two eNode Bs (required for handover)
Uu (UE to eNode B)
eNB
MME / S-GW MME / S-GW
eNB
eNB
S1
S1
S1
S1
X2
X2
X2
E-UTRAN
Uu
64. LTE Protocol Stack
64
PDCP → Packet Data Convergence Protocol
RLC → Radio Link Control
MAC → Medium Access Control
GTP-U→ GPRS Tunneling Protocol
65. 65
PDCP layer or layer 2 Packet Data Convergence Protocol is responsible for data
ciphering and IP header compression to reduce the IP header overhead. The service
provided by PDCP to transfer IP packets is called a radio bearer. A radio bearer is
defined as an IP stream corresponding to one service for one UE.
RLC layer or layer 2 Radio Link Control performs the data concatenation and then
generates the segmentation of packets from IP-Packets of random sizes which
comprise a Transport Block (TB) of size adapted to the radio transfer. The RLC layer
handles a retransmission scheme of lost data through a first level of Automatic
Repeat reQuests (ARQ).
66. 66
There are two types of LTE frame structure:
Type 1: used for the LTE FDD mode systems.
Type 2: used for the LTE TDD systems.
Type 1 LTE Frame Structure
The duration of one LTE radio frame is 10 ms. One frame is divided into 10
subframes of 1 ms each, and each subframe is divided into two slots of 0.5 ms each.
LTE Frame Structure
68. 68
Each slot contains either six or seven OFDM symbols, depending on the Cyclic
Prefix (CP) length. The useful symbol time is 1/15 kHz= 66.6 mircosec. Since normal
CP is about 4.69 microsec long, seven OFDM symbols can be placed in the 0.5-ms
slot as each symbol occupies (66.6 + 4.69) = 71.29 microseconds.
When extended CP (=16.67 microsec) is used the total OFDM symbol time is (66.6
+ 16.67) = 83.27 microseconds. Six OFDM symbols can then be placed in the 0.5-ms
slot.
69. Detailed time domain structure
69
TCP: 160Ts (5.1us) for first symbol;v144Ts (4.7us) for other six symbols
TCP-e: 512 Ts (16.7 us) for all symbols
Need for two different CP:
1. To accommodate environments with large
channel dispersion
2. 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
73. 73
• LTE – radio resource = “time-frequency chunk”
• Resource Block (RB) = 12 subcarriers in one TS (12*15KHz x 0.5ms)
• Time domain
1 frame = 10 sub-frames
1 subframe = 2 slots
1 slot = 7 (or 6) OFDM symbols
• Frequency domain
1 OFDM carrier = 15KHz
74. 74
Time Unit Value
Frame 10 ms
Half-frame 5 ms
Subframe 1 ms
Slot 0.5 ms
Symbol (0.5 ms) / 7 for normal CP
(0.5 ms) / 6 for extended CP
Basic timing unit: Ts 1/(15000 * 2048) sec » 32.6 ns
75. OFDMA time-frequency scheduling
• Minimum allocateable resource in LTE is
Resource Block pair
• Resource block pair is 12 subcarriers 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
75
76. 76
Type 2 LTE Frame Structure
The frame structure for the type 2 frames used on LTE TDD is
somewhat different. The 10 ms frame comprises two half frames, each
5 ms long. The LTE half-frames are further split into five subframes,
each 1ms long.
The subframes may be divided into standard subframes of special subframes.
The special subframes consist of three fields;
DwPTS - Downlink Pilot Time Slot
GP - Guard Period
UpPTS - Uplink Pilot Time Stot.
77. LTE OFDM
77
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 15 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) ~ 23.552 ns
78. Downlink reference signals
• For coherent demodulation – terminal needs channel estimate for each subcarrier
• Reference signals – used for channel estimation
• There are three type of reference signals
1. Cell specific DL reference signals
2. UE specific DL reference signals
3. MBSFN reference signals
78
79. Cell specific reference signals
• DL transmission may use up to four antennas
• Each antenna port has its own pattern of reference signals
• Reference signals are transmitted at higher power in multi-
antenna case
• Reference signals introduce overhead
– 4.8% for 1 antenna port
– 9.5% for 2 antenna ports
– 14.3 % for 4 antenna ports
• Reference symbols vary from position to position and
from cell to cell – cell specific 2 dimensional sequence
• Period of the sequence is one frame
79
Four port TX
Two port TX
One port TX
80. UE Specific Reference Signal RS
• UE specific RS – used for beam forming
• Provided in addition to cell specific RS
• Sent over resource block allocated for DL-SCH (applicable only for
data transmission)
80
Note: additional reference signals increase
overhead. One of the most beneficial use of
beam forming is at the cell edge – improves
SNR
81. 81
Relays and Repeaters
Relaying is used in order to deploy cells in areas where no (or very
expensive) wired backhaul exists. It is often used to improve the
coverage and throughput, for example, in urban, and rural scenarios.
Figure below shows a typical scenario where a Relay Node (RN) is
connected to an eNodeB wirelessly. The eNodeB that the RN is
connected to is called in that case, a Donor eNodeB (DeNB).
