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Basics, Network Entry Procedures,
and Bandwidth Request/Grand
Mechanism for IEEE Std. 802.16
Chen-Nien Tsai
Institute of Computer Science and
Information Engineering
National Taipei University of Technology
2007.10.8
Outline
►A
Brief Introduction to IEEE Std. 802.16.
► Overview of IEEE 802.16
► MAC/PHY Basics
► Network Entry and Initialization
► Bandwidth Request/Grand Mechanism
► Summary
2
Introduction to IEEE Std. 802.16
► The
central aim of IEEE 802.16 technology
is to support broadband access.
 Providing service at a rate of at least 1.544
Mbps. (ITU definition)
► Broadband
Wireless Access (BWA)
 Broadband extension of the wireless access
concept.
► Wireless
Broadband Access
 A wireless implementation of broadband access
concepts.
3
Introduction to IEEE Std. 802.16
► IEEE
Std. 802.16 is called the wirelessMAN®
standard for wireless metropolitan area
networks.
 Supports networks that are about the size of a
city.
 Not limited to urban applications.
 Some of the most likely applications are in rural
areas.
►Replace
last-mile.
4
Wireless Technologies
Bandwidth
1 Gbps
IEEE 802.15
100 Mbps
10 Mbps
1 Mbps
IEEE 802.11
IEEE 802.16
3GPP/3GPP2/ETSI
802.11n
802.15.3
High rate
WPAN
802.15.1
Bluetooth
<1m
PAN
Wi-Fi
802.11a/g
WiMAX
802.16
3G
Wi-Fi
802.11b
10m
4G
2.5G
100m
LAN
PAN: Personal area network
LAN: Local area network
Up to 50km
MAN
Up to 80km
WAN
MAN: Metropolitan area network
WAN: Wide area network
5
IEEE 802.16 Project Timeline
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
IEEE Std 802.16-2001
IEEE Std 802.16a
IEEE Std 802.16c
IEEE Std 802.16-2004
IEEE Std 802.16-2004/Cor1
IEEE P802.16-2004/Cor2
IEEE P802.16Rev2
IEEE Std 802.16e (mobile)
IEEE Std 802.16f (MIB)
IEEE P802.16g (management)
IEEE P802.16h (coexistence mechanism)
IEEE P802.16i (management)
IEEE P802.16j (multihop relay)
IEEE Std 802.16k-2007
IEEE P802.16m
IEEE Std 802.16/Conf01
IEEE Std 802.16/Conf02
IEEE Std 802.16/Conf03
IEEE Std 802.16/Conf04
IEEE Std 802.16.2-2001
6
IEEE Std 802.16.2-2004
Completed
In progress
Superseded Standards
IEEE Standard Styles
► Amendment
 contains new material to be incorporated into
an existing IEEE standard.
 Designated by a lowercase letter after the
primary standard number.
 802.16c, 802.16a. (amendment to 802.16-2001)
► Corrigendum
 allows corrections but prohibits new features.
 802.16-2004/Cor1, 802.16-2004/Cor2.
7
IEEE Standard Styles
► Revision
 Base standard and its published amendments
are editorially merged.
 802.16-2004 (802.16-REVd), including 802.162001, 802.16c, 802.16a.
 802.16Rev2 (under development)
8
Overview of IEEE 802.16
IEEE 802.16
► Scope:
 Specifies the air interface of fixed BWA systems.
►Including
the medium access control (MAC) layer and
multiple physical (PHY) layer specifications.
► Purpose:
 Enables rapid worldwide deployment of costeffective BWA products.
 Facilitates competition in broadband access by
providing alternatives to wireline broadband
access.
10
Basic Network Architecture
Wireless link
Wired link
Core network
Users
Base Station (BS)
SS
Subscribe Station (SS)
SS
11
BS and SS
► Base
station (BS)
 A generalized equipment set providing
connectivity, management, and control of the
SS.
► Subscribed
station (SS)
 A generalized equipment set providing
connectivity between subscriber equipment and
a BS.
12
Typical Deployment Scenarios
Mesh Node
13
Reference Model
14
Service-Specific Convergence Sublayer
► Functions
 Classification.
 Header suppression.
► Two
CS specified
 ATM CS.
 Packet CS.
15
MAC Common Part Sublayer
► Functions




System access.
Bandwidth allocation.
Call admission
Connection management.
► Two
operation modes
 Point-to-multipoint (PMP)
 Mesh
16
Security Sublayer
► Functions
 Authentication
 Secure key exchange
 Encryption
17
Physical Layer
► Four




PHY specified
WirelessMAN-SC PHY
WirelessMAN-SCa PHY
WirelessMAN-OFDM PHY
WirelessMAN-OFDMA PHY
18
Physical Layer
► For
10-66 GHz (IEEE 802.16-2001)
 WirelessMAN-SC PHY
►Single-carrier
► For
modulation.
2-11 GHz (IEEE 802.16a)
 WirelessMAN-SCa PHY
►Single-carrier
modulation
19
Physical Layer
► For
2-11 GHz (IEEE 802.16a)
 WirelessMAN-OFDM PHY
►256-carrier
OFDM (orthogonal-frequency division
multiplexing)
►Multiple access is provided through TDMA (timedivision multiple access).
 WirelessMAN-OFDMA PHY
►2048-carrier
OFDM.
►Multiple access is provided by assigning a subset of
the carriers to an individual receiver.
20
MAC/PHY Basics
MAC Support of PHY
► Several
duplexing techniques are supported
by the MAC.
 Time division duplexing (TDD)
►UL
and DL transmission occur at different times and
usually share the same frequency.
 Frequency division duplexing (FDD)
►UL
and DL channels are located on separate
frequencies.
►Full duplex
►Half duplex
► FDD
or TDD?
22
Framing
► Each
frame has a DL subframe and UL
subframe.
 DL subframe begins with information necessary
for frame synchronization and control.
 In the TDD case, the DL subframe comes first,
followed by the UL subframe.
 In the FDD case, UL transmission occur
concurrently with the DL frame.
23
TDD Frame Structure
PS (Physical slot): a unit of time for allocating bandwidth.
Rate: symbol rate for SC and SCa PHY, nominal sampling frequency for
OFDM and OFDMA PHY.
24
FDD Bandwidth Allocation
25
TDD Downlink Subframe
Burst profile for DIUC
= 0 is well-known
Downlink
Interval
Usage Code
is a code
identifying a
particular
burst profile
synchronization
BW allocation and other channel
information for DL/UL
26
Downlink transmission
► Preamble
 Synchronization and equalization.
► Frame
control
 DL-MAP
►How
and when the DL data are transmitted.
 UL-MAP
►How
and when the UL data are transmitted.
 DCD/UCD
►Channel
description for UL/DL
27
TDD Uplink Subframe
28
Uplink Transmission
► Three
classes of bursts may be transmitted
in a UL subframe:
 Contention opportunities for initial ranging.
 Contention opportunities for BW requests.
 Contention-free periods assigned by BS to
individual SSs.
29
Connections and Addressing
► Each
SS has a unique 48-bit MAC address.
 It is used only during the initial ranging process
or authentication process.
 Not carried in every MPDU.
 How to identify src. and dest.?  CID
► 802.16
MAC is connection-oriented.
 Connection is a unidirectional mapping between
BS and SS MAC peers.
 A connection identifier (CID) is a 16-bit value
that identifies a connection.
►A
maximum of 65535 connections are supported for
30
each DL and UL.
Connection Types
► Basic
connection
 Assigned to each SS after successful ranging.
 To transport delay-intolerant basic MAC
messages.
 Identify the SS for managing per-SS functions.
(BW grants in UL-MAP)
► Primary
management connection
 Assigned to each SS after successful ranging.
 To transport delay-tolerant basic MAC messages.
31
Connection Types
► Secondary
management connection
 Assigned to each “managed” SS during the
registration process.
 To transport higher layer management
messages (SNMP, TFTP, and DHCP).
► Transport
connection
 Created and changed by Dynamic Service series
messages (DSA, DSD, and DSC).
 To transport user data.
32
MAC Management Messages
33
Connection Identifiers
► Initial
Ranging CID.
► Basic CID.
► Primary Management CID.
► Secondary Management CID.
► Transport CID.
► AAS Initial Ranging CID.
► Multicast Polling CID.
► Padding CID.
► Broadcast CID.
34
Well-known Addresses and Identifiers
m is the maximum possible number of SSs that can be supported
35
MAC Headers
► Stand-alone
MAC header
 6 bytes.
 The smallest possible information unit that can
be transported between two nodes. (with the
exception of HARQ MAPs)
 None of the stand-alone headers can be used to
encapsulate any payload.
 It is a misnomer to call them “headers.”
 BW request header and signaling header
(defined in other std.) are stand-alone MAC
headers.
36
BW Request Header Format
000: incremental BR
001: aggregate BR
Bandwidth request in bytes
Encryption Control
Header Check Sequence
37
MAC Headers
► Generic
MAC header
 Followed by the optional variable-size payload.
 Payload may consist of:
►MAC
subheaders
►Management messages
►Special payload
►Padding
38
Generic MAC Header Format
Subheader type
CRC Indictor
Encryption Key Sequence
Header Check Sequence
39
MAC Subheaders
► For
generic MAC header only
Packing and Fragmentation subheaders are mutually
exclusive.
Type
5 4 3 2 1 0
Mesh subheader
ARQ Feedback payload
Extended Type
Indicate whether the Packing or Fragmentation
Subheaders is extended.
Fragmentation subheader
Packing subheader
Downlink: FAST-FEEDBACK allocation subheader
Uplink: Grant Management subheader
40
Network Entry and
Initialization
Once the SS has powered up, it begins the network
entry and initialization process. After completing the
steps of the process, the SS has all the addresses and
parameters it need to communicate with the rest of the
network.
Network Entry and Initialization
► The
procedures for entering and registering
a new SS or a new node to the network.
► The procedures described here apply only to
PMP mode.
Hey, I want to join
the network.
BS
New SS
42
Phases
► Scanning
and synchronization to the DL
► Obtain transmit parameters
► Initial ranging
► SS basic capability negotiation
► SS authorization and key exchange
► Registration
► Establish IP connectivity
► Establish time of day
Optional
► Transfer operational parameters
► Establish provisioned connections
43
Scanning and synchronization to the DL
► Achieve
PHY synchronization
 Scan the possible channels of the downlink
frequency band of operation until it finds a valid
downlink signal.
► Then
try to acquire the channel control
parameters for the DL and the UL.
 How?
44
Obtain Transmit Parameters (1/4)
► Achieve
MAC synchronization.
 The SS achieves MAC synchronization once it
has received at least one DL-MAP message.
► Obtain
downlink parameters
 Retrieve parameters from the DCD messages.
 DCD messages contain:
►Frame
duration, TTG size, RTG size, downlink center
frequency, BS ID, and more.
►Downlink burst profiles.
45
Obtain Transmit Parameters (2/4)
► SS
want to know when BS broadcasts
channel parameters.
 SS can know it from DL-MAP.
 Note that DL-MAP is encoded with well-known
parameters.
► SS
want to know the downlink channel
parameters.
 SS can know it from Downlink Channel
Descriptor (DCD) Message.
46
Obtain Transmit Parameters (3/4)
► Obtain
uplink parameters
 Retrieve parameters from the UCD messages.
 UCD messages contain:
►Uplink
center frequency, bandwidth request
opportunity size, ranging request opportunity size,
and other PHY specific parameters.
►Uplink burst profiles.
► Receive
the UL-MAP
 So that SS can perform initial ranging.
 Initial ranging opportunities.
47
Obtain Transmit Parameters (4/4)
► Now
SS know downlink channel parameters,
then it want to know uplink channel
parameters.
 SS can know it from Uplink Channel Descriptor
(UCD) Message.
► The
next question is when SS can send
requests to perform following procedures.
 SS can know it from UL-MAP
48
Message Flows
BS
Send DL/UL-MAP
Send UCD/DCD
Wireless channel
DL/UL-MAP
Power on sequence complete
DL/UL-MAP
Send DL/UL-MAP
DL/UL-MAP
Send DL/UL-MAP
Send UCD
SS power on
UCD/DCD
Send DL/UL-MAP
Send DCD
SS
DCD
DL/UL-MAP
UCD
Establish PHY synchronization
Wait for UCD
Obtain parameters for UL channel
Send DL/UL-MAP
DL/UL-MAP
Extract slot info for uplink
Send DL/UL-MAP
DL/UL-MAP
Wait for transmission opportunity
to perform ranging
Start ranging process
49
Initial Ranging (1/3)
► What
is ranging?
 Ranging is the process of acquiring the correct
timing offset and power adjustments.
 RNG-REQ/RNG-RSP messages.
► Two
types of ranging
 Initial ranging: allow SS to join the network.
 Periodic ranging: allow SS to adjust
transmission parameters and maintain the
quality of RF communication link.
50
Initial Ranging (2/3)
► Initial
Ranging accomplishes the following:
 The time advance of SS transmissions is
adjusted to make the SS appear collocated with
the BS.
 The transmission power of the SS is adjusted
for optimal reception at the BS.
 The SS is allocated its Basic and Primary
Management CIDs.
51
Initial Ranging (3/3)
► If
two SS send there RNG-REQ in the same
slot (opportunity)
 Collision.
 Call for Contention Resolution Algorithm.
 Binary exponential backoff is specified in the
spec.
52
Basic Capability Negotiation
► SS
informs BS of its basic capabilities by
transmitting an SBC-REQ message with its
capabilities set to “on”.
► BS responds with an SBC-RSP message with
the intersection of SS’s and BS’s capabilities
set to “on”.
► Capabilities includes:
 Bandwidth allocation support, max. transmit
power, current transmit power, modulation type
support, and more.
53
Authorization and Key Exchange
► Perform
authorization and key exchange
procedures.
► Details are skipped.
54
Registration
► The
process by which SS is allowed entry
into the network.
► SS sends a REG-REQ message to BS.
► BS responds with a REG-RSP message.
► The SS is allocated its Secondary
management CID if the SS is managed.
► Also negotiate the version of IP and the QoS
parameters for the secondary management
connection.
55
Establish IP connectivity
► SS
and BS shall negotiate IP version during
REG-REQ/RSP exchange if the SS is
managed.
► After registration, SS shall invoke DHCP
mechanisms in order to obtain an IP
address and any other parameters needed
to establish IP connectivity.
56
Establish Time of Day
► That
the SS and BS have the current date
and time is required for time-stamping
logged events by the management system.
► The
protocol by which the time of day shall
be retrieved is defined in IETF RFC 868
(Time protocol).
57
Transfer Operation Parameters
► If
the SS has a configuration file, the name
is indicated in DHCP response.
► SS shall download the configuration file
using TFTP (Trivial File Transfer Protocol).
► SS notify the BS by transmitting a TFTPCPLT message when the file download has
completed successfully.
► BS responds a TFTP-RSP message.
58
Establish Provisioned Connections
► In
the case of a managed SS:
 The reception of the TFTP-CPLT message
triggers the BS to start connection setup.
► In
the case of a unmanaged SS:
 The successful completion of registration serves
as the trigger.
► Both
are BS-initiated.
► After at least one service flow has been
activated, the SS is capable of sending and
receiving user data.
59
Dynamic Service Establishment
► BS-initiated
Wireless channel
DSA-RSQ
SS
DSA-RSP
BS
DSA-ACK
60
Dynamic Service Establishment
► SS-initialed
DSA
 The standard does not go into details on what
actually triggers the DSA.
 Triggering is just assumed to happen,
stimulated by the upper layers when needed.
Wireless channel
DSA-RSP
This allows BS
to take it time
determining
whether to
admin the
service flow
DSA-ACK
61
DSA-RSQ
SS
DSX-RVD
BS
Bandwidth Request/Grand
Mechanism
The BW request/grand mechanism for the
IEEE 802.16 standard was chosen to be
efficient, low-latency, and flexible.
Requests
► The
mechanism that SS use to indicate to
the BS that they need uplink bandwidth
allocation.
► Requests are made on a per-connection
basis.
► Grants are made to the SS (Basic CID), not
to the connection.
► No explicit acknowledgments of requests.
63
Requests (1/2)
► Contention-based





bandwidth requests.
Transmit during contention period.
Broadcast polling.
Multicast group polling.
Focused contention transmission. (OFDM PHY)
CDMA-based bandwidth requests. (OFDMA PHY)
► Contention-free
bandwidth requests.
 Unicast polling.
 PM bit.
64
Requests (2/2)
► Bandwidth
stealing
 SS uses a portion of allocated BW for a
connection to send another BW requests rather
than sending data.
► Piggyback
Request
 The bandwidth request is piggybacked onto a
MAC PDU on an existing connection with
allocated BW.
65
Grants
► GPC
mode
 Grand Per Connection mode.
 Only optionally allowed in IEEE Std 802.16-2001.
 No longer specified in IEEE Std 802.16-2004.
► GPSS
mode
 Grand per Subscriber Station mode.
 Improves efficiency and latency. (smaller MAP)
 An addition scheduler is required to allocate the
granted bandwidth in each SS.
66
The Problems
► The
reality at the SS and the perception at
the BS can get out of sync.:
 BS does not hear a BW request.
 SS does not hear the allocation in the MAP.
 BS scheduler decides it does not have BW right
now for the particular service.
 SS used BW for a purpose different from that
originally requested. (e.g., bandwidth stealing)
67
Aggregate Requests
► BW
request/grant mechanism is designed to
be self-correcting.
► After a period, if the SS still needs BW for a
service, it simply asks again.
 SS issues an aggregate request.
 To avoid BS’s perception becoming further
askew from reality by duplicate requests.
 An aggregate request tells BS that the current
state of SS’s queue for that service, allowing BS
to reset its perception of that service’s needs.
68
Incremental Requests
► There
is a chance that a repeated aggregate
request crosses the grant for that same
bandwidth in the same frame.
 It can cause wasted allocations to the SS.
 It can be easily avoided by adding the concept
of incremental requests.
 BS just add this BW requests to it current
perception of the BW needs for that service.
69
Incremental or Aggregate?
► In
general, the airlink should be reliable.
► Therefore:
 Most BW requests typically would be
incremental.
 Only periodic aggregate requests to ensure BS
does not deviate too far from reality.
70
Other BW Request Options.
► SI
(Slip Indicator) bit.
 SS can set this bit, requesting BS to slightly
increase the rate at which it automatically
allocates BW to SS. (up to 1% additional BW)
► PM
(Poll Me) bit.
 SS can set this bit, indicating it has a BW need
on another connection.
 When BS sees the PM bit set, it knows the SS
needs to make a BW request and may poll it
immediately.
71
Usage Rules
Service
Type
Polling
Contention
Requests
UGS
PM bit
Not allowed
rtPS
Unicast Not allowed
PiggyBack
Requests
Bandwidth
Stealing
Not allowed Not allowed
Allowed
Allowed
nrtPS
All
Allowed
Allowed
Allowed
BE
All
Allowed
Allowed
Allowed
72
Summary
► MAC/PHY
Basics
 Frame structure
 Connections Types
 Header formats
► Network
Entry Procedures
 After completing the procedures, SS can
communicate with the network.
► Bandwidth
Request/Grand Mechanism
 Contention-based bandwidth requests
 Contention-free bandwidth requests
73
Summary
► More




about IEEE 802.16
QoS
Scheduling
Mesh mode
PHY details
74
References
[1] IEE Std 802.16-2004, IEEE Standard for Local and
Metropolitan Area Networks—Part 16: Air Interface for
Fixed Broadband Wireless Access.
[2] Carl Eklund et al., WirelessMAN: Inside the IEEE 802.16
Standard for Wireless Metropolitan Area Networks, IEEE
Press, 2006.
[3] http://www.ieee802.org/16/.
75
The End
Backup Materials
Duplexing
► Duplexing
defines how bidirectional
communication is achieved between two
devices or between a BS and a set of client
devices in a PMP system.
 Frequency Division Duplexing (FDD)
 Time Division Duplexing (TDD)
►Half-duplex:
transmit or receive but not both
simultaneously.
►Full-duplex: transmit and receive simultaneously.
78
Multiplexing
► Refers
to a mechanism in which a single
device transmits to multiple devices on a
single channel.
 Frequency Division Multiplexing (FDM)
►The
transmitting device divides the time domain into
multiple slots to communicate with multiple devices.
 Time Division Multiplexing (TDM)
►The
transmitting device uses different frequencies to
communicate with multiple devices.
 Orthogonal FDM (OFDM)?
79
Multiple Access
► Refers
to the way that multiple devices
access the medium, regardless of whether
the communication is many-to-one or manyto-many.




Time Division Multiple Access (TDMA)
Frequency Division Multiple Access (FDMA)
Orthogonal FDMA (OFDMA)
Code Division Multiple Access (CDMA)
80
Message Formats
上窮碧落下黃泉
Downlink Channel Descriptor
Message
► Define
the characteristics of a DL physical
channel.
82
DCD Channel Encoding (partial)
83
SC Downlink_Burst_Profile
84
DCD Burst Profile Encodings – SC
(partial)
85
DIUC Allocation – SC
86
Uplink Channel Descriptor Message
► Define
the characteristics of a UL physical
channel.
87
UCD Channel Encoding (partial)
88
SC Uplink_Burst_Profile
89
UCD Burst Profile Encodings – SC
(partial)
90
UIUC Allocation – SC
91
DL-MAP Message
92
SC DL-MAP IE
93
UL-MAP Message
94
SC UL-MAP IE
95
RNG-REQ and RNG-RSP
96
SBC/REQ and SBC-RSP
97
REG-REQ and REG-RSP
98
DSA-REQ and DSA-RSP
99
DSA-ACK
100
Service Flow Encodings
101
Grant Management Subheader
102
FDD or TDD?
Advantages of FDD Systems
► Continuous
UL and DL Transmissions.
 Reduce delay for MAC, ARQ, and channel
information feedback.
► Higher
Immunity to System Interference.
 Due to a large guard band.
 BS-to-BS and SS-to-SS (or MS-to-MS)
interference are generally negligible.
 Note that there are still interferences between
BS and MS.
104
Issues and Challenges of FDD Systems
► Feedback
Required for CSIT Acquisition.
 CSIT: Channel State Information at the
Transmitter.
 UL and DL channels are generally uncorrelated,
so the quality of CSIT will degrade.
► Inflexible
Traffic Allocation.
 Data traffic and Internet service have more
variation in traffic symmetry.
 It would be desirable if the system could
allocate bandwidth dynamically with regard to
traffic demand.
105
Issues and Challenges of FDD Systems
► Restrictive
Band Allocation.
 FDD systems require a pair of frequency
channels, it makes the FDD systems harder to
fit into the scarce resource of spectrum.
► Guard
Band.
 It represents a waste of resource.
► Higher
Hardware Cost.
 Requires a separate oscillator of different
frequency, an expensive duplexer, and a sharp
RF filter.
106
Advantages of TDD Systems
► Channel
Reciprocity.
 Channel state information at the receiver
provides CSIT.
 Better CSIT quality.
► Dynamic
Traffic Allocation/Traffic Asymmetry.
 Can distribute the bandwidth between UL and
DL easily by altering their subframe durations.
107
Advantages of TDD Systems
► Higher
Frequency Diversity.
 Diversity is a well-known technique to enhance
the system reliability in fading channels.
 DL and UL signals have wider bandwidth, which
corresponds to an increase in frequency diversity.
► Unpaired
Band Allocation.
 Only one single contiguous channel is needed.
► Lower
Hardware Cost.
 The sharing of a single oscillator and the
absence of a duplexer.
108
Issues and Challenges of TDD Systems
► Guard
Time between DL/UL Transitions.
 Reduces the efficiency of the system.
► Duplexing
Delay in MAC and ARQ.
 The traffic in both directions is discontinuous,
and there is a delay between consecutive UL/DL
subframes, called the duplexing delay.
► Outdated
CSIT.
 The estimated CSIT may be outdated due to
duplexing delay.
109
Issues and Challenges of TDD Systems
► Cross-Slot
Interference.
 This interference arises when neighboring TDD
BSs either have different traffic symmetries or
do not synchronize their frames.
 A major challenge in TDD systems.
► Interoperator
Interference.
 Different operators neither coordinate in
network planning nor synchronize their frames
and traffic asymmetry.
 Would cause strong adjacent channel
interference.
110
FDD or TDD?
► TDD
has received significant attention
because:
 Traffic asymmetry of high-bit-rate multimedia
application.
 The flexibility of unpaired spectrum.
► To
alleviate cross-slot interference, the
employment of sectored antennas and time
slot grouping are very effective.
111
FDD or TDD
► More
detailed discussion can be found in:
Petwer W. C. Chan et al., “The Evolution
Path of 4G Networks: FDD or TDD,” IEEE
Communications Magazine, vol. 44, issue 12,
Dec. 2006, pp. 42-50.
112
Review of the OFDM System
► OFDM
stands for Orthogonal Frequency
Division Multiplexing.
► It was proposed in mid-1960s and used in
several high-frequency military system.
► It is a multicarrier transmission technique.
 Divides the available spectrum into many
subcarriers, each one being modulated by a low
data rate stream.
113
Single carrier and Multicarrier
Transmission
► Single
carrier transmission
 Each user transmits and receives data stream
with only one carrier at any time.
► Multicarrier
transmission
 A user can employ a number of carriers to
transmit data simultaneously.
114
Single carrier and Multicarrier
Transmission
Single carrier transmission
Multicarrier transmission
cos(2 f1t )
s (t )
bi
cos(2 f 2t )
s (t )
bi
cos(2 fct )
∑
S/P
cos(2 f N t )
N oscillators are required
115
116
Service Classes
► UGS
(Unsolicited Grant Service)
► rtPS (Real-Time Polling Service)
► nrtPS (Non-Real-Time Polling Service)
► BE (Best Effort)
117