Ofdma Basics

1. Study paper on Scalable OFDM Access (SOFDMA)


1.0 Overview

This paper provides an overview of Orthogonal Frequency Division Multiplexing
(OFDM), Orthogonal Frequency Division Multiplexing Access (OFDMA) and the
Scalable OFDMA (SOFDMA) and its advantages. It is assumed that readers already have
some background in wireless communication. SOFDMA is the air interface defined for
portable/mobile Wi-MAX systems by IEEE in IEEE 802.16e(2005) standard.

This paper starts with the description of the wireless communication channel and the
challenges faced in non-line-of-sight (NLOS) links. It then describes the features of
orthogonal frequency division multiplexing (OFDM), orthogonal frequency division
multiple access (OFDMA), and Scalable OFDMA (SOFDMA). Wireline and wireless, fixed and mobile communications or networking technologies have chosen OFDM
techniques to achieve higher data rate (which is what is needed in the broadband).
Examples of such technologies are: ADSL, Wi-Fi (802.11a/g), DAB, Wi-MAX. This
technology is also being considered for 802.11n, 802.20 and 4G. In this paper references
are made for the use of SOFDMA in Wi-MAX system defined in IEEE 802.16e (2005).

2.0 Wireless Channel Impairments

Wireless channel is always very unpredictable with challenging propagation situations. It
is very different from wireline channel. In an ideal radio channel, the received signal
would consist only a direct path signal which would be a reconstruction of the transmitted
signal. However, in a real radio channel, the signal would be modified during
transmission in the channel. The unique characteristic of wireless channels is the multipath. There are other serious impairments also present to the channel, namely
propagation path loss, shadow fading, Doppler spread, time dispersion or delay spread,
etc.

2.1 Multi path Propagation Effects

There may or may not be a direct line-of-sight path between the transmitter and the receiver and electromagnetic waves from the transmit antenna travel via several different
paths until they reach a receiver. The propagation through these multiple paths are
referred to as multi path propagation as shown in Figure-1.

2. Base
Station
Mobile
Station
NLOS
LOS
NLOS
Figure 1: Multipath Phenomenon LOS = Line of Sight
NLOS = Non Line of Sight


Multi path presents a challenge for any communication system and results in additional
complexity of system design. The length of each path is different and so the signals
coming to a receiver over each path experiences different time delays, resulting in “delay
spread”. The wireless channel is thus characterized by the delay spread which depends on
the terrain type, environment (e.g. urban, suburban, rural), and other factors.

2.2.1 Channel Fading

Frequency selective and Time Selective Fading.

In radio transmissions, the channel spectral response is not flat. In the frequency domain
large delay spreads translate into frequency-selective fading. Signals on some frequencies
arrive at the receiver in phase while signals at some other frequencies arrive out of phase.
This results in frequency selective fading as shown in Fig. 2. NLOS channels may also
vary in time significantly, due to moving transceivers in mobile communications. Also
time variation of NLOS channels is caused by other moving objects in the paths of signals. This results in time selective fading as shown in Fig. 2.

3. Figure 2 .Multi path Fading Channel.

2.2.2 Delay Spread

The received radio signal from a transmitter consists of typically a direct signal, plus
reflected signals. The reflected signals arrive at a later time than the direct signal because
of the extra path length, giving rise to a slightly different arrival time of the transmitted
pulse, thus spreading the received energy. Delay spread is the time spread between the
arrival of the first and last multi path signal seen by the receiver. In a digital system, the
delay spread can lead to Inter-Symbol Interference (ISI). This is due to the delayed multi
path signal overlapping with the following symbols. Fig.3 shows ISI due to delay spread
on the received signal. As the transmitted bit rate is increased the amount of inter symbol
interference also increases.


Figure 3. Explanation of Inter Symbol Interference

3.0 OFDM Basics

The nature of WLAN applications demand high data rates. The systems have to deal with
unpredictable wireless channels at high data rate. The channel distortions at high data rate
is significant and it is not possible to recover the data with a simple receiver. A Complex
receiver is needed to correctly estimate the channel to recover the originally transmitted

4. data. The idea of multiple carrier transmission is used for combating the hostility of such
channels. OFDM is a special form of multi carrier transmission. To understand how
OFDM, OFDMA and SOFDMA works, it is useful to start with FDM (Frequency
Division Multiplexing).

3.1 FDM (Frequency Division Multiplexing)

In FDM system, signals from multiple transmitters are transmitted simultaneously (at the
same time slot) over multiple frequencies. Each frequency range (sub-carrier) is
modulated separately by different data stream and a spacing (guard band) is placed
between sub-carriers to avoid signal overlap.


Figure 4 (a): FDM (Frequency Division Multiplexing)
……
Frequency












3.2 OFDM (Orthogonal Frequency Division Multiplexing )

OFDM is a multiplexing technique that divides the bandwidth into multiple frequency sub carriers. OFDM also uses multiple sub-carriers but the sub-carriers are closely spaced
to each other without causing interference, removing guard bands between adjacent sub- carriers. Here all the sub carriers are orthogonal to each other. Two periodic signals are
orthogonal when the integral of their product, over one period, is equal to zero. The use
of OFDM results in bandwidth saving as seen in the figure 3.


Frequency
……
Figure 4 (b): OFDM (Orthogonal Frequency Division Multiplexing)

5. Saved bandwidth
Frequency
Frequency
OFDM Spectrum
FDMA Spectrum
Figure 4 (c) : Bandwidth comparison of OFDMA and FDMA


Figure 5 below gives an example of orthogonal sub carriers in OFDM system.


















Carrier Frequency (Hz)
Power (W)
Figure 5: Example of orthogonal sub carriers in OFDM system.
A B C D E

3.3 OFDM Principal

A block diagram of an OFDM Transmitter is as shown in figure 6. In this system, a very
high rate data stream is divided into multiple parallel low rate data streams ( this
increases symbol duration).

6. CarrierFEC
Pre-FEC
Transmitted
signal
Received
signal
Channel De-Orthogonalization
Demodulatio
QAM
Modulation
OFDMA
Symbol
Mapping
Sub-carrier
allocation

IFFT
Radio
Channel
Fi
gure 6: Block diagram of a OFDM modulator.
Channel Orthogonalization

Each smaller data stream is then mapped to individual data sub-carrier and modulated
using some sorts of PSK (Phase Shift Keying) or QAM (Quadrature Amplitude
Modulation). i.e. BPSK, QPSK, 16-QAM, 64-QAM. The term “OFDM” is frequently
followed by the number that depicts the potential number of sub carriers (including
guard-band sub carriers) in the signal (e.g. OFDM-256).

The use of OFDM results in higher spectral efficiency. Besides this an OFDM system
such as Wi-MAX is more resilient in NLOS environment. It can efficiently overcome
interference and frequency-selective fading caused by multi path because equalizing is done on a subset of sub-carriers instead of a single broader carrier. The effect of ISI (Inter
Symbol Interference) is suppressed by virtue of a longer symbol period of the parallel
OFDM sub-carriers than a single carrier system and the use of cyclic prefix (CP)-
explained later.

3.4 OFDMA (Orthogonal Frequency Division Multiple Access).

OFDM in its primary form is considered as a digital modulation technique and not a
multi user channel access technique. It is utilized for transferring one bit stream over one communication channel using one sequence of OFDM symbols. However, OFDM can be
combined with multiple accesses using time, frequency or coding separation of the users.
OFDMA employs multiple closely spaced sub-carriers. The sub-carriers are divided into
groups of sub-carriers. Each group is named a sub-channel. The sub-carriers that form a
sub-channel need not be adjacent








Frequency
……
Figure 7: Orthogonal Frequency Division Multiplexing Access
Sub-carriers with the same color represent a sub-channel.

7. OFDMA provides multiplexing operation of data streams from multiple users onto the
downlink sub-channels and uplink multiple access by means of uplink sub-channels.

The multiple access is achieved by assigning different OFDM sub - channels to different
users. In the downlink, a sub-channel may be intended for different receivers. In the
uplink, a transmitter may be assigned one or more sub-channels.

3.4.1. The Cyclic Prefix

The increased symbol duration in OFDMA improves the delay spread while the Inter
Symbol Interference (ISI) is completely eliminated by introduction of a Cyclic Prefix
(some data). CP is a repetition of the last samples of the data portion that is appended at
the beginning of the data payload. The ISI is completely eliminated as long as the CP
duration is longer than the channel delay spread.

A drawback of the CP is that it introduces overhead, which effectively reduces bandwidth
efficiency. Since OFDM signal power spectrum has a sharp fall off at the edge of
channel, a larger fraction of the allocated channel bandwidth can be utilized for data
transmission which compensates the loss in efficiency due to the cyclic prefix. The
concept of CP in OFDMA is explained in figure 8.


Ts
Tu
Tg
Cyclic
Prefix
Data Payload
Figure 8: Cyclic Prefix
T
g
Useful Symbol
Period
Total Symbol
Period

















3.4.2. OFDMA symbol Structure and sub channelization.

Both OFDM and OFDMA symbols are structured in similar way. In OFDMA each
symbol consists of sub-channels that carry data sub-carriers carrying information, pilot
sub-carriers as reference frequencies and for various estimation purposes, DC sub-
carrier as the center frequency, and guard sub-carriers or guard bands for keeping the
space between OFDMA signals as shown in figure 9..

8. …
Channel
Figure 9: Structure of sub-carriers in OFDMA
Data subcarriers
… ……
Pilot Subcarriers
DC Subcarriers
Guard
subcarriers

3.4.3. Sub-channelization

Active (data and pilot) sub-carriers are grouped into subsets of sub-carriers called sub-
channels Sub-channelization defines sub-channels that can be allocated to subscriber
stations (SSs) depending on their channel conditions and data requirements. Using sub-
channelization, within the same time slot, a BS can allocate more transmit power to SSs
with lower SNR and less power to user devices with higher SNR. Sub-channelization
also enables the BS to allocate higher power to sub-channels assigned to indoor SSs
resulting in better in-building coverage. Sub-channelization in the uplink can save a user
device transmit power because it can concentrate power only on certain sub-channel(s)
allocated to it. This power-saving feature is particularly useful for battery-powered user
devices. The concept of sub-channelization is explained in figure 10.


Figure 10 : Sub-Channelization in OFDM and OFDMA
Sub- carriers
Time
OFDM
Time
Sub- channe
OFDMA

9. 4.0 Scalable OFDMA (SOFDMA)

Scalable OFDMA is the OFDMA mode is used in Mobile Wi-MAX defined in IEEE
802.16e. Scalability is supported by adjusting the size of FFT size while fixing the sub-
carrier frequency spacing in 10.94 kHz. It supports channel bandwidths ranging from
1.25 MHz to 20 MHz. SOFDMA adds scalability to OFDMA. With bandwidth
scalability, Mobile Wi-MAX technology can comply with various frequency regulations
worldwide.

4.1 Basic Principals

In SOFDMA,
• Sub-carrier spacing is independent of bandwidth.
• The number of sub-carriers scales with bandwidth.
• The smallest unit of bandwidth allocation, based on the concept of sub-channels,
is fixed and independent of bandwidth and other modes of operation.
• The number of sub-channels scales with bandwidth and the capacity of each
individual sub-channel remain constant.


4.2 OFDMA Scalable parameters

Smaller FFT size is given to lower bandwidth channels, while larger FFT size to wider
channels. By making the sub-carrier frequency spacing constant, SOFDMA reduces
system complexity of smaller channels and improves performance of wider channels. In
order to keep optimal sub-carrier spacing, the FFT size should scale with the bandwidth.
This concept is introduced in Scalable OFDMA (SOFDMA).This results in the property
that the number of sub-channels scales with FFT/bandwidth. Various scalable parameters
in SOFDMA along with the fixed parameters are given in the table below.

PARAMETERS

VALUES
System Channel Bandwidth (MHz) 1.25 5 10 20
Sampling frequency Fp in MHz 1.4 5.6 11.2 22.4
FFT Size (Nfft) 128 512 1024 2048
Number of Sub-Channels 2 8 16 32
Sub-Carrier Frequency Spacing 10.94KHz
Useful symbol Time ( Tb=1/f) 91.4 microsecond
Guard Time (Tg=Tb/8) 11.4 microsecond
OFDMA symbol duration (Ts=Tb+Tg) 102.9 microsecond
Number of OFDMA Symbols (5ms Frame) 48


In addition to variable FFT sizes, SOFDMA supports features such as Advanced
Modulation and Coding (AMC), Hybrid Automatic Repeat Request (H-ARQ), high-
efficiency uplink sub-channel structures, Multiple-Input-Multiple Output (MIMO) in DL

10. and UL, as well as other OFDMA default features such as a variety of sub-carriers
allocation and diversity schemes. Discussing these features is beyond of the scope of the
paper.

4.3 SOFDMA frame structure

There are two types of frame structure, FDD and TDD. TDD has many advantages over
the FDD. The figure below explains the OFDM frame structure for a TDD
implementation.




















FCH
UL
MAP
(conl)
DL
MAP
UL
MAP
DL Burs#7
DL Burs#6
DL Burs#5
DL Burs#3
DL Burs#1 DL Burs#4
DL Burs#2
Coded symbol write order
OFDM Symbol Number
0 1 3 5 7 9 … … N-1
Sub-
channel
Logical
Number
Preamble
DownlinkSubframe
Guard
Burst 1
Burst 2
Burst 3
Burst 4
Burst 5
Ranging
ACK-
CH
Fast feedback (CQICH)

0 … … … M-1
1





s-1
s
s+1








N
UplinkSubframe

Figure 11: OFDMA frame Structure


TDD enables adjustment of the DL/UL ratio to effectively support the asymmetric
DL/UL traffic, while FDD DL/UL always have fixed and equal DL and UL bandwidths.
Recommended number of DL/UL OFDM symbols can flexibly realize a range of
asymmetric DL/UL traffic ratio. For a 10MHz Bandwidth, number of OFDM symbols in
UL and DL are as given in the table below.


Description

Base Station Values
Number of OFDM symbols in DL/UL for a 10MHz Bandwidth

(35:12),(34:13),(33:14),(32:15),(31:16),
(30:17), (29:18), (28:19), (27:20), (26:21),

11. 5.0 Advantages and Disadvantages of SOFDMA System

5.1 Advantages

5.1.1. Combating ISI and Reducing ICI

When signal passes through a time-dispersive channel, the orthogonality of the signal can
be lost. CP helps to maintain orthogonality between the sub carriers. Initially guard
interval-empty space between two OFDM symbols served as a buffer for the multi path
reflection. But the empty guard time introduces Inter Carrier Interference (ICI) that is
crosstalk between different sub carriers. A better solution is cyclic extension of OFDM
symbol or CP. It ensures that the delayed replicas of the OFDM symbols will always
have a complete symbol within the FFT interval (often referred as FFT window).

At the receiver side, CP is removed before any processing starts. As long as the length of
CP interval is larger than maximum expected delay spread, all reflections of previous
symbols are removed and orthogonality is restored.

5.1.2. Spectral Efficiency

In the case of OFDM, a better spectral efficiency is achieved by maintaining
orthogonality between the sub-carriers.

5.1.3. Some Other Benefits of OFDM System

- Low cost due to its simplicity.
- It is possible to significantly enhance the capacity by adapting the data rate per
sub-carrier according to SNR of that particular sub-carrier.
- OFDM is more resistant to frequency selective fading than single carrier systems.
- OFDM can be used for high-speed multimedia applications with lower service
cost.
- Smart antennas can be integrated with OFDM. MIMO systems and space-time
coding can be realized on OFDM and all the benefits of MIMO systems can be
obtained easily. Adaptive modulation and tone/power allocation are also
realizable on OFDM.


5.2 Disadvantage of SOFDMA system


5.2.1. Strict Synchronization Requirement

OFDMA is highly sensitive to time and frequency synchronization errors. Demodulation
of an OFDM signal with an offset in the frequency can lead to a high bit error rate.

12. 5.2.2 Peak-to-Average Power Ratio (PAPR)

Peak to Average Power Ratio (PAPR) is proportional to the number of sub-carriers used
for OFDM systems. An OFDM system with large number of sub-carriers will thus have a
very large PAPR when the sub-carriers add up coherently. Large PAPR of a system
makes the implementation of Digital-to-Analog Converter (DAC) and Analog-to-Digital
Converter (ADC) to be extremely difficult. The design of RF amplifier also becomes
increasingly difficult as the PAPR increases.

6.0 Conclusion

It may be concluded that OFDMA is a very efficient technique for broadband data
transmission over radio frequencies. It can be implemented digitally with simplicity and
at low cost. Therefore, it is being adopted in almost all the new wireless technologies.


7.0 References

1) http//www.wimaxforum.org/

2) IEEE 802.16e standards

3) Various internet sites on Wi-Max


8.0 ABBREVIATIONS & ACRONYMS

AAS Adaptive Antenna System
ADSL Asymmetric Digital Subscribers Line
AMC Adaptive Modulation and coding
CDMA Code Division Multiple Access

CP Cyclic Prefix
DAB Digital Audio Broadcasting
DL Downlink
BPSK Bipolar Phase Shift Keying
DPSK Differential Phase Shift Keying
FEC Forward Error Correction

13. FDM Frequency Division Multiplexing
FFT Fast Fourier Transform
FDD Frequency Division Duplexing
H-ARQ Hybrid Automatic Repeat Request
ICI Inter Carrier Interference
IEEE Institute of Electrical and Electronics Engineers
IFFT Inverse Fast Fourier Transform
ISI Inter Symbol Interference
LOS Line-of-Sight
MIMO Multiple Input- Multiple Output
NLOS Non-Line-of-Sight
OFDM Orthogonal Frequenc y Division Multiplexing
OFDMA Orthogonal Frequency Division Multiple Access
PSK Phase Shift Keying
PAPR Peak to Average Power Ratio
QAM Quadrature Amplitude Modulation
QPSK Quadrature Phase-Shift Keying
SNR Signal-to-Noise Ratio
S-OFDMA Scalable Orthogonal Frequency Division Multiple Access

SS Subscriber Station
TDD Time Division Duplex
UL Uplink
Wi-MAX Worldwide Interoperability Microwave access
WLAN Wireless Local Area Network