advandce of sofdma

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Copyright 2005 Intel Corporation. All rights reserved. Page 1 of 10

The Advantages of SOFDMA for WiMAX

Vladimir Bykovnikov

Intel Corporation

Abstract

SOFDMA has several advantages when used in NLOS wireless networks. The

paper outlines these advantages and shows the evolutionary path from OFDM

to SOFDMA.


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Table of Contents

Table of Contents................................................................................................................................2

1. Overview.................................................................................................................................2

2. Wireless Channel ....................................................................................................................2

3. OFDM.....................................................................................................................................33.1 Signal Structure...................................................................................................................3

3.2 Multipath Immunity............................................................................................................3

3.3 Diversity..............................................................................................................................43.4 Easy Support of Multiple Antennas....................................................................................4

4. OFDMA ..................................................................................................................................5

4.1 Subchannels and Multiple Access ......................................................................................54.2 Subcarrier Permutation Schemes ........................................................................................5

4.3 Sub-channelization Gain.....................................................................................................6

4.4 Efficient Scheduling............................................................................................................7

4.5 Frequency Reuse Factor of One..........................................................................................7

5. SOFDMA................................................................................................................................75.1 SOFDMA Profiles ..............................................................................................................7

5.2 Other Complementary Features of SOFDMA ....................................................................8

6. Summary.................................................................................................................................8

7. Acknowledgements.................................................................................................................9

8. References...............................................................................................................................9


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Fig. 2. Frequency- and time-selective fading

3. OFDM

3.1 Signal StructureOFDM stands for Orthogonal Frequency Division Multiplexing. An OFDM signal is comprised of a

number of orthogonal subcarriers (also referred to as tones). Each subcarrier can be modulatedindependently by BPSK, QPSK, 16QAM, or 64QAM modulation and carry different pieces of

information. All subcarriers within an allocation are modulated using the same modulation but the

modulation can be adapted for each subscriber according to the signal-to-noise ratio (SNR) at the receiver.Besides data subcarriers there are pilot subcarriers, DC subcarrier, and guard-band subcarriers. Pilot

subcarriers are used to measure channel characteristics in order to aid signal recovery at the receiver.

Guard-band subcarriers are nulled in order to satisfy spectrum mask requirements and to reduce

interference with adjacent channels. The structure of OFDM signals is illustrated in Fig. 3.

Fig. 3. Structure of OFDM signals (frequency domain)

OFDM signals can be easily constructed in the frequency domain and transformed to the time domain

with the relatively simple procedure. The term OFDM is frequently followed by the number that depictsthe potential number of subcarriers (including guard-band subcarriers) in the signal (e.g. OFDM-256).

3.2 Multipath ImmunityOne of the reasons for choosing OFDM for WiMAX is its natural immunity to multipath. This is the key

to success for NLOS wireless communication systems. As shown above, in NLOS channels, signals fromthe transmitter travel via multiple paths and arrive at the receiver with different delays. This leads to inter-

symbol interference (ISI) which is an issue for many traditional single-carrier broadband systems (higher

data rates in single-carrier systems translate to smaller symbol durations and more severe ISI). OFDMmakes the symbol duration significantly larger (e.g. 1024 times larger for OFDM-1024 than for single-

carrier systems with the same data rate). That greatly reduces the relative size of the delay spread

compared to symbol duration in time domain, and greatly reduces the effect of that ISI on communications.Additionally, OFDM encodes data bits using forward error correction (FEC) coding and distributes them


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across several subcarriers. If frequency-selective fading causes errors in the reception of a few subcarriers,

the data bits in those subcarriers are recovered using FEC. Finally, OFDM lends itself easily to the use of

diversity techniques to combat frequency-selective fading as described in the following section. In theseways, OFDM eliminates the need for the complex equalizers that are common in CDMA and TDMA

systems today.

3.3 DiversityIn wireless communication fading reduces link range and hence additional margin is required in the linkbudget to ensure proper signal recovery in the receiver. For example for SUI-3/omni channel [4] in orderto have a probability of fading less than 10 -3, a narrowband system sacrifices about 30 dB of link budget.

OFDM is a wide-band system that with the help of coding (forward error correction) and interleaving

provides significant fade-margin improvement (frequency diversity). The improvement depends onchannel conditions and bandwidth (e.g. for 2-MHz bandwidths in SUI-3/omni channels [4] the

improvement is about 15 dB). With natural frequency diversity of OFDM, even if some data is received in

error on the subcarriers with frequency-selective fading, the entire packet can often be reconstructed byusing the data from the rest of subcarriers and applying error correction.

OFDM also makes easy use of diversity techniques to provide further improvement in fade margin such asmultiple antennas (spatial diversity), repetition (time diversity), and space-time coding (STC) (a

combination of spatial and time diversity).

The combination of frequency, time, and space diversity methods results in significant fade marginimprovement.

3.4 Easy Support of Multiple AntennasThere is a theoretical limit (the Shannon limit) to the data rate of a channel that depends on the bandwidth

and signal-to-noise ratio (SNR) of that channel. Please note that the limit applies to a single channel.

However if the receiver and/or transmitter have more than one antenna and there is significant multipathin the communication channel, signals from/to antennas travel several different paths. Each combination

of transmit and receive antennas can be considered as a separate channel. In an environment with severemultipath, where the channels are independent, the throughput can be greatly enhanced in accordance with

the number of transmitter-receiver combinations in the system. Thus, the system throughput can

significantly exceed the theoretical limits for single-antenna systems! This concept is known as MIMO(Multiple Input Multiple Output).

Also multiple antennas can be used in an adaptive antenna system (AAS) to increase the link budget by

forming a beam on the transmit side in the direction of a subscriber (beam-forming) and/or by combiningsignals from multiple spatially-separated antennas in the receiver (MRCMaximum Ratio Combining, an

effective spatial-diversity combining method ).

Multiple antennas represent a great potential to increase spectral efficiency/link budget and it can be

implemented in two different waysin the time domain or in the frequency domain. Time-domain

implementation requires complex time/space processing and a lot of computational resources thatcurrently is hardly viable for portable/mobile devices. Frequency-domain implementation is much simpler

because it reduces computation to simple weighting of complex numbers. In contrast, CDMA systems

utilize time-domain processing that is a barrier for MIMO and AAS. OFDM systems naturally utilize


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frequency-domain processing, which greatly reduces the cost and unlocks the enormous potential for

MIMO and AAS.

4. OFDMA

4.1 Subchannels and Multiple AccessOFDMA (Orthogonal Frequency Division Multiple Access) is a multiple access method based on OFDMsignaling that allows simultaneous transmissions to and from several users along with the other

advantages of OFDM. While OFDM-256 is used in fixed WiMAX (based on IEEE 802.16-2004, [1]),

OFDMA mode has many advantages to be considered for nomadic/mobile usage and is used in IEEE802.16(e)

In OFDMA, subcarriers are assigned to subchannels that in turn can be allocated to different users. Thisprovides high-granularity bandwidth allocation as illustrated in Fig. 4.

Fig. 4. Two-dimensional scheduling in OFDMA

4.2 Subcarrier Permutation SchemesThere are two approaches to allocating subcarriers to subchannels:

1) distributed subcarrier allocation, and

2) adjacent subcarrier allocation.

The two approaches are shown on Fig. 5. In the distributed-subcarrier-allocation approach, a subchannel

uses different subcarriers randomly distributed across the channel bandwidth. The distributed-subcarrier

approach maximizes frequency diversity and averages inter-cell interference. This is the best approach formobile environment where channel characteristics change fast.

In the adjacent-subcarrier-allocation approach, a subchannel uses adjacent subcarriers which can be

adaptively selected by the scheduler (subcarriers with the highest signal-to-interference-plus-noise ratio(SINR) are chosen, and subcarriers in deep fades are avoided). This approach creates a loading gain,

and its easier to use with beam-forming AAS. The limitation of this approach is that it requires relativelystable conditions, where the characteristics of the channel change slowly (low-speed or nomadic usage).


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SINR

SIN

R

SINR

SINR

SIN

R

Fig. 5. Adjacent and distributed subcarrier allocation

OFDMA implements several permutation schemes to support both approaches. The partial usage ofsubcarriers (PUSC) and full usage of subcarriers (FUSC) permutation schemes defined in the standard

have the advantages of distributed subcarrier allocation and are well suited for mobile, fast-moving

subscribers. On the other hand, the adaptive modulation and coding (AMC) scheme uses adjacent

subcarriers that maximizes efficiency for stationary/nomadic users by creating loading gain and enablingbeam-forming AAS. The different schemes can be adaptively used for different subscribers within the

frame thereby maximizing overall system efficiency. Adjacent and distributed subcarrier allocation

schemes are compared in Table 1. PUSC and AMC are recommended in WiMAX.

Table 1: Comparison of adjacent and distributed subcarrier allocation schemes

Parameter Adjacent subcarrier allocation (AMC) Distributed subcarrier allocation (PUSC,

FUSC)

Gain Sub-channelization gain and loading gain Sub-channelization gain and benefits of

frequency diversity

Scheduling Requires advanced scheduler that allocates

subchannels according to channel characteristics

Simplified scheduler, doesnt use info about the

channel

Efficiency in

multipath channel

Almost no data lost Requires more redundancy (overhead) for

forward error correction

Channel Can be used in stationary channel Can be used in fast-changing channelAAS & MIMO Easier implementation More complicated implementation

Usage Fixed, portable, nomadic, pedestrian speed Mobile fast-moving subscribers

4.3 Sub-channelization GainSub-channelization gain is one of the unique OFDMA features that can significantly improve uplink linkbudgets. Imagine a situation where a subscriber uses 1 of 16 subchannels and concentrates all its

transmitter power in the subchannel. In this situation the power of each subcarrier in the subchannel will


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be increased by 10*log10(16) = 12 dB. This improvement in uplink budget can be used to increase uplink

link range, compensate for indoor penetration loss, or save power for the subscriber. It comes at almost no

cost in terms of system capacity, since the rest of the subchannels can be used by other subscribers.

Sub-channelization is a feature of OFDMA. It is also supported in the OFDM-256 PHY on the uplink

defined in IEEE-802.16-2004 [1]. This brings the advantage of sub-channelization gain to fixed WiMAX

systems and enables the use of indoor CPEs and nomadicity.

4.4 Efficient SchedulingWith OFDMA, packets can be scheduled across both frequency (subchannels) and time (symbols). It

provides an added dimension of flexibility resulting in higher granularity in resource allocation compared

to time-division multiplexing (TDM) systems (e.g. OFDM). More degrees of freedom in schedulingimprove fairness, quality of service (QoS), and bandwidth efficiency (an improvement of about 25-50%

can be achieved).

4.5 Frequency Reuse Factor of One

OFDM works well in the channels with relatively high SINR. In multi-cell deployments, in order to avoidinter-cell interference, basic OFDM requires directional antennas or relatively high frequency-reuse

factors and careful radio-frequency (RF) planning. OFDMA with its various subcarrier allocation schemes

(FUSC and PUSC) improves performance in multi-cell deployments by averaging the interference acrossmultiple cells. The interference becomes a function of cell loading and can be significantly reduced by

efficient scheduling. OFDMA systems, on the other hand, are very flexible in terms of RF planning and

support a variety of frequency reuse schemes.

Two of the most promising schemes are 1x3x1 and 1x3x3. Both schemes use three-sector base-stations

and require only one RF channel for all sectors and base-stations hence opening the door for operators

who have limited amount of spectrum. Frequency reuse 1x3x1 eliminates the need for any frequency

planning. That is a significant advantage especially for heavy urban areas where RF planning is verydifficult. Scheme 1x3x3 uses different (orthogonal) sets of tones (called segments) for each sector of a

base-station thereby reducing inter-cell interference and minimizing outage area. This scheme alsosimplifies RF planningone need only assign segments to sectors while using the same RF channel among

all base-stations.

5. SOFDMA

5.1 SOFDMA ProfilesWhen designing OFDMA wireless systems the optimal choice of the number of subcarriers per channel

bandwidth is a tradeoff between protection against multipath, Doppler shift, and design cost/complexity.Increasing the number of subcarriers leads to better immunity to the inter-symbol interference (ISI) caused

by multipath (due to longer symbols); on the other hand it increases the cost and complexity of the system

(it leads to higher requirements for signal processing power and power amplifiers with the capability ofhandling higher peak-to-average power ratios). Having more subcarriers also results in narrower

subcarrier spacing and therefore the system becomes more sensitive to Doppler shift and phase noise.

Calculations [7] show that the optimum tradeoff for mobile systems is achieved when subcarrier spacing is

about 11 kHz.


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Unlike many other OFDM-based systems such as IEEE 802.11a/g WLANs, the 802.16 standard [1]

supports variable bandwidth sizes for NLOS operations. In order to keep optimal subcarrier spacing, the

FFT size should scale with the bandwidth. This concept is introduced in Scalable OFDMA (SOFDMA)[2,7]. Possible SOFDMA profiles are shown in Table 1. Please note that in order to reduce system

complexity and facilitate interoperability the decision was made to limit the number of profiles for

WiMAX. At the present time only two FFT sizes, 512 and 1024, are recommended in WiMAX.

Table 1: OFDMA scalability parametersParameters Values

System bandwidth (MHz) 1.25 5 10 20

FFT size (NFFT) 128 512 1024 2048

Subcarrier frequency spacing 11.1607 kHz

Useful symbol time (Tb=1/ f) 89.6 s

Besides the fixed (optimal) subcarrier spacing, SOFDMA specifies that the number of subcarriers per

subchannel should be independent of bandwidth too. This results in the property that the number of

subchannels scales with FFT/bandwidth.

The basic principals of SOFDMA are clearly outlined in [7]:

Subcarrier spacing is independent of bandwidth. The number of subcarriers scales with bandwidth.

The smallest unit of bandwidth allocation, based on the concept of subchannels, is fixed andindependent of bandwidth and other modes of operation.

The number of subchannels scales with bandwidth and the capacity of each individual subchannel

remains constant.

5.2 Other Complementary Features of SOFDMAIn addition to variable FFT sizes, SOFDMA supports features such as Advanced Modulation and Coding(AMC), Hybrid Automatic Repeat Request (H-ARQ), high-efficiency uplink subchannel structures,

Multiple-Input-Multiple Output (MIMO) in DL and UL, as well as other OFDMA default features such as

a variety of subcarrier allocation and diversity schemes. Discussing these features is beyond of the scopeof the paper. An overview of these features can be found in [7] and detailed descriptions can be found in

the IEEE 802.16 standards [1-3].

6. Summary

There are many challenges in NLOS communication channels (e.g. delay spread, intersymbol interference,

time-selective fading, and frequency-selective fading caused by multipath propagation). OFDM is

extremely well suited to overcoming those challenges. OFDMA is a multiple-access method that allows

simultaneous transmissions to and from several users and provides several more advantages (e.g. sub-channelization gain, loading gain, more efficient scheduling, and a frequency reuse factor of one).

OFDMA is also very well suited for use with AAS and MIMO which can significantly improve

throughput, increase link range, and reduce interference. SOFDMA uses a subchannel structure thatscales with bandwidth, providing an optimal tradeoff between protection against multipath and Doppler

shift, and design cost/complexity. All of these advantages contribute to a spectral efficiency that is much

higher than competing systems, and the potential for even greater improvements in the future.


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7. Acknowledgements

This paper benefited greatly by helpful reviews and discussions with Fu-sheng Cheng, Kuo-Hui Li, GregDesBrisay, Jitendra Raichura, Andrew Tang, Shailender Timiri, Masud Kibria, Kranti Singh, and Hassan

Yaghoobi.

8. References[1] IEEE Std 802.16-2004 Part 16: Air Interface for Fixed Broadband Wireless Access Systems, IEEE

New York, 2004.

[2] IEEE P802.16e/D9 Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems:

Amendment for Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation

in Licensed Bands, IEEE, New York, June 2005.

[3] IEEE P802.16-2004/Cor1/D3 Corrigendum to IEEE Standard for Local and Metropolitan Area

NetworksPart 16: Air Interface for Fixed Broadband Wireless Access Systems, IEEE, New York, 2005.

[4] Channel Models for Fixed Wireless Applications, IEEE 802.16 Broadband Wireless Access Working

Group, http://ieee802.org/16 , IEEE, New York, 2003.

[5] Donald C. Cox., 910 MHz Urban Mobile Radio Propagation: Multipath Characteristics in New York

City, IEEE Transactions On Communications, vol. com-21, no. 11, November 1973.

[6] Morais, Douglas H., Fixed Broadband Wireless Communications: Principles and Practical

Applications, 1st

ed., Prentice Hall, January 2004.

[7] Hassan Yaghoobi, Scalable OFDMA Physical Layer in IEEE 802.16 Wireless MAN, Intel

Technology Journal, Volume 8, Issue 3, 2004.

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