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7/23/2019 Runcom OFDMA Tutorial.pdf http://slidepdf.com/reader/full/runcom-ofdma-tutorialpdf 1/12  OFDMA Tutorial - Theory, principles, design considerations and applications Yigal Leiba –Runcom Technologies, Ltd., Rishon-Lezion, Israel ABSTRACT OFDMA is a new modulation method for the 2 nd  generation of Broadband Wireless Access (BWA) systems that combines both upstream access and modulation together. OFDMA is based on modulating multiple orthogonal sub-carriers. Unlike traditional multi-carrier modulation schemes, several transmitters of the multiple access system modulate the sub-carriers simultaneously. OFDMA offers up to an 18dB gain in the upstream link budget and up to a 12dB gain in the downstream link budget relative to traditional multiple access technologies. These gains are complemented by the capability to operate in severe non-line-of-sight conditions. Armed with these advantages, OFDMA enables deployment of integrated indoor BWA subscriber units (SU). This article discusses the principals and design considerations for the downlink and uplink operation of OFDMA based systems. The topics discussed are: SU synchronization, media access control (MAC) layer aspects and handling of real-world  problems such as phase noise and power-amplifier (PA) backoff. OFDM basics OFDM motivation and history During the past few years data rates demanded from fixed wireless networks increased from approximately 10Kbps to 10Mbps and beyond. This increased data rate demand focused interest on modulation techniques that operate efficiently over broadband channels. Operation over broadband channels implies ability to operate even in fading channels in which significant multipath is present. Figure 1 illustrates a simulated delay profile (channel model according to [1], [2]) and frequency response of a 6MHz MMDS channel with  S µ 2 RMS delay spread, using omni-directional antennas. When using Single Carrier (SC) modulation over a multipath channel, channel delay spread may be longer than the symbol duration. This situation introduces Inter-Symbol Interference (ISI) at the receiver. In order to demodulate the data, a SC system would have to employ an equalizer. The equalizer functions as a filter with a frequency response that is close to the inverse of the channel frequency response. Filtering the received signal with the equalizer reduces ISI. The number of taps required from the equalizer may be as large as twice the delay spread introduced by the channel (measured in symbol durations).

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OFDMA Tutorial - Theory, principles, design considerations andapplications

Yigal Leiba –Runcom Technologies, Ltd., Rishon-Lezion, Israel

ABSTRACT

OFDMA is a new modulation method for the 2nd

 generation of Broadband WirelessAccess (BWA) systems that combines both upstream access and modulation together.

OFDMA is based on modulating multiple orthogonal sub-carriers. Unlike traditional

multi-carrier modulation schemes, several transmitters of the multiple access system

modulate the sub-carriers simultaneously.

OFDMA offers up to an 18dB gain in the upstream link budget and up to a 12dB gain in

the downstream link budget relative to traditional multiple access technologies. Thesegains are complemented by the capability to operate in severe non-line-of-sightconditions. Armed with these advantages, OFDMA enables deployment of integrated

indoor BWA subscriber units (SU).

This article discusses the principals and design considerations for the downlink and

uplink operation of OFDMA based systems. The topics discussed are: SU

synchronization, media access control (MAC) layer aspects and handling of real-world

 problems such as phase noise and power-amplifier (PA) backoff.

OFDM basics

OFDM motivation and historyDuring the past few years data rates demanded from fixed wireless networks increased

from approximately 10Kbps to 10Mbps and beyond. This increased data rate demand

focused interest on modulation techniques that operate efficiently over broadbandchannels.

Operation over broadband channels implies ability to operate even in fading channels in

which significant multipath is present. Figure 1 illustrates a simulated delay profile

(channel model according to [1], [2]) and frequency response of a 6MHz MMDS channel

with   S µ 2 RMS delay spread, using omni-directional antennas. When using Single Carrier

(SC) modulation over a multipath channel, channel delay spread may be longer than the

symbol duration. This situation introduces Inter-Symbol Interference (ISI) at the receiver.

In order to demodulate the data, a SC system would have to employ an equalizer. The

equalizer functions as a filter with a frequency response that is close to the inverse of the

channel frequency response. Filtering the received signal with the equalizer reduces ISI.

The number of taps required from the equalizer may be as large as twice the delay spreadintroduced by the channel (measured in symbol durations).

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The computational load associated with the equalizer is proportional to the square of the

number of its taps. For the simulated 6MHz channel shown in Figure 1 and with a symbol

rate of 5MSymbol/S, about 200 taps are required to equalize the channel for 64-QAM

modulation.

0 5 1 0 1 5 2 0-5 0

-4 5

-4 0

-3 5

-3 0

-2 5

-2 0

-1 5

-1 0

-5

0

T i m e [ m i c r o s e c o n d s ]

   D  e   l  a  y  p  r  o   f   i   l  e   [   d   B   ]

-3 -2 -1 0 1 2 3-3 0

-2 5

-2 0

-1 5

-1 0

-5

0

F r e q u e n c y [ M H z ]

   C   h  a  n  n  e   l   t  r  a  n  s  m   i  s  s   i  o  n   f  u  n  c   t   i  o  n   [   d   B   ]

 

Figure 1. Simulated delay-spread and channel frequency response for a 6MHz MMDS channel

Multi-carrier modulation techniques, specifically Orthogonal Frequency Division

Multiplexing (OFDM), are capable of operating with severe multipath, and can avoid ISI problems associated with SC modulation techniques.

OFDM modulation chain

OFDM overcomes the ISI problem by modulating multiple narrow-band sub-carriers in

 parallel. Since each sub-carrier occupies a narrow bandwidth, it experiences a non-

frequency selective channel and consequently no ISI. Not all the sub-carriers carry data.

Some of the sub-carriers are modulated with a constant pattern known to both transmitterand receiver. These carriers are called ‘pilot-carriers’ and are used in the demodulation

 process. Other sub-carriers at the edges of the frequency band are not modulated, and

serve as a guard band. The guard band ensures that spectral density masks for out of band

emissions is met. The modulated sub-carriers are multiplexed using an Inverse FourierTransform (IFFT). The resulting time-domain waveform is padded with a cyclic prefix

intended to contain the multipath effects at the receiver. The size of the cyclic prefixtypically can be selected to be41 ,

81 ,

161  or

321  of the OFDM symbol. For a 6MHz MMDS

channel and 2048 sub-carriers FFT this allows handling peak channel delay spread up to

S µ 74 .

In SC modulation every symbol is transmitted using the entire channel bandwidth, and all

received symbols have the same signal to noise ratio (SNR) that is the average channel

SNR. In OFDM modulation different sub-carriers are expected to undergo differentattenuation due to the frequency selectivity of the channel. As a result, the more severely

attenuated sub-carriers carry information less reliably. This phenomenon can result in an

irreducible bit error rate (BER) even when the average channel SNR is high. To

overcome this problem forward error correction (FEC) and interleaving are applied to the

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transmitted information prior to modulation. Figure 2 depicts a typical OFDM transmit

 processing chain.

Scrambler FEC Interleaver    Constellation

Mapper 

Frame

generation

Pilotinsertion

IFFT  Cyclic prefix

insertion

Input

bits

OFDM

waveform

Figure 2: Typical OFDM transmitter chain 

OFDM vs. OFDMA

What is OFDMA?

OFDMA is a combination of modulation scheme that resembles OFDM and a multipleaccess scheme that combines TDMA and FDMA. OFDMA typically uses a FFT size

much higher than OFDM, and divides the available sub-carriers into logical groups calledsub-channels. Unlike OFDM that transmits the same amount of energy in each sub-

carrier, OFDMA may transmit different amounts of energy in each sub-channel.

To understand the OFDMA concept we can look at an example, specifically the OFDMA

specification in IEEE 802.16a draft standard ([3]). In this standard two OFDMA schemes

can be used. The mandatory OFDMA scheme is based on 2048 sub-carriers and there is

also on optional scheme based on 4096 sub-carriers. For the 2048 sub-carriers scheme, inthe uplink there are 1696 used sub-carriers. The rest of the sub-carriers are use as a guard

 band to guarantee the OFDM brick-wall spectral mask. The used sub-carriers are divided

in 32 sub-channels, each containing 53 sub-carriers. SU are allocated one or more ofthese sub-channels for each transmission burst. Multiple SU can access the channel

simultaneously by transmitting on different sub-channels.

In the downlink, the transmission power allocated for a sub-channel can be boosted by

6dB or attenuated by 6dB relative to the nominal transmission power. The transmission

 power of all the sub-channels has to be conserved, therefore boosting one sub-channelimplies attenuating another.Figure 3 illustrates how multiple SU transmit simultaneously on different sub-channels in

an OFDMA uplink.

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GI

OFDMA Symbol #1

GI

OFDMA Symbol #2

GI

OFDMA Symbol #3

GI

OFDMA Symbol #4

   F  r  e  q  u  e  n  c  y

   (  s  u   b  -  c  a  r  r   i  e  r   )

Time

(OFDMA symbol)

SS #1SS #2

SS #3SS #4SS #5SS #6

Physical sub-carrier allocation

Logical sub-channel allocation

SS #1 SS #1 SS #1 SS #1

SS #5 SS #5 SS #5 SS #2

SS #2 SS #2

SS #3 SS #3

SS #4 SS #4

SS #6 SS #6

Group #1

Group #2

Group #NG

 Figure 3: OFDMA uplink - time/frequency view

OFDMA gain concentration

OFDMA has some features that make it ideal for BWA systems, and superior to other

modulation and multiple access schemes. One such feature is power-concentration. Since

a SU is typically allocated less than all the available sub-channels for a transmission

 burst, it concentrates its output power on part of the channel bandwidth. This power-

concentration is similar to what happens when using traditional Frequency Division

Multiplexing (FDM).For the 32 sub-channels example mentioned before, 32 SU may be allocated one sub-

channel each simultaneously. Each SU transmits only on those sub-carriers belonging toits allocated sub-channel. The BST receiver receives a signal to noise ratio (S/N) that is

 better then the equivalent OFDM case, in which only one SU transmits during a burst, by

a factor of 32. This concentration effect translates to a 15dB improvement in S/N without

sacrificing capacity, or impacting the average data rate provided by each SU.

Power-concentration is not limited to the uplink. In the downlink, although there is a

single transmitter, power can be concentrated in certain sub-channels at the expense of

diluting other sub-channels. Transmissions for SU suffering for high RF path loss can bedone on sub-channels in which the transmission power is boosted. The current IEEE

802.16a draft standard ([3]) allows ±6dB of power change for each downlink sub-

channel.

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 Adaptive modulation and FEC

OFDMA lends itself to adaptive modulation and FEC, as each sub-channel may carry its

own modulation and FEC scheme. The modulation and FEC are selected according to theRF path loss associated with each SU, and thus allow efficient use of the availablechannel bandwidth.

In IEEE 802.16a draft standard ([3]) for example, QPSK, 16-QAM and 64-QAM

modulations are allowed. The FEC scheme is concatenated Reed Solomon (RS) and

convolutional code, with optional turbo product codes.

OFDMA and space-time coding

Space-time diversity techniques are easily combined with OFDMA. The combining can

 be done in the frequency domain, according to the S/N ratio available on each sub-carrier.

Figure 4 illustrates the S/N improvement that can be gained by space-time diversity

techniques. The solid line is the result of maximal ratio combining of combining the twodotted lines, representing signals received from two independent reception paths.

Analysis and measurements in cellular systems ([4]) shows that a separation as small as

10λ (120cm at 2.5GHz) is sufficient to bring significant diversity gain.

-3 -2 -1 0 1 2 3-3 0

-2 5

-2 0

-1 5

-1 0

-5

0

Frequency [MHz]

   N  o  r  m  a   l   i  z  e

   d   S   /   N

   [   d   B   ]

 

Figure 4: Maximal ratio combining of two channel multipath profiles

OFDM/OFDMA PHY implementation

Coarse timing synchronization

Coarse timing synchronization is typically the first step in the demodulation process. Atthis stage parameters such as carrier frequency offset, and the channel transmission

function are unknown. OFDM does not require a training sequence in order to

synchronize to a continuous OFDM symbol stream. Synchronization is based on the

inherent redundancy in the OFDM symbol that is created by the presence of the cyclic

 prefix. This redundancy, and the knowledge of the expected FFT size enables correlatingthe received signal with itself and detecting the correlation peaks (see [5]). When large

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offsets between the sampling clocks of the transmitter and receiver may exist, an

additional scanning mechanism can be added to select the best correlation.

Carrier frequency offset estimation

Carrier frequency offset estimation can preformed in two stages ([5]). One stage is a pre-

FFT algorithm that processes the raw sampled signal, and can determine frequency

offsets that are smaller than the FFT carrier spacing. The second stage is a post-FFTalgorithm based on searching a known-in-advance sequence in the FFT bins. This

sequence is carried by some of the sub-carriers, called ‘fixed-pilots’. Once the fixed-pilot

sequence is identified, the remaining frequency offset that is an integer multiple of FFT

carrier spacing can be determined.

Channel estimation

In the downlink, channel estimation takes advantage of another sort of pilot-carriers

called ‘moving-pilots’. These sub-carriers, whose location changes every OFDMAsymbol according to a fixed pattern, are modulated with a known in advance sequence.

The receiver locates the moving-pilots and can estimate the channel at the frequency ofeach pilot by comparing the expected pilot the value to the measured value. Extrapolating

the data obtained from the moving-pilots enables the channel estimation and elimination

of any phase noise errors that are common to all the sub-carriers.

In the uplink, channel estimation is based on a preamble, as well as on the moving-pilots.

The preamble duration is one OFDMA symbol.

Fine timing synchronization

Fine timing estimation can be done according to the estimated channel impulse response(CIR), where the objective is to sample the OFDM symbol relative to the guard interval,such that most of the transmitted energy, scattered by the channel delay spread, is

captured.

Demodulation and FEC decoding

Demodulation of the information carried by each OFDM sub-carrier is straightforward.

Each sub-carrier is multiplied by the inverse of the estimated CIR at its frequency, and

the result is sent to the FEC decoding engine for extraction of the bits. The FEC decoder

operates on each OFDMA sub-channel separately, thus allowing different FEC schemeon each sub-channel. Each sub-channel should contain enough sub-carriers, such that

sub-carriers with good SNR compensate for those with degraded SNR (due to multipath).

Many FEC schemes can be used with OFDMA, typically a concatenation of Reed-Solomon and block convolutional code, or block-turbo-codes are used ([3], [6]).

Multiple access issues

In the uplink direction, OFDMA differs from most traditional modulation schemes by the

fact that multiple transmitters must be synchronized in time and frequency in order to

receive a valid OFDMA symbol at the BST. SU synchronization is therefore a critical

component of the OFDMA modulation scheme, and it is based on the SU tracking theBST carrier frequency and BST symbol timing. The SU corrects its local timing and

frequency references as part of the downstream demodulation process. The corrected

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references are used in the uplink transmission process, ensuring that all SU are

synchronized to the BST. The SU must also compensate for RF propagation delays, this

is done by a ranging algorithm in which the BST instructs the SU how much time it has

to advance its transmission in order to get to the BST at the right time.

OFDMA hurdles

In spite of its many advantages, OFDM modulation has been considered for a long time

costly to implement from an RF point of view. Two main reasons for this are phase noise

requirements and power amplifier (PA) linearity requirements.

Phase noise

Any oscillator used in an RF chain contains spectral components at frequencies other

than its intended oscillation frequency. These spectral components are generally referred

to as ‘phase noise’. A typical VCO embedded in a phase locked feedback loop (PLL) willexhibit a noise spectrum that is flat up to the loop bandwidth, and decreases at

20dB/decade above the loop bandwidth ([7]). The one-sided phase noise curve can be

approximated by the formula,

( )2

0

1     

  +

=

 B f  

 P  f   P   

Where B is the loop bandwidth, and 0 P  determines how good is the VCO.

To see how phase noise affects an OFDM receiver, we can look at a simple case where

the channel is not frequency selective. We can use the following formula ([8]),

( ) ( ) ( )∑ ∑∑−

=

=

=  

−⋅+⋅+≈

1

0

1

0

1

0

2exp N 

k r r 

 N 

m

 N 

m

k k k    mk r  N 

 jm s N  jm s

 N  j s y   π φ φ   

Where N is the number of OFDMA bins, k  y is the received symbol in bin k, k  s is the

transmitted symbol in bin k, and ( ){ }m j   φ ⋅exp  is the local oscillator (LO) impaired by

 phase noise. What can be understood from this formula is that the phase noise is

composed from two components, one is a component that is common to all sub-carriers

 bins within a OFDMA symbol, and another that is not common an creates interference

and loss of orthogonality between the sub-carriers. The common component can be

compensated for, as it is common to the known pilot-carriers.

 Now we can attempt to derive the required phase noise performance from the LO ([9]).For a typical 6MHZ MMDS channel, the OFDMA symbol duration specified in IEEE

802.16a draft standard ([3]) is S µ 32298 , meaning that only noise components above

3.5KHz have to be considered when calculating the phase noise. Integrating overequation (1) results in

( )∫ ∞

 

  

  

  

 −⋅⋅=⋅=

0

00 arctan

2 f  

 PN  B

 f   B P df   f   P  N 

  π  

 

If we choose for instance B=1KHz, we can draw a limiting curves for the LO phase noise

 performance based on a minimum C/N due to phase noise alone, that is 6dB below the

minimum C/N threshold (see [6], Annex A). The 6dB below the minimum C/N threshold

(1)

(2)

(3)

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creates an equivalent noise degradation (END) of 1dB. We require C/N of 27.7dB for

QAM-64 at FEC rate ¾, 22.7dB for 16-QAM at FEC rate ¾ and 11.4dB for QPSK at

FEC rate ½. These C/N figures are for a Rayleigh channel. The resulting limiting curves

shown in Figure 5,

102

103

104

105

106

-110

-100

-90

-80

-70

-60

-50

-40

-30

Frequency [Hz]

   P   h  a  s  e  n  o   i  s  e

   l  e  v  e   l   [   d   B  c   /   H  z   ]

QPSK

16-QAM

64-QAM

Figure 5: Limiting phase noise curves

PA linearity

When feeding a linear PA with a constant envelope waveform such as a sine wave, the

PA is only linear up to some specified output power, above which it becomes saturated

and cannot be considered linear anymore. When the PA is fed with a waveform that does

not have a constant envelope, it only saturates on the momentary power peaks of this

waveform and remains linear for the rest of the time. If the ratio between the waveform

 peak power to its average power (PAPR) is high, the requirement that the amplifier does

not saturate on the waveform peaks causes the average power output by the amplifier to be well below the peak power it can handle. The ratio between the peak power the PA

can handle to its actual operating point as determined by the PAPR is called backoff. AsPA cost rises with its peak handling power capability, modulations with high PAPR are

considered inefficient in their use of the PA.

QPSK 16-QAM 64-QAM

SC-QAM (β=0.25) 4.9dB 6.9dB 7.4dB

SC-QAM (β=0.15) 6.1dB 7.4dB 7.9dB

OFDMA-2048 11.0dB 11.4dB 11.7dB

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Table 1: PAPR is comparison between modulation methods

Operation in the non-linear region of the PA is avoided because it creates interference

 both inside and outside the band. The problem of interference outside the band is similarin OFDMA and SC, and may be alleviated by about 3dB by employing filtering

techniques ([10]). Inside the band the clipping effect is quite different for OFDMA

relative to SC modulation. While for SC the clipping causes distortion directly in the

constellation plane, in OFDMA there is an FFT operation between the clipping and the

constellation plane, so the distortion is spread across all the OFDMA sub-carriers.

Figure 6: Transmitted constellation map with low backoff

Figure 6 shows SC QAM-64 with square root raised cosine (SRRC) pulse shaping(rolloff factor 0.15) versus OFDMA-2048 QAM-64. The SC constellation is shown with

6.8dB backoff, while the OFDM constellation is shown with 8.8dB backoff. The PA

simulated by the Rapp model ([11]). Although the constellation for OFDMA seems

worse, it is better in the sense that it is easier to the FEC to correct the kind of distortion present in it, than to correct the distortion present in the SC constellation. Note also thatwhen the transmission channel is frequency selective and the SC modulation will use

equalizers (specifically decision-feedback equalizers), the distortion in the transmitted

constellation might introduce BER even for a channel with good S/N.

MAC aspects unique to OFDMA

Narrowband channels

Efficient operation with narrowband channels is one of the major challenges in the design

of a BWA system. An example where this challenge is met in reality might be a BWA

uplink operating in a 6MHz MMDS channel at 16QAM, where the uplink can burst to

about 15Mbps, while a single voice over IP (VOIP) channel requires only 8Kbps. To

understand the reason for the inefficiency we can assume that an SU transmits 10 bytes ofdata every 10mS. The transmission of these bytes, and additional 10 bytes of protocol

overhead will take about S µ 10 . To this figure we need to add a preamble for the modem,

and a guard interval between transmissions. The duration of both these intervals is

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 proportional to the channel peak delay spread. Even if we substitute the conservative

number of S µ 5 for each, we arrive at an efficiency of 50%.

With OFDMA parallel processing capability the situation is much better. For instance,

suppose a SU requiring a VOIP service is assigned one sub-channel out of the 32

available in IEEE 802.16a draft standard ([3]). The duration of the data transmission will

now be 32 times longer, namely S µ 320 , while the overhead remains the same. The

efficiency will now be 97%.

Contention based multiple access

The MAC layer of most multiple access networks needs some contention access

mechanism. While slots for access can be allocated in advance for a known traffic pattern, changes in the traffic pattern require some mechanism to signal their presence

and change the allocations accordingly. The IEEE 802.16a draft standard ([3]) for

instance defines a mechanism called BW requests. These BW requests may be used oncontention basis, thus allowing SU that is not allocated any BW a chance to signal it

requires some BW allocation.

Clearly, the more bursty and unpredictable the nature of the traffic, the more important

 becomes the contention mechanism. The problem is that classical contention-accessalgorithms (such as slotted Aloha with exponential backoff) are inefficient, and collisionstake long times to resolve, thus increasing the network latency.

OFDMA greatly improves the contention access mechanism both by virtue of its parallel

 processing capabilities (BW requests send on different sub-channels will not collide), and

 by a novel PHY layer capability called CDMA BW request. The idea behind CDMA BW

requests is to assign some set of OFDMA sub-channels for BW requests. A SU wishing

to request BW modulates the sub-carriers in these sub-channels with a specific CDMAcode taken from a pool of available codes. Due to the processing gain feature of CDMA

codes, several codes may be identified without ambiguity at the same time. The chancesof collision in contention based access when using CDMA codes is greatly reduced (by a

factor of about 50 in IEEE 802.16a draft standard for example), and the net result is better

network efficiency and lower latency.

System aspects of an OFDMA BWA system

Outdoor to indoor operation

Outdoor to indoor operation has become a major desire of BWA operators. This mode of

operation enables SU with integrated antennas to be purchased and installed by thesubscriber. For the BWA operator this translates to reduced SU cost, reduced installation

cost and competitiveness with other broadband access technologies (DSL, Cable).

Outdoor to indoor operation involves using smaller, omni-directional antennas, and

mandates that the RF signal be able to propagate inside the home. On the system level,the implications are that the RF path loss and the multipath delay spread are greatly

increased.

OFDMA is the best-suited modulation method for this type of operation as it enables

handling severe multipath conditions efficiently. OFDMA handles the increase in RF

 path loss by using its gain concentration feature and by employing space-time coding

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techniques (e.g. antenna diversity). Highly robust FEC schemes such turbo-codes

complement the other these techniques, and improve OFDMA robustness.

Interference immunity

An OFDMA system is fairly resistant to most forms of external interference. A narrow-

 band interference source for example, might block some of the OFDMA sub-carriers.

However, due to the fact that OFDMA sub-channels use sub-carriers spread across theentire frequency band, and due to the robust FEC scheme, this loss of sub-carriers will

generally not introduce significant errors. A broadband bursty interference source may

 block reception for a short while. In spite of this, the long duration of each OFDMA

symbol, and the FEC block interleaving over several OFDMA symbols will prevent

significant errors from this type of interference source as well.

When interference in the BST between OFDMA system cells is concerned, OFDMA

 behaves very much like frequency hopping spread-spectrum (FHSS) system. In each cell

of an OFDMA system, the OFDMA sub-carriers are divided in G N  groups (see Figure 3).

The OFDMA sub-channels are composed of different sub-carriers, such that one sub-

carrier is selected from each group and the number of sub-carriers in a sub-channel is

CHN SUB N 

. Typically, a SU that is the victim of interference transmits on a random sub-

channel. The interfering SU at another BST does the same in statistically independent

manner.

The probability of collision in K out of the sub-carriers is given by,

( ) K  N 

CHN SUB

 K 

CHN SUB

G

G

 N  N  K 

 N  K  P 

−−

 

  

 −⋅

 

  

 ⋅

 

  

 =

11

Substituting 53=G N   and 32=−CHN SUB N   we find that the average number of colliding

sub-carriers is 65.1)(   = AVG N Collision

. This is equivalent to a spreading gain of

dB AVG N 

 N 

Collision

G 15)(= .

Conclusions

OFDMA works reliably in severe multipath environments encountered under non line of

sight propagation conditions. OFDMA can provide system gains up to 18dB in the

uplink, and 12dB in the downlink relative to traditional modulation schemes.

Consequently, OFDMA achieves superior coverage, and is potentially the modulation

and multiple-access scheme that will enable outdoor-to-indoor operation. Outdoor toindoor operation substantially reduces SU installation and distribution costs, and enables

mass deployment of BWA systems.OFDMA is an optimal solution for 2

nd generation BWA networks. This technology is

cost-effective, standards-based and practical. Solving the phase-noise and PA linearity

 problems in OFDM is comparable to solving it in traditional modulation schemes, anddoes not significantly affect implementation costs. Since OFDMA is incorporated into the

DVB-RCT and IEEE 802.16a (draft) standards, the prices of OFDMA related

technologies are expected to drop. OFDMA technology is not just a theoretical technique,

FPGA based OFDMA systems from Runcom are already operating successfully in

several ! sites worldwide. Commercial ASIC’s and modules will be available in 2002.

(4)

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McGraw-Hill, 1997

[8] “Phase Noise and Sub-Carrier Spacing Effects on the Performance of an OFDMCommunication System”, A. G. Armada et.al, IEEE Comm. Letters, vol. 2, no. 1,

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