Experimental Broadband Ofdm System Field Results for Ofdm and Ofdm With Frequency Domain Spreading

8/3/2019 Experimental Broadband Ofdm System Field Results for Ofdm and Ofdm With Frequency Domain Spreading

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Experimental Broadband OFDM SystemField Results for OFDM and OFDM with Frequency Domain Spreading

Vijay Nangia, Kevin L. Baum

Motorola Labs – Communication Systems Research Laboratory

1301 E. Algonquin Road, Schaumburg, IL 60196 USA

Abstract — This paper presents link performance results of a

series of field experiments conducted for OFDM and spread-

OFDM/MC-CDMA in the context of a fourth-generation cellular

system (4G) downlink. The experimental system is a 20 MHz

2x2 MIMO-OFDM system operating at a carrier frequency of

3.676 GHz. Measurements were conducted in a moderate density

urban and suburban setting outside Chicago, Illinois which

exhibits varying degrees of delay spread and signal to noise ratio,

with vehicle speeds ranging from 0 to 65 miles per hour. Coded

performance (r=1/2 64-state convolutional coding) is compared

for various link configurations: no receive diversity, 2-branch

diversity, Alamouti transmit diversity, and 2x2 MIMO forQPSK, 16-QAM, and 64-QAM constellations. Results indicate

that OFDM typically outperforms spread-OFDM, especially for

higher order modulations. Moreover, the performance of dual-

stream MIMO is sensitive to the channel condition number.

Keywords—OFDM, SOFDM, MC-CDMA, Alamouti, MIMO,

Experimental 4G System

I. I NTRODUCTION

An experimental mobile broadband communication systemhas been developed [1][2] for evaluating a number of physicallayer technologies for 4th generation mobile cellular systems,such as Orthogonal Frequency-Division Multiplexing(OFDM), advanced error-control coding techniques, adaptivearray signal processing, and aggressive frequency re-use. Theexperimental system is a 20 MHz 2x2 MIMO-OFDM systemoperating at a carrier frequency of 3.676 GHz. The systemconsists of a base station with two transmit antennas and atwo-antenna mobile receiver.

This paper presents and compares link performance resultsof a series of field experiments for two promising broadbandtransmission technologies - OFDM and OFDM withfrequency domain spreading. The latter is sometimes calledspread-OFDM (SOFDM), OFDM-CDMA, or multicarrier CDMA (MC-CDM(A)) [5]-[9]. For convenience, we will usethe term SOFDM in the sequel. OFDM is well known for its

ability to maintain orthogonality between the transmittedsymbols even in severe delay-spread channels, through the useof a cyclic extension. The orthogonality between symbolseliminates intra-cell interference [3], thus making it possibleto boost data rate and system capacity with higher-order constellations when users are near the base station (i.e.,adaptive modulation). Moreover, since each subcarrier experiences flat fading, numerous adaptive antenna and space-time coding techniques that have been developed under a flatfading framework (such as Alamouti transmit diversity [4])

are directly applicable to a wideband OFDM system. It is wellknown that OFDM requires channel coding and interleavingacross subcarriers to exploit the frequency diversity of thechannel [11].

SOFDM is a transmission method that combines spreadingand code division multiplexing concepts with OFDM toexploit the channel frequency diversity regardless of thechannel coding scheme. In SOFDM, the original data streamis spread in the frequency domain over the differentsubcarriers using a spreading code, and the original data ratecan be maintained by code-multiplexing N data symbols to N

orthogonal codes. In the downlink, the use of orthogonalcodes like Walsh-Hadamard or DFT codes guarantees nointra-cell interference (MAI) in a flat fading channel (i.e., thesame channel gain on all occupied subcarriers). However, in afrequency selective delay-spread channel, the orthogonality between the codes is lost due to different channel gains on thedifferent chips of a spreading code. Different linear equalizer/detection techniques have been investigated in theliterature [8] to deal with the lost orthogonality, such as ORC(Orthogonality Restoring Combining), EGC (Equal GainCombining), and MMSE equalization.

The paper is organized as follows. Section II gives anoverview of the experimental system setup, and Section IIIdescribes the OFDM and SOFDM system models. Section IV

describes the procedure used to analyze the experimental data.Section V presents the performance results for OFDM andSOFDM for numerous transmit/receive configurations,including the baseline 1x1 configuration, 1x2 receivediversity, 2x1 transmit diversity, 2x2 transmit-receivediversity, and 2x2 dual-stream MIMO. Finally, Section VIsummarizes the overall results of the experiments.

II. EXPERIMENTAL SYSTEM

This section presents an overview of the OFDMexperimental system. The OFDM signal consists of 751 subcarriers spaced 25 kHz apart, resulting in a signal bandwidth of 18.775 MHz. Nulls are placed at each end of thespectrum to provide a guard band for D/A conversion and

filtering. A 1024-point IFFT transforms this frequencydomain data into the time-domain, and the resulting time-domain signal is sampled at 25.6 MHz. A 25% cyclicextension (256 samples) is added to the time-domain signal tocombat multipath and inter-symbol interference. The system parameters are summarized in Table I. For SOFDM, thespreading factor is 748, and all 748 codes (with equal power per code) are used (multicode).

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Figure 1. Base, mobile, and receiver for experimental system.

TABLE I

SYSTEM PARAMETERS

Channel Bandwidth 20 MHz

Number of subcarriers 751

Number of data subcarriers 748

Subcarrier spacing 25 kHz

Cyclic extension length 10 µs

FFT size 1024

Modulation QPSK, 16-QAM, 64-QAM

Coding R = 1/2

64-state convolutional

Interleaving Bit Interleaving for OFDM (over the subcarriers of one OFDM

symbol period) No interleaving for SOFDM

The system transmits from one or two antennas, at a base

station located on top of a building, to one or two receivers

located in a test vehicle (shown in Figure 1). The transmitted

waveform, which steps through the different constellations,the different antenna scenarios (baseline, diversity, and

MIMO), and the different transmission techniques (OFDM

and SOFDM), is stored in a programmable arbitrary waveform

generator, which acts as a source to continuously transmit the

encoded signals. Two 80° beamwidth 18 dBi gain antennas

mounted 160 ft above the average surrounding terrain radiate

the RF signals at 66 dBm EIRP (effective isotropic radiated

power).

The receiver, mounted in a test vehicle, consists of twoindependent branches with 5 dBi gain omnidirectionalantennas on the roof approximately 2.7 m above the ground.The received RF signals are down-converted to 38.4 MHz IF

signals and sampled at 51.2 MHz with 12 bits of resolution.To maintain suitable input levels at the A/Ds, each receiver gain is automatically adjusted based on the received signalstrength and timing. The complex digitized (at 25.6 MHz) baseband time-domain samples are periodically captured for off-line analysis.

III. SYSTEM MODEL

This section describes the system models of the OFDM andSOFDM downlink transmission schemes.

I n f o r m a t i o n

b i t s

C o n v o l u t i o n a l

e n c o d e r

I n t r a - B l o c k

i n t e r l e a v e r

C o n s t e l l a t i o n

m a p p i n g

S/P IFFT P/S a d d

p r e f i x

channel Σ

noise

d e c o d e d

b i t s

S o f t d e c i s i o n

V i t e r b i

d e c o d e r

d e

- i n t e r l e a v e r

S y m b o l t o

b i t m a p p i n g

S/PFFTP/S

r e m o v e

p r e f i x

c h

a n n e l g a i n

c o r r e c t i o n

Figure 2. OFDM system model.

I n f o r m a

t i o n

b i t s

C o n v o

l u t i o n a

l

e n c o

d e r

W H T /

D F T

C o n s

t e l l a t i o n

m a p p

i n g

S/P IFFT P/S a

d d

p r e

f i x

channel Σ

noise

d e c o

d e

d

b i t s

S o

f t d e c

i s i o n

V i t e r b

i

d e c o

d e r

I W H T / I D F T

S y m

b o

l t o

b i t m a p p

i n g

S/PFFTP/S

r e m o v e

p r e

f i x

e q u a

l i z e r

Figure 3. SOFDM system model. WHT is a Walsh-Hadamard Transform,

IWHT is an Inverse Walsh-Hadamard Transform.

Block diagrams of the overall OFDM and SOFDM systemmodels for one transmit and one receive antenna (1x1) caseare shown in Figure 2 and Figure 3. For SOFDM, performance is evaluated with both Walsh-Hadamard andDFT-matrix orthogonal spreading codes. The benefit of DFTspreading codes is a low transmit peak-to-average power ratio,which comes at the expense of higher processing requirementsat the transmitter/receiver due to non-binary complex-valuedcodes. Also, note that frequency interleaving of the coded bitsis not needed in the case of SOFDM because thespreading/despreading process averages the channel quality

across all subcarriers and spreading codes (SOFDM link simulations showed virtually identical performance with or without interleaving).

A mathematical description of the model is presented in[12]. The received signal is demodulated using conventionaldemodulation techniques for OFDM [10][11], and SOFDM[7]-[9]. As shown in Figure 2 and Figure 3, within eachsymbol block, the cyclic prefix of N p samples is discarded andthe remaining samples are DFT’d into the frequency domain.Let r [k ,n] denote the received samples corresponding to the nth symbol block after the cyclic prefix has been discarded. Thus,

],[],[],[

0

nk nn N l k x pnk r

p L

l

pl ++−=∑=

, (1)

where pl is the sampled channel pulse response and pl isassumed to be zero for l <0 and l > L p. Also, n[k ,n] is the noiseat sample k of symbol block n. The DFT of the sequencer [k ,n] can be written as

],[],[][],[ nk N nk X k P nk R += . (2)

For the OFDM/SOFDM system, linear frequency-domainequalization is employed on each frequency bin according to

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],[][],[ bk Rk wbk Z = , (3)

where for OFDM,

][

1][

k P k w = , (4)

and for SOFDM, w[k ] will be chosen according to either anEGC (Equal-gain chip combing) or MMSE criterion,

(EGC)|][|

][][

*

k P

k P k w = , (5)

(MMSE)/][][

][][

22*

*

xnk P k P

k P k w

σ σ += , (6)

where 2nσ is the variance of n[k ,n] and 2

xσ is the variance of

x[k ,n]. After computing (3), for SOFDM, the long-code isremoved from the equalized frequency-domain signalfollowed by despreading and gain correction. Finally, soft-decision Viterbi decoding is performed. The above equationscan be easily extended to the multiple receive antenna andMIMO cases [13][14]. Note that the OFDM implementation

is computationally less complex than SOFDM, since OFDMuses a simple equalizer and has no spreading/despreadingmatrix operations.

IV. MEASUREMENT A ND A NALYSIS

Outdoor measurements were performed on various driveroutes around the base site. The test area contains a mixtureof single and multistory residential and commercial buildingsas well as some undeveloped spaces. The area surroundingthe base is divided into sectors, and each sector is covered bya pair of 80° beamwidth antennas (A1 and A2). Test vehiclespeeds ranged from 0 to 65 mph, depending on location, andsnapshots of data were collected at approximately 2.5 secondintervals (a snapshot contains 500 OFDM symbol periods).

The 25.6 MHz complex baseband time-domain samplescaptured by the experimental system are processed off-line.The channel response is determined with a frequency-domainmulti-transmit-antenna channel estimator designed using thefact that all of the transmitted data on both transmit antennasis known. The channel estimator uses a least-squares multi-user technique similar to that presented in [13]. In order toevaluate the performance at lower SNR, white noise of theappropriate power is added to each received signal. Thereceived signal is then equalized using (4), (5), (6) for the 1x1case. For the dual-stream MIMO case, a linear MMSEapproach is used [14] for joint equalization and streamseparation. Log-likelihood ratios are then generated and sent

to the convolutional-decoder. Frame and bit error statistics aregathered for each symbol interval in the snapshot, and theseresults are then averaged over all snapshots.

V. EXPERIMENTAL R ESULTS

Results are presented in terms of decoded BER as afunction of Eb/No (Energy per information bit to noise power spectral density) for OFDM and SOFDM with severaltransmit/receive antenna combinations, including the baseline1x1 configuration, 1x2 receive diversity, 2x1 transmit

diversity, 2x2 transmit-receive diversity, and 2x2 dual-streamMIMO for QPSK, 16-QAM and 64-QAM constellations.

First, the impact of the receiver equalization/despreadingalgorithm for SOFDM is investigated. Figure 4 shows the performance of SOFDM with equal-gain chip combining(EGC) and linear MMSE detection for the single transmit-single receive (1x1) configuration. EGC performance iscomparable to MMSE for QPSK (within 0.5 dB), but it

degrades severely (high BER floor) for the 16-QAM and 64-QAM constellations. The poor performance of EGC for higher-order modulations is due to its inability to equalize theamplitude variations in the frequency domain channel. Hence,EGC is not a viable receiver for SOFDM with high-order modulations and only the linear MMSE receiver will beconsidered for the remaining results of the paper.

Figure 5 compares the performance of OFDM andSOFDM for the baseline 1x1 (single transmit and receiveantenna) configuration. For SOFDM, results are presented for both Walsh-Hadamard and DFT spreading (SF=748). Theresults indicate that OFDM outperforms SOFDM and that theOFDM performance gain increases as the order of theconstellation is increased. These results support the trends

predicted by simulations [12]. Also, the performance of SOFDM is invariant to the type of orthogonal spreading codesused. Therefore, only Walsh-Hadamard spreading results will be presented for SOFDM in the remainder of the paper.

Figure 6 and Figure 7 show the results for the 2-branchreceive diversity (1x2) and Alamouti 2x1 transmit diversitycase respectively. The performance gains of OFDM stillincrease with the constellation size. However, the gains areless pronounced compared to the 1x1 configuration. Figure 8shows the results for the 2x2 transmit-receive diversity case.The results show that as the diversity order increases, the performance difference between OFDM and SOFDMdecreases. Figure 9 compares the performance of 16-QAMOFDM for several transmit/receive antenna combinations: baseline 1x1, 1x2 receive diversity, 2x1 transmit diversity,2x2 transmit-receive diversity. The results indicate thatreceive diversity is more effective than transmit diversity.This result was expected, since the receiver collects 3 dB moreenergy when two receive antennas are used (results are plotted based on Eb/No per receive antenna).

Figure 10 shows the results for open-loop, independentstream 2x2 MIMO with linear MMSE detection and channelswith an average reciprocal condition number, κ −1, greater than0.2. The reciprocal condition number, κ

−1, of the 2x2 channelmatrix, is equal to the ratio of the smallest to the largestsingular values of the matrix, and is related to the degree of separability of the two streams. The higher the reciprocal

condition number ( 10 1 ≤≤ −κ ) the higher the theoreticalcapacity supported by the channel. The reciprocal conditionnumber, κ −1, is computed for each 2x2 channel matrix and isaveraged over time and frequency. Typically, high delayspread channels exhibit high κ −1, while the lowest κ −1 arefound when the delay spread is low. Figure 10 also shows thatOFDM outperforms SOFDM for dual-stream 16-QAM whilethe performance of SOFDM is comparable to OFDM for dual-stream QPSK. Comparing MIMO QPSK to Alamouti2x2 16-QAM in Figure 8 shows that for identical data rates,

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transmit diversity performed a couple dB better than MIMOfor the field data. Although this result highlights the benefitsof 2x2 diversity, non-linear receiver processing (which is notused here) can further improve the performance of MIMO.

Figure 11 shows the effect of the average reciprocalcondition number, κ −1, on the performance of 2x2 MIMO-OFDM for different condition number thresholds,τ. For a particular threshold, τ0, all channels with κ −1 > τ0 are

included in the BER counting process (note that 75% of thedata has κ −1 > 0.1, and 50% has κ −1 > 0.2). The results showthat the performance of dual-stream MIMO improves as τ0 isincreased, as it improves the likelihood of separating the twostreams. Thus, the condition number of the channel has asignificant impact on the performance of dual-stream MIMO.

VI. CONCLUSIONS

This paper evaluated the performance of OFDM andspread-OFDM (SOFDM) using 20 MHz field experiments in awide-area suburban system context. Results were obtained for several transmit/receive antenna combinations, including the baseline 1x1 configuration, 1x2 receive diversity, 2x1 transmitdiversity, 2x2 transmit-receive diversity, and 2x2 dual-streamMIMO. In the results, OFDM typically outperformedSOFDM, and by a substantial margin in many cases. Thistrend was most pronounced for the baseline single-antennacase, and for higher order modulations. The improved performance and lower implementation complexity indicatethat OFDM is very promising for 4G systems. Receivediversity was found to be more effective than transmitdiversity, and the condition number of the channel had asignificant impact on the performance of dual-stream MIMO.For SOFDM, both DFT spreading and Walsh-Hadamardspreading were found to have virtually identical performance.

ACKNOWLEDGMENTS

The authors would like to thank the Motorola Labs 4Gxteam for developing the experimental system and for collecting the received waveforms over numerous drive routes(especially Mickael Batariere, James Kepler, Thomas Krauss,Sandeep Mukthavaram, Igor Lisica, Jeffrey Porter, and KeithBlankenship). Thanks are also extended to Tim Thomas for providing the channel estimation.

R EFERENCES

[1] M. Batariere, J. Kepler, T. Krauss, S. Mukthavaram, J. Porter, F. Vook ,“An experimental OFDM system for broadband mobilecommunications,” IEEE VTC-2001/Fall , Atlantic City, NJ, pp. 1947-1951, October 7-11, 2001.

[2] I. Lisica, M. Batariere, J. Porter, J. Kepler, T. Krauss, F. Vook,“Experimental broadband mobile OFDM system: description and initialresults,” 2001 Mobile and Portable Research Group WirelessSymposium, Blacksburg, VA, June 6-8, 2001.

[3] K. Baum, V. Nangia, K. Ramasubramanian, “A link performance studyof DS-CDMA and OFDM for 4

thgeneration cellular,” 2001

International Conference on 3G Wireless and Beyond , May 2001.

[4] S. M. Alamouti, “A simple transmit diversity technique for wirelesscommunications,” IEEE Journal on Select Areas in Communications,vol. 16, no. 8, Oct. 1998.

[5] S. Abeta, H. Atarashi, M. Sawahashi, F. Adachi, “Coherentmulticarrier/DS-CDMA and MC-CDMA for broadband packetwireless,” IEEE VTC 2000/Spring , pp. 1918-1922, May 2000.

[6] S. Abeta, H. Atarashi,, M. Sawahashi, “Forward link capacity of coherent DS-CDMA and MC-CDMA broadband packet wireless accessin a multi-cell environment,” IEEE VTC 2000/Fall , Sept. 2000.

[7] N. Yee, J. P. Linnartz, G. Fettweis, “Multi-carrier CDMA in indoor wireless radio networks,” IEEE PIMRC ’93, pp. 109-113, 1993.

[8] S. Kaiser, “On the performance of different detection techniques for OFDM-CDMA in fading channels,” IEEE Globecom ’95, 1995.

[9] S. Kaiser, “Performance of multi-carrier CDM and COFDM in fadingchannels,” IEEE Globecom ’99, pp. 847-851, 1999.

[10] J. A. C. Bingham, “Multicarrier modulation for data transmission: anidea whose time has come,” IEEE Communications Magazine, Vol. 28, pp. 5-14, May 1990.

[11] H. Sari, G. Karam, I. Jeanclaude, “Transmission techniques for digitalterrestrial TV broadcasting,” IEEE Communications Magazine, Vol. 33,

pp. 100-109, Feb. 1995.

[12] V. Nangia, K. Baum, “OFDM and OFDM with frequency domainspreading: observations for higher-order modulations,” 2002

International Conference on 3G Wireless and Beyond , May 2002.

[13] T. Thomas, F. Vook, K. Baum, “Least-squares multi-user frequency-domain channel estimation for broadband wireless communicationsystems,” 37 th Allerton Conf., Monticello, IL, September 1999.

[14] R. T. Compton Jr., Adaptive Antennas: Concepts and Performance,Englewood Cliff, NJ: Prentice Hall, 1988.

6 8 10 12 14 16 18

10−3

10−2

10−1

Eb/No (dB)

D e c o d e d B E R

1x1, r=1/2 64−state Conv

MMSEEGC

QPSK

16−QAM 64−QAM

Figure 4. SOFDM: MMSE vs. EGC receiver.

6 8 10 12 14 16 1810

−4

10−3

10−2

Eb/No (dB)

D e c o

d e d B E R


OFDMSOFDM−WHTSOFDM−DFT

QPSK

16−QAM

64−QAM

Figure 5. OFDM vs. SOFDM: 1x1 (baseline).

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0 2 4 6 8 10 1210

−4

10−3

10−2

10−1

Eb/No (dB)

D e c o d e d B E R


OFDMSOFDM

QPSK

16−QAM

64−QAM

Figure 6. OFDM vs. SOFDM: 1x2 (Rx diversity).

2 4 6 8 10 12 1410

−4

10−3

10−2

Eb/No (dB)

D e c o d e d B E R

Alamouti 2x1, r=1/2 64−state Conv

OFDMSOFDM

QPSK

16−QAM

64−QAM

Figure 7. OFDM vs. SOFDM: Alamouti 2x1.

−2 −1 0 1 2 3 4 5 6 7 8 910

−4

10−3

10−2

10−1

Eb/No (dB)

D e c o d e d B E R

Alamouti 2x2, r=1/2 64−state Conv

OFDMSOFDM

QPSK

16−QAM

64−QAM

Figure 8. OFDM vs. SOFDM: Alamouti 2x2.

2 4 6 8 10 1210

−4

10−3

10−2

Eb/No (dB)

D e c o d e d B E R

OFDM, 16−QAM, r=1/2 64−state Conv

1x1Al.2x11x2Al.2x2

Figure 9. 16-QAM OFDM vs. antenna config: 1x1, Al2x1, 1x2, Al2x2.

2 4 6 8 10 12 1410

−4

10−3

10−2

Eb/No (dB)

D e c o d e d B E R

Mimo 2x2, Cond. Number Thresh=0.2, r=1/2 64−state Conv

OFDMSOFDM

QPSK

16−QAM

Figure 10. OFDM vs. SOFDM: 2x2 MIMO.

2 4 6 8 10 12 14 16

10−3

10−2

10−1

Eb/No (dB)

D e c o d e d B E R

OFDM Mimo 2x2, r=1/2 64−state Conv

Cond. Thresh=0Cond. Thresh=0.1Cond. Thresh=0.2

QPSK

16−QAM

Figure 11. OFDM 2x2 MIMO vs. channel condition number threshold.

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Experimental Broadband Ofdm System Field Results for Ofdm and Ofdm With Frequency Domain Spreading