Harris Mobile Antenna

8/13/2019 Harris Mobile Antenna

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MOBILE DVB-T USING ANTENNA DIVERSITY RECEIVERS

Gerard FARIA Scientific Director

Harris Broadcast Communications, France

ABSTRACT

Even if the European standard for Terrestrial Digital Video Broadcasting(DVB-T) has been designed for Portable and Fix reception, it offers varioustrade-off between bitrate and robustness which make it able to deliver VideoServices to Mobile receivers. This has been demonstrated in many publicexhibitions, verified during several comparative tests of DTV systems and itis even used for regular Services in Singapore.

On the track of the MOTIVATE project, the MCP project performed in March

2001 a Laboratory test campaign, using a new type of DVB-T receiversespecially designed for the mobile reception of DVB-T signal.

These new receivers take profit of a dual antenna reception to cope with thedifficult situation of the reception of a high bitrate digital signal on the move.

The laboratory tests proved the figures coming from the theoretical analysisand furthermore, the results have been verified by a limited field trialcampaign.

Taking successively the receiver & the network points of view, this reporthighlights the major aspects to consider for broadcast services to Mobile

receivers, using the DVB-T standard.

INTRODUCTION

The European standard for Terrestrial Digital Video Broadcasting (DVB-T) even designed forportable and fix reception, offers several transmission modes making it able to deliver VideoServices to mobile receivers, then to fully justify the spectrum required by such services.

During the MOTIVATE project, the DVB-T capabilities to deliver DTV programmes toreceivers in motion has been deeply analysed. At the end of this project, prototypes ofreceivers using two antennas have shown encouraging advantages when implementingmobile DVB-T services.

These good preliminary results have drive several MCP partners to design antenna diversityreceivers for DVB-T and to perform systematic tests on their implementation.

The MCP projects organised a test campaign in March 2001, to verify the theoreticalanalysis and to evaluate the gain brought by the antenna diversity technique to the DVB-Tmobile capability.

Laboratory tests have been performed on three receivers representative of varioustechnologies (i.e.: Experimental, Professional and Consumers) and the gain brought byAntenna diversity has been verified on the field.

Major results and conclusions are reported in this document.


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MOBILE DVB-T: THE RECEIVERS POINT(S) OF VIEW

Mobile reception can be defined, as being the reception of a DVB-T signal while in motion,using an omnidirectional antenna situated not less than 1.5 metres above the ground level.

This situation corresponds to a particular difficult environment as it has a large influence on

the DVB-T receiver behaviour and accordingly on the Quality of Service delivered to theusers.

Most problems faced by a receiver in motion come from the reception antenna, which isgenerally not in line-of-sight with the transmitter antenna. Instead of receiving from thetransmitter, through a fix roof-top antenna, a direct-path contribution where some echoes(Rice or Gaussian) are added, the antenna in motion (portable or mobile) receives onlyechoes, resulting in a multipath channel (Rayleigh) which varies highly with the time, thefrequency and the location.

Additionally, in case of mobile reception, the frequency shift experienced by each echo(Doppler shift), depends on the speed of the mobile but also on the arrivals direction of the

incoming echo. The angular dispersion of the echoes implies a different Doppler shift foreach echo. This "Doppler spread" is characterised by the Doppler spectrum of the channeland it causes inter-carrier interference, as it breaks the orthogonality law of the COFDMsignal.

The difficulties to receive DVB-T signals in motion are summarised in Table 1.

Mo b il e Po r t ab l e

Low coherencebandwidth

Delicate channel equalisation (channel transfer functionestimation)

Rayle

igh Deep-fades,flat fading Loss of all or a large number of carriers in an OFDM symbol

Low coherencetime

People moving in a room cancause picture loss due todeep or flat fading

Timevariations

oftheChannel

Low spatialcoherence

Delicate channel equalisation(tracking of the variations of thechannel transfer function)

Accurate placement of thereceiver to avoid loosing all ora large number of carriers

Orthogonality Loss due to

Doppler spread

Additional noise

(Inter-Carrier Interference dueto "FFT-leakage")

Table 1: Problems of Mobile reception

ANTENNA DIVERSITY : WHY?

Basically, an antenna diversity receiver receives the emitted signal on several antennashaving distinct characteristics (e.g. location, radiation pattern and/or polarisation).

In such situations, when one antenna is experiencing bad reception conditions (deep or flatfading) the other should probably not. Then, by choosing the right antenna at the right

moment, the diversity receiver behaves like receiving continuously a perfect signal.


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ANTENNA DIVERSITY : HOW?

In fact, the implementation is a little bit more sophisticated than a simple switch betweenantennas as it takes advantages of the COFDMs frequency/time partitioning implemented inthe DVB-T system.

! In the time-domain, DVB-T carries successive independent OFDM symbol isolated by

the way of a Guard Interval.! In the frequency domain, each OFDM symbols carries a set of independent carriers,

isolated by their orthogonality.! On top of that transmission frame organisation, a DVB-T receiver implements coherent

demodulation and channel estimation.! These features allow the demodulator to continuously weigh the credibility of each carrier

inside each OFDM symbols.Within an antenna diversity receiver, the demodulation process is performed on eachantenna and, before being decoded using the standard DVB-T decoding algorithm, thequality (likelihood) of each demodulated carrier is compared and selected by the way of alinear combination.

The result of this process is illustrated in Figure 1. Three spectrums are superposed there,two correspond to the ones received by two distinct antennas, the third results from thecombination process (known as Maximum Ratio Combining MRC) realised by the AntennaDiversity receivers.

This combined signal is the one experienced by the COFDM decoder and Figure 1 clearlyhighlights that, as fading does not occur on the same carriers from one antenna to the other,the fades are remarkably soften by the combination process.

RECEIVERS BEHAVIOUR

In mobile situation, the major difficulty comes from the Doppler shift affecting the echoesreceived. Globally, the behaviour of the receiver in Mobile situation can be characterised bythe curve presented in Figure 2.

This curve has a "C/N floor", giving information about the minimum signal requirement for agood mobile reception, and presents an upper Doppler limit, giving information on the"maximum speed" reachable by the receiver.

Globally, up to a given Doppler limit (or inter-carrier interference level), the DVB-T receiversare able to perform sufficient channel correction to demodulate the DVB-T signal. When theDoppler frequency (i.e.: the speed of the mobile) further increases, the recoveryperformance decreases drastically up to a point where no demodulation remains possible.

TESTED RECEIVERS

Early in 2000, the MCP partners decided to design DVB-Ts diversity receivers and to testthem to evaluate the real gain of the antenna diversity technique to the mobile DVB-T. Threecouples of receivers have been tested in March 2001, representing three different designsand categories of receivers (i.e.: Experimental, Professional and Consumer usage).

Moreover, each couple of receivers provides Single antenna and Dual antenna reception.This has been particularly appreciated, as the aim was not to compare variousimplementations, but to evaluate the antenna diversity gain and to verify that it is

homogeneous for each given implementation.


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Figure 1: Antenna Diversity Reception spectrums

Figure 2: Mobile behaviour curve


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TEST METHODOLOGY

The tests conduced in laboratory have used the test-bed and the test methodology definedby the Motivate project [3,4]. Two channel profiles: Typical Rural Area reception (RA6) andTypical Urban reception (TU6), defined by the COST207 project for GSM, have been used

to simulate in laboratory the terrestrial channel propagation. The sixteen DVB-T modeslisted in the Table 3 have been experienced.

N FFT GI Constellation Code Rate Bit Rate

1 8K 1/8 QPSK 1/2 5,53 Mbps

2 8K 1/8 QPSK 2/3 7,37 Mbps

3 8K 1/8 QPSK 3/4 8,29 Mbps

4 8K 1/8 16-QAM 1/2 11,06 Mbps

5 8K 1/8 16 QAM 2/3 14,75 Mbps

6 8K 1/8 16-QAM 3/4 16,59 Mbps

7 8K 1/8 64-QAM 1/2 16,59 Mbps

8 8K 1/8 64-QAM 2/3 22,12 Mbps5 2K 1/8 QPSK 1/2 5,53 Mbps

6 2K 1/8 QPSK 2/3 7,37 Mbps

7 2K 1/8 QPSK 3/4 8,29 Mbps

8 2K 1/8 16-QAM 1/2 11,06 Mbps

9 2K 1/8 16 QAM 2/3 14,75 Mbps

10 2K 1/8 16-QAM 3/4 16,59 Mbps

11 2K 1/8 64-QAM 1/2 16,59 Mbps

12 2K 1/8 64-QAM 2/3 22,12 Mbps

Table 3: DVB-T modes experienced

PERFORMANCE OF THE DIVERSITY

The absolute results obtained by the Diversity Receivers depend on the DVB-T mode but, inall cases, the performances of the antenna diversity receivers were significantly improvedwhen compared to the single antenna reception situation.

For a given implementation (i.e.: identical technology for single and dual antenna receivers),the gain of diversity can be evaluated by the following two parameters:! gain in C/N min for low Doppler frequencies,

(minimum signal requirement for network planning),! gain in maximum Doppler frequency for a given C/N,

(maximum speed for the mobile receiver),

The Table 4 gives the average gains recorded for the three couple of receivers, in the twochannel profiles experienced.

Urban Profile Rural Profile

Gain (C/N) Gain (Fd,max) Gain (C/N) Gain (Fd,max)

Rx 1 -6,3 dB 107% -9,7 dB 130%

Rx 2 -6,3 dB 181% -8,3 dB 190%

Rx 3 -6,4 dB 126% -10,0 dB 100%

Average -6,0 dB 100% -9,0 dB 150%

Table 4: Diversity gain


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The gain brought by the introduction of antenna diversity in DVB-T receivers is verysignificant in terms of C/N (6dB or 9dB depending on the channel profile) and therefore isquasi independent of the receiver technology.

The gain in Doppler frequency (i.e.: maximum speed) is also significant but is moredependent of the receiver technology. Nevertheless, that gain is approximately 100% for

Urban profile and almost 150% for the Rural profile.The gains of diversity related to the Channel profiles are illustrated in Figure 3. The networkplanning will, without any doubt, enjoy these spectacular gains:

! in Rural area it becomes possible to extend the coverage area without increasing thetransmitter power.

! in Urban area, it will be probably appreciated to change the DVB-T mode to obtain morebitrate without reducing the coverage area of the transmitter, or to reduce the transmitterpower without changing the on-air DVB-T bitrate!

Another way to consider this C/N gain is to apply it for portable reception.

Due to the building penetration loss, the portable receivers suffer a weaker signal than thefix / outdoor reception (i.e.: rooftop antenna) ones. Moreover, portable receivers experiencetime varying and frequency varying channel due to the echoes occurring indoor or provokedby people moving around the receiver antenna.

In that case, the Diversity receivers offer not only a C/N advantage, they also avoid lookingfor the best antenna positioning Thanks to the space diversity naturally provided whenusing two antennas! (nb: only few 10 cm are needed between quarter lambda antennas).

MOBILE DVB-T: THE NETWORK POINT(S) OF VIEW

Mobile reception of a DVB-T signal is influenced by the terrestrial propagation channelcharacteristics, which depends on the geographical environment and on the robustness ofthe DVB-T modes used.

DVB-T MODES

Many standardised DVB-T modes are not suitable for mobile reception. But the DiversityReceivers change greatly the set of possible modes.

FFT size influenceThe Fast Fourrier Transform (FFT) size has a direct impact, for a given network C/N level,on the maximum reachable speed of the receiver.

Roughly, 2K modes can cope with 4 times higher Doppler shift than the 8K modes, due tothe 4 times larger carrier-spacing it produces. But Antenna Diversity receivers changeconsiderably the suitability of 8K modes, as illustrated in Figure 4.

In addition to the 6dB gain in minimum signal level required for reception in the Urbanprofile, the improvement in maximum Doppler frequency provided by Diversity receivers,makes the 8K mode suitable for Mobile DVB-T Services. For instance, in the Figure 4, ifimplemented in the channel 40 (626 MHz), the usage of antenna diversity receivers makesmaximum speed, of the 8K modes, increasing from 40 km/h to 80 km/h.

In Urban environment, that improvement makes the difference between a non-suitable and

a suitable DVB-T mode!


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Figure 3: Diversity gains for channel profiles

Figure 4: Diversity gains for 2K & 8K


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Guard Interval influence

For DVB-T Services addressing fixed and portable receivers, the guard interval is generallyviewed as a necessary evil. Although it consumes channel bitrate capacity, it remainsnecessary to make the DVB-T receivers able to cope with the delayed replicas of the original

signal: the echoes.In the mobile situation, where the receivers are supplied only by echoes, then have to copewith fast variations of the transmission channel characteristics, the guard interval constitutesanother disadvantage: it stifles the receivers channel estimation with the information neededfor tracking the fast channel variations.

Accordingly, MCP chooses to focus laboratory tests on a realistic guard interval: 1/8, whichhas a value able to cope simultaneously with the cases of Urban SFN and hilly Ruralprofiles. In other words, the 1/8 Guard Interval constitutes a typical case: networks using ashorter guard interval will enjoy an increase of available channel bitrate whilst the receiverswill achieve slightly better mobile performance.

Modulation influenceObviously, the amount of protection bits embedded in the DVB-T signal has a directinfluence on the receivers performance in mobile situations.

Even if it consumes useful bitrate, a strong protection is definitively required to help thereceivers to cope with the degradation experienced in a time varying multipath channel, likethe mobile one. This clearly means that code rates 1/2 and 2/3 are the most suitable onesfor mobile Services. Introducing Antenna Diversity receivers, the code rate 3/4 becomespossible to enjoy more useful bitrate.

Regarding the constellation used, the more dense the modulation scheme is, the morebitrate it makes available on air, but the more difficult it is for a receiver to demodulate the

DVB-T signal.

Accordingly, 4QAM and 16QAM modulations will give better mobile reception performancethan 64QAM which provides high bitrate capacity. That is always true, but the AntennaDiversity receivers increase the suitability of the 64QAM, even in the 8K DVB-T modes.

Examples of Dual antenna reception are shown in Figure 5. These example use the strongCoding Rate 1/2 and provides, in the worst case, a minimum maximum speed of 180 km/h,which is definitively comfortable for an Urban environment.

But the remarkable thing is the minimum signal level required (C/N) for correct reception, theantenna diversity receivers make:! the 64QAM-dual reception equivalent to a 16QAM-single reception! the 16QAM-dual reception equivalent to a 4QAM-single reception

That could be considered as a detail, until the DVB-T bitrate is considered: using Diversityreceiver increases, the available on-air bitrate by 5.53 Mbps and that means the customerswill enjoy ONE additional programme in the DVB multiplex!

That additional programme could be obtained without any change in the network planningand furthermore, it gives some additional margin on the maximum speed limit of the Service.

RF channel influence

The impairments occurring in the mobile environment mainly depend on the Doppler effect.This Doppler distortion evolves proportionally with the speed of the vehicle and the DVB-Tsignal centre-frequency.


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Figure 5: Diversity gains with Modulation

Figure 6: Influence of the RF channel


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As a consequence, the RF channel used to deliver Digital TV to Mobiles is of majorimportance in regard to the Service reception performance.

The Figure 6 gives a view of the speed limit variation (i.e.: at C/N min+3dB) as a function ofthe RF frequency channel, for the different DVB-T modes. Values related to the DAB system(Digital Audio Broadcasting ETS 300 401) have been also represented for information, as

DAB takes also advantage of the COFDM modulation scheme.The MCP laboratory tests made use of the UHF channel 40 (626 MHz). This channel issituated roughly in the middle of the DVB-T Service bands (Band IV & Band V). Accordingly,the C/N performances reported are averaged in regard to the set of usable RF channels.

In the Figure 6, it appears that the weakest DVB-T mobile mode (8K, 64QAM, 2/3) could bereceived at 45 kms/h in the upper channel of the Band V, whilst it would be usable at 200kms/h in the lower channel of the Band III.

It could be noted also, that the DVB 8K with diversity antenna receivers, is equivalent withthe DAB mode I, whilst the other DAB modes are comparable with the DVB-T 2K oneswhen using DVB-T diversity antenna receivers!

DVB-T MOBILE RECEPTION PERFORMANCE AT A GLANCE

The figures 7 & 8 clearly illustrate the major conclusions arising from the test campaignusing Antenna Diversity receivers.

MINIMUM C/N FOR DIVERSITY RECEIVERS IN MOTION

Figure 7 shows the curves representing the minimum C/N required for correct reception ofMobile DVB-T, using Single antenna receivers and Antenna Diversity receivers, thus forvarious DVB-T modulation and bitrate.

It can be noted that the mode 16QAM, 3/4 has no real interest as it provides a bitrateidentical to the 64QAM, 2/3 but requires much more C/N! Nevertheless, the 6dB gain of theDual-Antenna receivers makes the 64QAM, 2/3 mode requiring less than 25dB of C/N.

The Diversity receivers can receive globally 5 Mbps to 24 Mbps in mobile situationAccordingly, the most popular DVB-T mode (64QAM 2/3 = 24 Mbps) for fix and portablereception becomes available for mobile services accordingly

It also means that broadcasters willing to broadcast HDTV program to mobile (plus somedata or even an additional SDTV program) can implement that service if antenna diversityreceivers are used in the mobiles!

More and more bitrate can be made available for Antenna Diversity receivers at the expanseof the field strength (i.e.: the transmitter power) made available to it.

MAXIMUM SPEED FOR DIVERSITY RECEIVERS IN MOTION

Figure 8 shows the maximum reachable speed, as it will be when using CH40 (626 MHz),for each DVB-T mode. The histogram shows the various bitrate capacities depending on theGuard Interval chosen. The two curves represent the maximum speed at which this bitratewill be usable for a Diversity receiver in motion.

Generally speaking, the 2K modes offer services at four times the speed offered by the 8Kmodes. The 2K Modes offer 5 to 24 Mbps at respective maximum speed of 550 kms/h to250 kms/h. These performances correspond to one programme receivable in High-Speedtrains and up to five programmes (or an HDTV one !) receivable in cars.


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Figure 7: C/N required by DVB-T modulation

Figure 8: Maximum Speed vs DVB-T modes


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Due to these great speeds, the 2K modes seems well adapted for rural environment servicedelivery Unfortunately, the 2K modes offer also the smallest guard interval values, limitingthe possible size of the transmission cells (ie: the maximum delay of echoes) thus avoidingthe use of large Single Frequency Networks (SFN).

The paradoxical of the 2K Mobile DVB-T remained there, because high speed in small

transmission cells imply the usage of cell handover by the receiver and the Diversityreceivers cannot escape to implement that feature.

The advantage of the diversity receivers is more noticeable for the 8K modes, because theavailability of the range of bitrate moves from 10..35 kms/h to 50..120 kms/h, making themtruly suitable for services in urban area.

As the urban area are generally the first ones in which new DVB-T services are deployed,that means the diversity receivers could make the programmes, delivered there for fix andportable reception, available in motion.

Furthermore, as portable reception could be considered as a simple case of the mobile one,the Diversity receivers will bring to the indoor portable reception, their advantage in C/N gain

while avoiding any boring antenna positioning adjustment.

CONCLUSIONS

The study of DVB-T receivers using Antenna diversity techniques clearly demonstrates thatDiversity receivers provide a huge gain in the minimum signal level required fordemodulation and increase the maximum speed limit at which the DVB-T signal can bedecoded.

The Diversity receiver gains can be viewed and used variously by broadcasters. They caneither save operational costs, reducing the transmitter power or increase the DVB-T bitrateavailable on-air with their current transmitter setting.

They can also consider that Diversity receivers make the Services planned for Fix receptionavailable for Mobile reception, while improved for indoor Portable reception.

The latest generation of DVB-T demodulator chip implements already the necessaryinterfaces and processes to design dual antenna receivers, then this technology will certainlybe rapidly available on consumer products.

At least, the traditional trade-off between robustness and bitrate must now includerobustness, bitrate and maximum speed but the Diversity receivers make the constraintsconsiderably lighters, then ease the broadcasters difficulties to balance this extended trade-off!

ACKNOWLEDGEMENTS

The author wishes to thank the Motivate projects partners, the MCP projects partners andthe DVB forum partners for the discussions, which contributed to this presentation.

Special thanks shall be addressed to the engineer of France Telecom R&D who initiatedand demonstrated theoretically & practically the suitability of the Antenna Diversitytechniques for Mobile reception of DVB-T.

REFERENCES

1. MCP project Deliverable 14 Laboratory tests on DVB-T Diversity Receivers, April 2001.2. MCP project Deliverable 06 DVB-T Diversity Receivers, April 2001.

3. Motivate project Deliverable 06 - Reference Receiver model for Mobile DVB-T, January 2000.4. Motivate project - Report to the 36rd DVB Technical module meeting, TM2310 - January 2000.

Gerard FARIA ([email protected]) 2001

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