Wideband receiver for Digital Radio Mondiale by M. J. Bradley

Broadcasting within the traditional AM bands is set to change with the introduction of digital broadcasts. Wideband receiver architectures allow many of the advanced functions of standards such as Digital Radio Mondiale to be implemented and upgraded without changing the hardware. This article describes some of the design issues of wideband receivers and the performance of a practical receiver.

1 Introduction

Digital Radio Mondiale1f2 (DRM) is a new standard for digital broadcasting in the bands below 30 MHz (LW, MW and SW bands). The audio quality of AM broadcasts in these bands is limited by the relatively low channel bandwidth (‘generally 9 kHz in the LW/MW bands and 5 kHz in the SW band) and by the effects of co-channel interference and signal fading. The DRM system provides techniques to improve the audio quality of broadcasts by:

increasing the signal bandwidth that can be transmitted in a given channel bandwidth (source coding is used to increase the spectral efficiency) allowing the use of wider channel bandwidths and providing a number of mechanisms to limit the detrimental effects of the radio environment.

2 Receiver architectures for DRM

A variety of digital receiver architectures exist that are suitable for DRM, each with their merits and trade-offs. These range from single-channel narrowband super- heterodyne receivers through IF sampling receivers to multichannel wideband receivers. DRM has a number of channel bandwidths (ranging from 4.5 kHz to 20 kHz) so the receiver must be capable of digitising a range of

bandwidths. In practice, a conventional narrowband receiver will tend to digitise the largest channel bandwidth and implement the final channel selectivity by digital filtering. However, within the DRM system, there are a number of features that could benefit from alternative receiver architectures.

The propagation characteristics of the short- wave/high-frequency band vary over the course of the day, season of year and over the 11 year sunspot cycle. For instance, at night the effects of absorption in the ionospheric D layer reduce, leading to an increase in received signal strength or transmission range and often in the level of co-channel interference. Broadcasters transmit on different frequencies at different times of the day and year and often transmit the same programme on several frequencies in parallel to increase the probability of reception in the given target area. At present the user is normally required to retune the radio receiver as the broadcast frequency changes or as the signal strength undergoes short-term fades (in the order of a few seconds in duration).

One of the DRM system techniques designed to improve this situation is the provision of automatic frequency control. This enables receivers to retune to follow a signalled change in transmission frequency or to receive a better channel carrying the same programme. The system is similar in concept to RDS (Radio Data

low-noise IF anti-alias amplifier filter AGC filter

lowpass filter

~

control

Fig. 1 Single-channel narrowband digital receiver

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 2002 15

v demodulator and decoder

primary receiver

- secondary receivers

I . - I

7 7 r

I ----t- l I

audio data

Fig. 2 Multichannel narrowband digital receiver

System) commonly used by many FM car radios. DRM transmissions contain control data streams multiplexed with the digital audio data stream. The control data stream signals when a change of broadcast frequency will occur and at which other carrier frequencies the same programme is being transmitted. In the case of combating a fade in signal strength by changing to another channel, the receiver should provide a seamless transition in the audio data stream during the transfer. To accomplish this a single narrowband digital receiver (see Fig. 1) must be able to retune rapidly to the alternate channel during a non-essential period of the transmitted data stream, ensure that the same programme is being received, measure the quality of the alternate channel compared to the current channel and retune to the better channel without interrupting the audio data stream. The ratification, measurement and decision functions need not be carried out within the non-essential period of the transmitted signal (typically in the order of 40 ms); all that is required within this time period is the retuning to the alternate frequency, the collection of data on which to perform the measurements and a second retuning back to the initial frequency. Depending on the decision strategy, numerous measurements may be required to ensure that the alternate signal is better than the current signal over a longer time period. The control of the single narrowband receiver becomes more complex from the aspect of frequency and gain control and the measurement process becomes more difficult due to the short periods of time available for retuning and data collection.

An alternative method is to employ a multichannel digital receiver (see Fig. 2) in which a primary receiver demodulates and decodes the desired signal whilst the secondary receiver(s) perform($ measurements on alternate frequencies without disrupting the operation of the primary receiver. Digitising the wideband RF signal and implementing the multiple channels in the digital domain can simplify the architecture further (see Fig. 3) .

The DRM system does not require a wideband receiver; indeed a single-channel, narrowband receiver with a lower current consumption would be more appropriate for a low-cost (<&loo) handheld receiver. However, a wideband multichannel receiver provides greater flexibility for implementation of the many services provided by DRM.

The demodulation and decoding functions of a DRM receiver are implemented in some form of digital signal processing device. Handheld receivers may employ low- power DSP (digital signal processing), FPGAs (field programmable gate arrays) or custom ASICs (application- specific integrated circuits). The continuing performance increase of desktop personal computers (PC) is such that they are now capable of performing the digital signal processing aspects of a DRM receiver (e.g. demodulation, channel decoding and source decoding) in real-time. Combined with a wideband multichannel receiver, a flexible and reconfigurable software receiver architecture is possible. Additional functionality can be added incrementally to a software receiver as broadcasters implement more options available within the DRM system. For example, the receiver could remain on

16 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 2002

amplifier __ QL

/L - 30 MHz lowpass

filter

~

audio data demodulator - and decoder +b am* 'L

I 4

*&-p-J: - + control

Fig. 3 Multichannel wideband digital receiver

standby whilst scanning DRM signals listening for severe weather warnings, automatically record programmes of interest to disk, or search for programmes broadcast in a given language. Alternatively the PC could be reconfigured to demodulate and decode alternative digital radio formats for the AM bands such as that being proposed for primary use in the domestic US market3.

3 Wideband multichannel digital receivers

Wideband digital receivers covering the LW, MW and SW bands must be capable of digitising received signals with large variations in signal strength. The maximum instantaneous dynamic range required is determined by the noise power in the DRM channel bandwidth and the maximum combined received signal level across the 100 kHz to 30 MHz band.

External noise in these bands is much greater than the receiver thermal noise and arises from both natural phenomena and man-made sources of interference. The excess noise (above thermal noise) ranges typically from 12 dB at an electrically quiet rural site at 30 MHz up to 76 dB at a noisy urban site at the bottom of the HF band4s5

(see Fig. 4). The smallest DRM signal bandwidth is . 4-5 kHz, resulting in a thermal noise level of -137 dBm and a minimum receiver noise level of -125 dBm.

Individual broadcast signals can be received at levels of -13 dBm and above, especially in the LW and MW bands. In contrast, in the SW band the received signals can be very much lower commensurate with the lower external- noise floor. Across the 100 kHz to 30 MHz broadcasting bands, many strong signals exist and the average power

'

70

60

.% E- 50 a, - .! 40

8

c v)

30

20

10

0 1 10

frequency, MHz

100

business ~ residential - rural - galactic - quiet rural -

Fig. 4 Median levels of excess noise above thermal noise

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 2002 17

Fig. 5 DRM-capable multichannel widebai receiver

nd

of the combination of all these signals can increase the received signal level to +2 dBm. In addition the peak amplitude of the wideband signal must not saturate either the analogue circuitry or the analogue-to-digital converter (ADC) as the resulting distortion products can easily swamp individual smaller signals. The individual signals are statistically unlikely all to combine coherently and a typical crest factor of 10 dB is assumed to cover the sporadic in-phase addition of the signals. This increases the maximum instantaneous received signal level to +12 dBm. Thus, without further signal processing, the required instantaneous dynamic range of the receiver is 137 dBm. This is greatly in excess of current ADC technology and the dynamic range ,of the received signal must be reduced or the receiver designed with a poor

sensitivity in the mid and upper SW band. One technique for reducing the instantaneous dynamic range is to use an antenna that attenuates the LW and MW bands where stronger signals and higher noise floors exist.

High-speed and high-dynamic-range ADCs are now available as single integrated circuits. An example is the Analog Devices AD6644 14 bit, 65 MHz ADC6. These devices introduce low levels of distortion with the harmonic distortion products being less than -85 dB and spurious distortion terms -100 dB (relative to the full- scale ADC output). In a wideband receiver the multitude of individual signals on the ADC input gives rise to large numbers of low-level distortion terms at the ADC output, which effectively increases the noise floor. The distortion from high-level received broadcast signals in the LW and

0

- 20 s E -40 I1 n -60

-80

-1 00

B 9

m U

-1 20

-1 40

-1 60 0 5 10 15 20 25 3d 35

frequency, MHz

Fig. 6 Receiver harmonic distortion (wideband response, 256k samples)

MW bands can result in significant interference to the low-level received broadcast signals in the SW band unless the wideband received signal is carefully controlled. The paper by Davies7 provides further details of wideband receiver techniques.

The wideband digital data rate (typically in the region of 1 Gbit/s) is too high for real- time transfer and signal processing within a PC so the selection of a smaller number of lower bandwidth channels is performed by digital down- converters (DDCs). The output data rates of the DDCs are low enough to be easily transferred to a PC for the demodulation and decoding tasks. The DDC

18 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 2002

consists of a quadrature mixer followed by channelisation and decimation filtering. An example is the Analog Devices AD66246

usciiiaiur (LU) rrequency, niter bandwidth and decimation factor. The device has digital LO generators with distortion prod- ucts less than -100 dBc and with accurate phase quadrature

phases in the decimation filters

enabling parts of the timing synchronisation mechanism to be performed outside the PC. This reduces the oversampling rate and further lowers the data

outputs. The down-sampling

are adjustable in each chain,

-

that has four down-conversion chains on a single chip. Each chain has a programmable local- ---.IILL- fr n\ I #-,. A -60

F 0 -80 0 s

-1 00 m

-1 20

-1 40

U

-1 60 0 5 10 15 20 25 30 35

frequency, MHz

- 20

% -40 0

The accuracy of the system clock is important for a DRM wideband receiver as it determines both the LO frequency and the sampling rate. Static errors in the clock frequency result in both a carrier frequency offset and a warping of the subcarrier frequency spacing of the OFDM (orthogonal frequency division multiplex) signal used in DRM8. Jitter or phase noise on the clock results in phase noise on each of the OFDM subcarriers. Each of these factors reduces the performance of the receiver.

4 DRM-capable software radio prototype

A multichannel wideband receiver designed for receiving DRM broadcast signals is shown in Fig. 5. The digital signal processing is implemented in.a PC (not showh). The antenna connector is on the bottom left and the signal is fed into the analogue and ADC sections (inside the screened cans). The digital section consists of a Cchannel DDC, a large FIFO (first-in, first-out) store to enable snapshots of wideband data to be collected and an EPLD (electrically programmable logic device) implementing PC interface functions.

Fig. 6 shows the harmonic performance of the receiver. The plot is generated from a 256k-point FFT (fast Fourier transform) of the wideband ADC output. The ADC used is an Analog Devices AD6644. The 2.3 MHz input tone is -1.5 dB relative to the ADC full-scale

distortion products are 80 dB below the fundamental. Fig. 7 shows the intermodulation performance via a two- tone test. The ADC input is operating at maximum input signal level and all harmonics, intermodulation products and spurious terms are below -83 dB. These distortion products give the limit of the instantaneous dynamic range of the receiver. The range demonstrated in practice is large enough to receive broadcast signals across the 100 kHz to 30 MHz broadcast bands.

A typical digitised wideband spectrum is shown in Fig. 8. The large dynamic range of signals is evident together with the decreasing external-noise floor as the frequency increases.

The receiver is controlled via a PC interface (Fig. 9), which allows independent control of the four narrowband

- 20 O /

-40 0

11 -60 0

-80

- 100

-120

- 140

2 0 s m

-160 ’ 0 5 10 15 20 25 30 35

frequency, MHz

output and in-band harmonic Fig. 8 Measured wideband spectrum (256k samples)

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 2002 19

Fig. 9 PC receiver control interface

RakL-l= 7604

I 1200

Samples

- .I

channels and the analogue receive circuitry. The chosen signal demodulation is implemented in real time within the PC.

Further development of the wideband receiver for the DRM application will reduce the chip count by remov- ing the wideband data collection FIFO store and combining the downconverter and PC-interface EPLD. Thus the receiver will consist of an analogue gain control block, ADC and down-converter. The digital equivalent of today’s portable narrowband AM receiver that incorporates the digital signal processing functions related to DRM is likely to develop into three- or two-chip solutions.

5 Conclusions

Advances in analogueto-digital converter technology have permitted the design of practical wideband receivers aimed at receiving digital broadcast signals. In addition the processing power of personal computers has increased to a level at which the digital signal processing functions may be performed in software rather than in dedicated hardware. This allows flexible, reconfigurable receivers to be built providing users with many additional functions compared to a conventional ‘AM’ band receiver. Low-cost, low-power, handheld receivers will certainly be available and target the market for a basic digital radio. However these devices will probably have limited scope for implementing the complete functionality of the DRM standard.

Acknowledgments

The author would like to acknowledge the help of Jonathan Pearce and Terry Bristow in the development of the multichannel wideband receiver.

Martin Bradley graduated from the University of Durham in 1991 with a first class Honours degree in Electronic Engineer- ing. From 1991 to 1993 he worked for GEC-Marconi Research on digital signal processing for HF transceivers. In 1996 he gained a PhD from the University of Durham with a thesis entitled ‘Adaptive equalisation for fading digital radio communication chan- nels’. Since joining Roke Manor Research in 1996 he has worked on a variety of radio systems in the areas of HF, DECT, EDGE, 3G and GPS. His interests are in radio system design and digital signal processing.

Address: Roke Manor Research Limited, Roke Manor, Romsey, Hampshire, SO51 OZN, UK.

E-mail: martin.bradley@roke.co.uk

References

1 See the DRM Web site: http://www.drm.org 2 ETSI standard ETSI TS 101 980 (ETSI, 2001). See

3 See http://www.ibiquity.com 4 World distribution and characteristics of atmospheric radio

5 ‘Man-made radio noise’. CCIR Report 2583 @TU, Geneva, 1978) 6 See Analog Devices Web site: http://www.analog.com 7 DAVIES, N. C.: ‘A high performance HF software radio’. Proc.

8th Int. Cod. on HF Radio Systems andTechniques, ZEE C o n .

8 STOTT, J.: T h e effects of phase noise in COFDM’, EBU Tech.

http://www.etsi.org

noise’. CCIR Report 322-1 (ITU, Geneva, 1974)

Publ. 474 GEE, 2000), pp.249-256

Rev., Summer 1998, (276), pp.12-25

0 IEE: 2002 First received 30th July 2001 and in revised form 7th January 2002.

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