By Dr Sebastian Georgi
Dr. Sebastian Georgi is een van de systeemingenieurs voor het WMAS-systeem dat momenteel door Sennheiser wordt ontwikkeld. Hij doet al meer dan 10 jaar onderzoek naar draadloze breedbandtechnieken en hoe deze specifiek kunnen worden afgestemd op professionele audiotoepassingen. Georgi promoveerde aan de Technische Universiteit van Hamburg op het gebied van OFDM. Georgi heeft een sterke band met muziek en speelt fagot in een semi-professioneel orkest in Hannover.
Veel moderne draadloze systemen maken gebruik van breedbandtechnologie. In dit artikel wordt uitgelegd waarom. Professionele draadloze microfoonsystemen worden als voorbeeld gebruikt, omdat met de introductie van het nieuwe hoofdstuk Draadloze Meerkanaals Audiosystemen (WMAS) in de ETSI-norm EN 300 422-1, fabrikanten van dergelijke systemen nu de voordelen van breedbandtechnologie kunnen aanbieden. De huidige draadloze microfoons werken in het TV-UHF frequentiebereik en gebruiken meestal B = 200 kHz gemoduleerde bandbreedte. De meeste toepassingen vereisen een bereik van niet meer dan 100 m.
Is range the correct term in this application? Let’s calculate the free space propagation range of a typical wireless microphone: The receiver shall have a noise figure NF = 10 dB and the transmission scheme demands a signal-to-noise ratio of SNR = 10 dB. This results in a receiver sensitivity of
With a typical transmit power of Ptx = 10 dBm, an overall link budget of Ptx - Psens = 111 dB is available. Free space path loss is calculated as follows:
At a carrier frequency of fc = 500 MHz the range d is therefore:
The range d equals almost 17 km in this example, even without any antenna directivity. This does not match the daily experience of wireless microphone operators. One must conclude here that free space range is not the limiting factor. So, what is the limiting factor then?
RF channel model
Only a few wireless systems (satellite communication, radio relay systems) experience wireless RF channel behaviour of pure line of sight. Most systems suffer from multipath propagation caused by reflections. At the receiver, radio waves emitted by the same source arrive from different directions, and therefore interfere with each other. Sometimes arriving waves have the same phase, which leads to constructive interference, sometimes arriving waves have opposite phase due to different path lengths and the interference is therefore destructive. Even in outdoor scenarios at least two radio propagation paths exist, the direct line of sight and the reflection over ground. In most outdoor environments further obstacles add to the number of propagation paths. Operators of wireless microphone systems doing walk tests experience dropouts caused by fading, which recover with more distance to the stationary antenna.
In indoor scenarios the number of reflections is larger, making the RF channel behaviour more complex. Fading dropouts are regularly experienced, even close to stationary antennas. It can therefore be concluded that wireless microphone systems do not have an issue with range but with fast fading caused by multipath propagation. Is there a countermeasure available? Yes, there is. One solution is to use diversity reception: Operators of today’s wireless microphones put their faith in the probabilistic hope that when one receiver antenna is facing destructive interference, the other receiver antenna provides sufficient signal-to-noise ratio (SNR). However, practical experience shows that this is not always the case.
The better solution is the use of broadband signals. Interference strongly depends on the frequency of the carrier wave used. Destructive interference happens when phases of arriving radio waves have opposite signs. When the radio signal consists of a sufficiently wide frequency range, constructive and destructive interference is spread over the bandwidth and the strength of the receive signal is never fully faded.
Narrowband vs. broadband
Is a bandwidth of B = 200 kHz narrowband or broadband?
To distinguish these two types of wireless RF channel behaviour, signal bandwidth is only one parameter. The second parameter is the coherence bandwidth Bc of the wireless channel. It roughly describes in which bandwidth the RF channel’s transfer function can be considered equal. Both parameters have a counterpart in the time domain: the modulated bandwidth is roughly reciprocal to the length of each modulated symbol Ts. That means basically, the faster information symbols are transmitted, the more bandwidth the signal occupies. The coherence bandwidth is reciprocal to the maximum delay spread τmax of all paths the radio wave travels upon. That means the more the propagation paths differ in delay, the faster the channel’s transfer function varies over frequency. In audio terms, τmax is called reverberation time.
A narrowband RF channel is present when the modulated bandwidth is significantly smaller than the coherence bandwidth B << Bc or the symbol duration is significantly larger than the maximum delay spread Ts >> τmax. In this case the RF channel can be characterized by one multiplicative factor only, which makes equalization quite simple on the receiver side. On the other hand, the entire signal suffers from fading and the previously mentioned dropouts occur.
A broadband RF channel is present when the modulated bandwidth is significantly larger than the coherence bandwidth B >> Bc or the symbol duration is significantly smaller than the maximum delay spread Ts << τmax. In this case constructive and destructive interference happen simultaneously to the receive signal at different portions of the modulated spectrum – fading becomes frequency selective. This reduces the risk of dropouts dramatically. On the downside, more complex equalization and channel coding techniques need to be applied which increases system implementation complexity.
Orthogonal Frequency Division Multiplexing (OFDM) allows simple equalization even in broadband channels by using multiple carriers:
To finally answer the question if a bandwidth of B = 200 kHz is narrowband or broadband, the RF channel environment needs to be considered. Second-generation cellular communication system GSM uses B = 200 kHz as well, but inside a cell radius of up to several kilometres. Here, the wireless RF channel can be considered broadband. In the case of GSM, 26 out of 142 bits inside one frame are already used as training sequences to estimate and equalize the wireless RF channel.
In the case of wireless microphones operating in B = 200 kHz with a typical distance of 100 m, the RF channel behaves mostly narrowband, especially in outdoor scenarios.
The following pictures show real-time measurement data from an outdoor walk-test scenario. In each picture, the upper plot shows the RF receive power in colour representation (i.e., red is 50 dB attenuation) over time (x-axis) and frequency (y-axis). The lower plot shows the RF receive power over time (x-axis) for systems with different bandwidths from B = 200 kHz to B = 6 MHz, which are normalized to have identical transmit power. The effect of frequency-selective fading can be clearly seen. Systems using B = 200 kHz suffer from deep fading notches while B = 6 MHz system’s receive power is nearly steady.
Figure 3 shows measurement data at fc = 482 MHz centre frequency and Figure 4 shows a similar walk test but at fc = 1375 MHz centre frequency. The fading effect is comparable, only the time variance is increased due to roughly tripled doppler frequency caused by the same movement speeds.
Today, wireless microphone systems operate with a bandwidth of B = 200 kHz employing RF channels, which need to be considered narrowband. It has been shown that narrowband systems do not have an issue with range but with fast fading caused by multipath propagation on most applications. Therefore, a large fading margin is applied to the link budget and receive diversity is used to reduce the probability of dropouts caused by fading. Implementation is based on single-carrier digital modulation schemes, keeping system complexity low.
RF channels such as entire TV channels of bandwidth B = 6 MHz offer broadband properties in most scenarios. Dropouts can be avoided here by applying broadband transmission schemes such as OFDM. This eliminates the root cause for dropouts caused by fading. Although the free space distance of broadband channels is reduced due to higher thermal noise penalty, it is more than compensated for in real environments due to a much lower demand for fading margin.