Multipath fading effects on short-range links

Thursday, 19 May, 2011


This article introduces multipath fading and describes the results of experiments conducted by Alciom on short-range 2.4 GHz RF links in indoor environments.

Multipath fading occurs when a radio signal is transmitted between two nodes through several spatial channels, due to reflections and diffraction of radio waves on walls, floors, objects, etc.

In the time-domain, when a short RF burst is transmitted by the first node, this burst is received several times by the destination node, one time through each spatial path. Each reception is associated with a given attenuation and delay (corresponding also to a phase shift).

This induces two problems as the successive bursts are summed by the receiving antenna - fading (when the echoes are out of phase with each other, giving destructive or constructive interferences), and inter-symbol interference (when the duration of a binary bit is shorter than the time between the first and the last echoes).

Multipath fading is then a fast varying reception level of a radio signal in presence of multiple reflections. The received signal changes in amplitude when one of the nodes is moved, when the environmental conditions are modified (movement of an object which changes the reflection paths) or when the operating frequency changes.

Multipath fading is a complex phenomenon, more easily expressed with a statistical model. Two basic models exist:

  • Rayleigh fading, which is good estimation of multipath fading behaviour when there is no dominant line-of-sight propagation (nearly all power comes from the reflected path), classical for urban mobile networks as well as for tropospheric propagation;
  • Rician fading, which includes the presence of a line of sight channel plus reflections.

Many other complex models exist but all are statistical in nature.

Here is a typical Rayleigh fading pattern.

 

When one of the nodes is moved this plot shows the presence of deep fading situations, with variations usually in the 10 to 15 dB range but with occasional 30 dB or more attenuation. Distance between peaks and valleys is roughly half a wavelength (giving 6.25 cm at 2.4 GHz), as it moves from a signal in phase to a signal out of phase, equivalent to increasing the path length by half a wavelength.

Therefore multipath fading can give unexpected drops in signal quality in indoor or urban environments or even loss of communications even at very short distances. For example, a 25 dB drop is equivalent to a coverage distance reduced by a factor of 18: a system usually working with a 200 m range, like a classical Wi-Fi system, may not work on some locations 10 m from the transmitter.

Multipath fading is also a nightmare for RSSI-based distance measurement systems as fading gives false distance estimations.

On the frequency domain the same situation exists: multipath fading gives ‘good’ frequencies and other frequencies where high attenuation occurs.

An RF signal generator drives a fixed dipole antenna with a 10 mW 2.4 GHz CW signal. This signal is received by a vertical monopole antenna, fixed on a ground plane sliding on a 1 metre-long aluminium rail. The line-of-sight distance between the two antennas is 20 cm to 1.20 m, giving a theoretical attenuation of 15 dB from one end to the other.

The slider is pulled by a rope wound around a small drum. A 10-turn potentiometer records the position of the drum, giving a voltage which is then proportional to the position of the slider. This voltage drives the X position of an oscilloscope used in XY mode.

Lastly, the reception antenna is connected to a spectrum analyser configured in zero-span mode. Its video output is connected to the Y axis of the oscilloscope, proving directly a position-to-attenuation plot.

The measurements confirmed the heavy influence of multipath fading when reflections exist and in particular in the presence of metallic furniture. Drops of 10 to 20 dB will be very usual in office-style environments due to occasional reflectors, with drops down to 30 dB in case of large metallic reflectors in close proximity, meaning a reduction of a factor 1000 of the received signal power.

 
Fading in a ‘typical’ indoor office.

It should be emphasised that these results are with situations when a direct line-of-sight transmission exists. If all signals come from reflections then the relative effect of multipath fading will be even larger.

Fadings are related to the relative phases and amplitude of the received signals. Therefore, changing the frequency of the carrier RF frequency will change the position of the peaks and valleys. A frequency hopping system will be more robust to multipath fading than a fixed frequency system as long as the protocol allows lost frames, as at a given position some frequencies will be less attenuated than others.

It can be shown that, on average, the distance (in Hz) between two consecutive valleys (or two consecutive peaks) is roughly inversely proportional to the multipath time (time delay between the shortest and longest spatial paths). The wider the frequency separation the better.

The 2.4 GHz ISM band could be used from 2.4 to 2.48 GHz in most countries, giving a maximum 80 MHz frequency hopping distance which gives 12 ns spreading time or enough to counteract a 2 x 1.5 m reflection.

Frequency hopping could help to reduce the effects of multipath fading by around 50%, using the full 80 MHz ISM bandwidth available at 2.4 GHz. However, as soon as the hopping band is reduced then the improvement is far lower.

A spread spectrum modulation could also help as it has roughly the same effects on multipath fading as frequency hopping, but only as long as the frequency spread is very large. This then applies to UWB systems but not to usual 2.4 GHz systems like Wi-Fi or 802.15.4 which uses quite small modulation bandwidths. Tests done with a 2 MHz AWGN signal showed no visible improvements against a CW signal.

Another solution to fight against multipath fading is to use several transmission or reception antennas, which will allow the receiver to avoid ‘bad’ spots. In its most simple form the receiver is simply switched to the antenna providing the best signal, even if far more powerful solutions do exist.

Spatial diversity is a very efficient way to minimise the effects of multipath fading and best results were achieved with antenna separation of 3 to 5 cm, corresponding to a quarter to half a wavelength at 2.4 GHz.

Time diversity (ie, sending several times the same message) is equivalent to spacial diversity when one of the nodes is moving. Unfortunately, this doesn’t apply to fixed objects but this solution is very efficient for example for mobile systems, especially when associated to time-interleaving codes.

Another, often complementary solution is to use polarisation diversity, which means different polarisations for the receiving antennas. This helps for multipath fading but also for ‘polarisation fading’, meaning when polarisation of both antennas is in quadrature. The relative effects of multipath fading are far stronger in this situation, as the line-of-sight signal is not received.

A RAKE receiver is a radio receiver designed to counter the effects of multipath fading and, more importantly, inter-symbol interference. It implements several sub-receivers (‘fingers’) each assigned to a different multipath component. Through a learning sequence it identifies and extracts each multipath signal through a tuned delay line and correlator, providing both an improvement in signal quality and a reduction in inter-symbol interference.

However, this only applies for modulated signals with a pre-known pattern like CDMA transmissions. If a continuous wave is cancelled by destructive interference there is nothing a receiver can do to retrieve it.

Multipath fading can reduce the RF coverage of a transmission link by a factor of 20 or more if the system uses a fixed frequency and a single antenna.

Using either frequency hopping and/or spatial diversity and/or polarisation diversity, a designer can reduce its impacts typically down to ±5 dB.

Frequency hopping gives slightly lower improvements than antenna diversity in the 2.4 GHz band, due to limited available bandwidth. Best performances are achieved with 3 to 5 cm spatial diversity combined with polarisation diversity. Frequency hopping is also a plus of course.

Alciom

www.alciom.com

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