Detection of occupied frequency bands is the foundation for applications of dynamic spectrum access (DSA). In order to convince network operators that DSA is feasible in cellular frequencies, it has to be shown that a reliable detection of their primary signals is possible. In this section, we present an experimental validation of an algorithm and hardware, which can detect the presence of a Long Term Evolution (LTE) signal. In contrast to the classical mono antenna approach, an array of antennas is used, which allows to enhance the detection capabilities, particularly when besides the useful signal there is also interference.

Objectives of experiments

• Reliable sensing in real environment
• Performance gain of multi-antenna vs mono-antenna
• Effectiveness of primary and secondary synchonisation criteria
• Parametrization of sensing algorithm (detection threshhold)

Setup

The multi-antenna LTE sensing platform allows to acquire LTE (I, Q) data and to process them using advanced antenna processing algorithms. As described in the pictures below, the multi-antenna demonstrator is made of:

• A set of 5 antennas,
• A 4-channel receiver,
• A 4-channel acquisition board,
• A GPS system for positioning,
• A laptop for data storage and off-line multi-antenna signal processing.

Multi-antenna sensing platform block diagram

Filtering and gain control are applied to the signal in the multi-channel receiver unit, the multichannel acquisition board is used to convert the signals to digital domain and a control computer handles processing and evaluation of the digital samples. The multi-antenna LTE sensing platform is validated with lab tests by measuring sensitivity and co-channel interference rejection with real LTE eNBs. A hardware array simulator consisting of splitters, coupling modules and a set of cables of particular lengths is employed to virtually create a multiantenna, mono-path propagation channel with two directions of arrival.

Multi-antenna sensing platform

The main characteristics of the multi-channel receiver are: frequency bands - 1920-1980 MHz / 2110-2170 MHz); bandwidth - 5 MHz; Output intermediate frequency - 19.2 MHz; Noise factor (at maximal gain) <7 dB; Rx gain - 0 to 30dB (1 dB step); Frequency step - 200 kHz;  number of Rx channels – 4; Gain dispersion<1 dB; Phase dispersion<6°; Frequency stability <10^-7; Selectivity at ±5 MHz >50dB.

The main characteristics of the multi-channel acquisition are: Resolution - 12 bit; internal quartz clock - 15.36 MHz; Number of channels - 4; -3 dB bandwidth >25 MHz; Memory - 8MSamples (i.e. 2MSamples per channel).

Sensitivity tests

For sensitivity measurements, the platform depicted below is used. The level of the BTS is gradually lowered in order to estimate the sensitivity level when using one or four channels for detection processing.

Lab test platform for sensitivity measurements

Interference rejection tests

For interference rejection measurements, the platform described in the picture below is used. The second BTS is considered as the interfering BTS in the following. Its bandwidth was set to 20MHz in order to be able to highly load the OFDM sent symbols. The level of the first BTS is gradually lowered while the level of the second one does not change. The 80% detection limit level is searched when using one or four channels for detection processing.

Lab test platform for interference rejection measurements

Results

LTE detection sensitivity level for an 80% detection rate

The LTE detection sensitivity performance is summarized in the table above. It is slightly higher to what can be expected (6 dB with 4 antennas). This might be due to a lower sensitivity of the first channel compared to the other three.

LTE rejection capacity for an 80% detection rate

We can see that, with four antennas, the rejection gain is equal to 32 dB. 

For further details refer to: Nicola Michailow, David Depierre and Gerhard Fettweis: “Multi-Antenna Detection for Dynamic Spectrum Access: A Proof of Concept”,  QoSMOS Workshop at IEICE 2012

https://mns.ifn.et.tu-dresden.de/Lists/nPublications/Attachments/895/main.pdf