Wide Band Vs Narrow Band

A simple spectrum agile radio would be a OFDM base band section followed by a heterodyne or direct conversion RF front end. The spectrum agility is obtained by using some of the subcarriers (M out of N) in the OFDM. The sub-carriers to be used by a cognitive radio is determined by a Spectrum Manager which gets information about the primary users from the Spectrum Sensing Unit.

Things get complicated when we start thinking about increasing the range of sensing. If suppose we could sense a very wide range of frequencies (We will see how this is possible later), then what should be the architecture of the spectrum agile radio.

If we use the same radio as earlier, it would be require a wide band RF front end which is capable of handling a big baseband bandwidth. Let us take some numbers to understand the situation better.

The frequency range over which sensing is done is say 800 - 2400 MHz i.e. a total of 1600 MHz.

If the spectrum agility is provided only in the baseband (which is primarily a digital circuit or could even be software running on a DSP/microprocessor), then the baseband bandwith should be more than or equal to 1600 MHz.

The data converters needed (DAC at the transmitter and ADC at the receiver) should have a conversion speed of few Giga-samples/sec. In fact, it should be more than 3.2 GHz. Though such high speed data converters exist in literature, most of them have very low precision (around 4-6 bits) and are primarily flash based architectures. This means high power consumption and large area along with low SNR.

Secondly, the RF front end blocks i.e. mixer, PA and LNA (plus IF processing stage in case of heterodyne receiver) should also be wideband. The Q of these circuits would be of the order of 1 which is very difficult to achieve at 1600 MHz.

The carrier frequency that needs to be generated is fixed, 1600 MHz. Hence the LO generation is not a problem. (I donno much about the phase noise requirement. But I think it should be a relaxed specification as compared to the existing narrow band communication standards.)

Now let us look into the baseband part. If suppose the minimum size of a spectrum hole is 200 kHz, then the total number of holes in the 1600 MHz bandwidth is around 8000. The major block in OFDM is an FFT/IFFT processor. An 8192 point FFT processor would take large area and power when operating at 1600 MHz.

If we look closely at the problem in hand, we will realize that the actual bandwidth used by a SU is very small. It would be typically vary from 200 kHz (for a GSM type channel) upto 10 MHz (for a high data rate user). Secondly, it is highly unlikely that the 10 MHz bandwidth required by an SU is scattered over the whole 1600 MHz range. More often than not, we will find the required spectrum holes within a bandwidth of 100 MHz.

This means a narrow band RF frond end (upto 100 MHz) would be sufficient for the SU communication. In order to have adaptability over the whole 800-2400 MHz range, we need a tunable RF front end. Along with that the LO should also generate frequencies from 850-2350 MHz (i.e. roughly 160 steps). Another problem that arises is the Q of the tuning circuits. The Q increases as the carrier frequency moves from 800-2400 MHz.

The base band portion would have a bandwidth of 100MHz and a 256 point FFT processor which is feasible.

Spectrum Agile Radio

The most primitive way of obtaining a spectrum agile radio is to vary the carrier frequency of the SU whenever it needs to switch from one frequency to another. The spectrum manager tells the SU transmitter and receiver which frequency to use.
Along with the carrier frequency, the bandwidth of the SU also keeps changing as per user needs. So the radio should have circuits (primarily filters) whose bandwidth is controllable. In other words, tunable filters which are tough to design.

Another scenario that may occur in cognitive radios is the lack of contiguous unused spectrum. It is possible that there is enough bands of frequencies unused but they are scattered over the whole scan range, and the bandwidth requirement of the user is larger than the biggest spectrum hole. In this scenario, the above mentioned spectrum agile radio (which can change its carrier frequency and bandwidth) fails. The figure below depicts this case.

To use the scattered (small) spectrum holes, OFDM was proposed as the baseband modulation technique. In OFDM, the total baseband bandwidth is divided into sub-carriers (or sub-channels) and data is put onto those sub-carriers. FFT processor is used for doing OFDM modulation and demodulation.
To put data just in the spectrum holes, only those sub-carriers that lie in the hole carry SU data and the rest of them are filled with zeros. Since the transmitter and the receiver know which sub-carriers are being used, SU communication is possible as shown below.

Allocation of spectrum holes to multiple SUs is shown below

The power spectrum of the output of different secondary users look something like this.

What is it

Cognitive Radio is an intelligent wireless communication system that scans the radio spectrum, detects unused spectrum and uses it for communication. When the licensed user (also called Primary User (PU)) of that spectrum comes up (or reclaims the spectrum), the cognitive radio (also known as Secondary User (SU)) moves to some other available frequency. So it primarily consists of a spectrum sensing unit, spectrum manager and spectrum agile handset. The spectrum sensing unit has to be fast and hence is power consuming. The spectrum management unit also ensures that the SUs dont have to switch their frequencies too rapidly. This is done by keeping the PU appearance probability in mind.

Need of Cognitive Radio
Though all the available spectrum is being allocated to users, RF measurement shows that about 30% of the spectrum is actually being used, on an average, across time and space axes. There is lot of congestion in the free ISM bands and there is a lot of fight over spectrum allocation to private companies. Usually the government owns the spectrum and licenses it to others for commercial use. Military and maritime navigation has been licensed with the majority of the RF spectrum, though they are not much in use at all times and at all places. So lot of licensed frequencies remain under-utilized.

The concept of Cognitive radio aims to tap this potentially available spectrum for commercial purposes. I should also state that this is just one aspect in which cognition is applied. Cognitive radios in generic sense as defined by Joseph Mitola is a wireless equipment which understands the radio environment and predicts the user needs and is capable of adapting itself to the requirements thereby providing efficient use of resources.

Requirements
There is only one requirement that the cognitive radio should and MUST follow. It should not interfere with the licensed users communication, i.e. the QoS of the licensed user should not be compromised. In other words, SU should be able to detect the PU with 0% error and SU should back-off within a prescribed time limit if the PU comes up.

Challenges

• Detection of PU with 0% error means detecting a weak PU signal embedded in noise, i.e. presence of PU should be identified even if the power of PU is comparable to noise.
• Strong interferers near a weak PU band makes the detection even more tougher.
• Back off within a prescribed time limit forces the spectrum sensing to be fast and dynamic. The sensing has to be done continuously which consumes lot of power.
• The settling times of the transmitter and receiver while they change the channel limits the SU communication QoS.
• The range over which the cognitive radios can scan and operate is theoretically infinite but a wide range puts severe design challenges of the hardware.
• Power spilled over from SU to the adjacent channel may cause interference to the PU occupying that frequency. This spillage should be within the limits prescribed by the PU. This essentially translates to the guard bands in the SU channels.

Tech Blog

After almost 3 years of blogging, let me start off a blog on some technical stuff. As I am doing Phd now, let me discuss things related to that. To start with,
The title of the blog says it all - Budhimaan Taar-rahit Upakaran meaning Intelligent Wireless Equipment, in other words Cognitive Radio.
We will delve into this slowly in further posts.
Till then you read my non-technical blog
http://vgblogs.blogspot.com