Adaptive Baseband - Variable Bandwidth

A baseband architecture for cognitive radio using OFDM has been discussed in my previous post. The block diagram is again shown below.










Along with the tunable RF front-end and narrow baseband of around 100MHz as discussed in earlier posts, we can also think of making the baseband adaptive to cater to the needs of SUs with different bit rate requirements.

Let us consider the case of variable baseband bandwidth as shown in figure below.

Here the spectrum that is scanned for holes is from 800 MHz to 2.4 GHz. This range could be divided into 16 subranges of 102.4 MHz or 128 subranges of 12.8 MHz. Other divisions are also possible. The numbers 102.4 MHz and 12.8 MHz indicate the baseband bandwidth. The carrier frequency of the RF front end can upconvert/downconvert the baseband signal into any of the subranges.

The motivation for variable baseband bandwidth comes from the POWER perspective. As the baseband bandwidth increases, the power consumed by the data converters ADC/DAC increases for maintaining a particular SNR. In some ADC/DAC, like sigma-delta converters, the resolution and sampling frequency could be exchanged for one another, i.e. for the same power consumption, sampling frequency can be increased by compromising on resolution. In Sigma-Delta converters where the Effective number of bits (ENOB) is proportional to the Oversampling ratio (OSR).

Hence, the lowest power mode is the narrowest baseband bandwidth. That raises a question of the lowest possible baseband bandwidth. The lower bound depends upon the availability of the spectrum holes and the bit rate requirement of the SU.

Let me pose an example scenario. In the range 800 - 2400 MHz, the smallest size of the hole that is available is 200 kHz. This is dictated by the GSM band at 850/1800/1900 MHz. At any given time, the holes could be contiguous or scattered. If the spectrum holes are scattered in the spectrum, then the minimum baseband bandwidth required depends on the frequency separation of the farthest holes.

Consider the case when the holes are contiguous. Let the bit rate needed by the SU is 300kbps. The number of holes needed depends upon the SNR available in each of the hole. Let us assume that the 2 holes are allocated to the SU. Since the holes are contiguous, the baseband need not be OFDM and the baseband bandwidth can be 400kHz.

Is this possible?

The answer is NO because if the baseband bandwidth is 400 kHz, the number of subranges goes up to 4000. This puts a limitation on the frequency synthesizer in the tunable RF front end. The frequency synthesizer should be capable of generating frequencies in steps of 400 kHz in a range of 1600 MHz. This is infeasible with the present state-of-the-art.

Thus, OFDM is needed in the baseband even if the spectrum holes are contiguous. The infeasibilty of the frequency synthesizer restricts the minimum baseband bandwidth.

Moreover, the power consumed by the frequency synthesizer increases as the frequency steps get smaller and smaller (What is the relation? I donno). So the two conflicting terms, i.e. the power in the baseband and power in the frequency synthesizer, together decides the minimum baseband bandwidth supported by the architecture.


Summary
Known Facts

1. Minimum size of the spectrum hole = 200 kHz
2. Range of scanned spectrum = ~800 MHz to ~2.4 GHz ~ 1600 MHz
3. Power consumed in baseband processing increases with baseband bandwidth
4. Power consumed in frequency synthesizer increases with reduction in PLL step size

Unanswered Questions

1. Exact relation for power consumed vs baseband bandwidth
2. Exact relation for power consumed vs PLL step size

Baseband

The baseband part of cognitive radio is generally Orthogonal Frequency Division Multiplexing (OFDM). The basic block diagram of OFDM is given below.

The input bit stream is read 'k' bits at a time and mapped into symbol constellation, i.e. a symbol stream of complex numbers. This stream is parallelized into M channels. These M channels are then mapped to form one OFDM symbol. This OFDM symbol has M data sub-carriers, P pilot sub-carriers and N-(M+P) zeros. The zeros are inserted at dc and at the edges. Zero at dc is to avoid corruption of data due to dc offsets and LO leakage. The pilot data is inserted to estimate the channel for equalization and frequency synchronization. The number and location of pilot carriers depends on the method used for channel estimation. The signal at the input of IFFT is the frequency domain representation of the signal that is sent over the channel. A typical signal is shown below.

The frequency domain signal is converted into time domain signal by the IFFT block. A cyclic prefix is added to the data stream which is then serialized.

At the receiver side the data is parallelized and the cyclic prefix is removed. The FFT block converts the signal into frequency domain. Equalization is performed in the frequency domain, which is nothing but division of each channel by the gain of the transfer function of the transmission medium for that particular frequency. This data is then serialized and symbol demodulation is performed to obtain the bit stream.

If we make a tunable narrowband RF front end then the baseband bandwidth F_B would be around 100 MHz. See the last post for details about the need for narrowband front end.

In the case of cognitive radio, each SU uses only some of the data carriers. The spectrum manager will provide the SU with the locations of these data carriers. All other locations (frequencies) will be filled with zeros. These will include frequencies where PU is present.

Issues
1. Adjacent Channel Interference: When a SU is putting data in a sub-carrier, the resulting spectrum is not confined in that frequency alone. There is small amount of power transmitted in all the adjacent sub-carriers.

The resultant baseband spectrum is actually a sinc function. This will cause interference to the PU (as well as other SU). The interference is significant only at the adjacent channels as the sinc function decays pretty fast. The tolerable interference to the adjacent channel has to be defined and that may vary from one PU band to another. So for each sub-carrier, the amount of power permissible has to be calculated.

Guard bands, windowing techniques, insertion of cancellation carriers etc. are possible solutions for reducing adjacent channel interference.

2. Pilot carrier positioning: In the case of normal OFDM system, pilot carriers are usually scattered uniformly among the sub-carriers and the channel estimation at data carriers is obtained by interpolation from the pilot carriers. In cognitive radio, since only few of the sub-carriers are used for data (rest are zeros), a method for finding the location of pilot carriers needs to be investigated.

One of the possible solutions is to distribute them among each cluster of spectrum holes, i.e. there should be at least some P% of sub-carriers should be pilot carriers in each cluster of holes being used.

Another solution is to use long training sequences for channel estimation. In this technique, all the sub-carriers (being used by an SU) are used to send NT number of training symbols which are used to estimate the channel in each of the sub-carriers. This technique is useful only in slow fading channels. In case of fast fading channels, prediction of channel estimation will have to be employed with the training symbols correcting the predicted estimate from time to time.

3. Interference from PU: The received baseband signal of the SU will contain data from the PUs and other SUs. An FFT of the signal is taken, which gives the frequency domain information of the signal. Only those sub-carriers where the transmitter has put the data is used as the output and the rest is ignored.
A question arises whether the PU will add noise to the SU. The other SUs will not be an issue as they will be orthogonal.

4. Synchronization: In normal OFDM systems, the transceiver and receiver need to be time and frequency synchronized to avoid errors in demodulation. In case of many SUs sending orthogonal signals in the same band, the carriers of all the SUs need to be synchonized.

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