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