2.1 Cognitive and Software Defined Radio
2.1.1 Multi-Standard Cognitive Radios
Building cognitive radios to act as secondary users (SUs) requires that they are able to find and transmit in unoccupied spectrum assigned to primary users (PUs), and this must be done without causing harmful interference to the PUs. Other incumbent users (IUs) must also be avoided. Apart from the critical issues of sensing for unused spectrum and allocating bands for transmission, the lower pri- ority of SUs presents a problem in terms of transmission capability and quality of service. If the spectrum bands allowed for a CR are fully occupied by PUs and IUs transmission might be blocked. Multi-standard cognitive radios can operate in multiple frequency bands with different specified standards, providing greater flexibility.
Multicarrier modulation techniques offer an ideal opportunity for such systems due to their regularity and parameterisation. OFDM and Filter Bank MultiCarrier (FBMC) are two types of multicarrier modulations. In order to understand how FBMC is distinct from OFDM, it is best to study multicarrier systems in which the output signal can be expressed in the continuous time domain as in Equ. 2.1.
This can be a unified formulation for both OFDM and FBMC.
s(t) =X
n N−1
X
k=0
xk[n]h(t−nT)ei2π(t−nT)fk, (2.1)
where xk[n] denotes the sample of the kth subcarrier data symbol in the nth symbol of continuous multicarrier symbols, fk is the kth subcarrier in a set of N used subcarriers, T is the multicarrier symbol duration, and h(t) is a prototype filter.
This transceiver in a multicarrier system can be modelled as a block diagram, shown in Figure 2.1. As can be seen in the discrete-time domain, N data symbols at synthesis are up sampled by a factor of L, which is calculated by TT
S, withTS denoting the sample period of the output sequence s[n]. They are then filtered by a prototype filter h[n]. The output of each data stream is modulated by the
h(t) x0(t)
h(t) x1(t)
h(t) xN-1(t)
...
s(t)
h'(t) x0(t)
x1(t)
xN-1(t) h'(t)
...
s(t)
Multicarrier Synthesis Multicarrier Analysis
(a)
h[n]
x0[n]
h[n]
x1[n]
h[n]
xN-1[n]
.. .
s[n]
h'[n] x0[n]
h'[n] x1[n]
h'[n] xN-1[n]
.. .
s(t)
Multicarrier Synthesis Multicarrier Analysis
(b)
L
L
L
L
L
L
ei(N-1)2πt/T
ei2πt/T e-i2πt/T
e-i(N-1)2πt/T
e-i(N-1)2πn/L
e-i2πn/L ei2πn/L
ei(N-1)2πn/L
h'(t)
Figure 2.1: Block diagram of a multicarrier modulated system, (a) in the countinuous-time and (b) in discrete-time
frequency of multiple carriers and summed for transmission. The signal in the receiver is demodulated and then filtered by a bank of matched filters h0[n], and down sampled by a factor of L. When critical sampling applies L = N and the prototype filterh[n] is selected as a rectangular pulse in the time domain, i.e. asinc pulse in the frequency domain, this multicarrier system becomes a conventional OFDM system.
FBMC is different from OFDM in the selection of the prototype filters h[n], and matched filters h0[n] which are chosen and designed depending on the adopted FBMC modulation technique.
By using well-designed filters for each subcarrier, FBMC can be a more effective solution in comparison to OFDM in term of ICI cancellation and spectral leakage suppression because non-adjacent subcarriers are almost completely separated by the bank of matched filters. OFDM has many important and desirable features over the FBMC. OFDM was originally developed focusing on a low-complexity im- plementation. The low complexity of OFDM is achieved thanks to a fundamental
assumption in which subcarriers of the OFDM symbol are perfectly synchronized and orthogonal with consecutive subcarriers. Thus the subcarriers are used for modulation at the transmitter using an IFFT block; inversely, they are separated by using an FFT block at the receiver. By contrast, FBMC is more complex than OFDM. The demand for well-designed filters in FBMC results in increasing com- plexity and resource requirements. Moreover, while employing MIMO in OFDM to increase the system’s capacity and spectral efficiency is somewhat straightforward, the development of MIMO-FBMC systems is relatively more complex.
OFDM modulation has been the dominant technique adopted for many wireless standards and has been investigated in terms of spectral sensing and carrier al- location for CRs. OFDM system implementation is simple, low cost, and can be more effectively parameterised. A single baseband implementation can be made to flexibly support multiple standards like 802.11 [25], 802.16 [26], and 802.22 [27], as well as supporting future OFDM-based standards.
This capability requires the ability to switch baseband processing from one stan- dard to another. This in turn means the need to perform variable length FFT/IFFT operations, insert cyclic prefixes of configurable length, and handling different pi- lot vectors as well as different preambles. As a result, the processing modules should be designed to support all requirements of the different standards.
Two additional challenges must be addressed. OFDM systems typically can only tolerate a small carrier frequency offset (CFO) leading to strict constraints on the design of the RF front-end. In a multi-standard system, the RF front-end accesses a wide range of frequencies depending on the standard in operation. Such a precise and yet wide ranging frequency requirement makes RF front-end design difficult and requires very expensive components. CRs also demand small spectral leakage for both in-band and out-of-band transmitted signals to avoid causing harmful in- terference to primary users, while OFDM signals have intrinsically large side lobes leading to a potentially large degree of spectral leakage. Hence, synchronisation and leakage management are more pressing issues in multi-standard radios.
The interface to higher layer processing is another important factor. Many hard- ware radio platforms are extremely difficult to design for or to modify. Hence, only hardware experts can use them. While detailed optimisation of low level blocks is important, providing a general interface for implementing higher layer processing is also important. This ensures that radio experts can use the system to investigate cognitive radio techniques without the need for specific advanced low-level FPGA expertise. Our work tries to offer well-designed and documented parameterised signal processing blocks in hardware, with a high level manage- ment interface to enable radio designers to benefit from the dynamic capabilities of FPGA platforms.