Chapter 7 Conclusion 159
7.2 Future Work
At the end of this dissertation, a number of open issues are discussed. Next-generation wireless systems are required to support higher rates, better reliability, and higher mobility while targeting lower cost, lower power consumption, and higher levels of integration [89]. In order to support higher data rate, the trend is to operate at higher carrier frequencies and use higher-order signal constellations which are more sensitive to analog front-end impairments. In this study, we have studied joint effects of IQ-M and CFO. However, there are other issues, such as dynamic acquisition errors of analog-to-digital convertors and amplifier nonlinearity, which could be handled in the digital domain. It is worthy of developing smart signal processing for analog front-end impairments.
Due to the development of new wireless technologies and the improvement of existing ones, the number of users, the demand for spectrum efficiency, and the demand
for higher data rate are increasing. However, the spread of the technology brings different interference sources or jamming [90], [91]. Interference or jamming can interfere with others’ radio and degrade the communication quality. In order to minimize the jamming level and recover the transmitted signal, an appropriate anti-jamming scheme must be applied.
In practice, a communication system can be modeled by a layered approach, such as physical layer and medium access control (MAC), where each layer has a specific role.
The role of the physical layer is to deliver information bits across a wireless channel in an efficient and reliable manner given a limited bandwidth or transmit power. On the one hand, the MAC layer is response for the resource management among multiple users. In traditional designs, there is no cross-optimization across layers. However, optimizing the individual layer is not always the best approach from a system performance perspective. In order to achieve cross-layer optimization, an adaptive cross-layer approach is thus required [92].
With the rapid evolution of wireless standards and increasing demand for multi-standard products, the need for flexible baseband solutions is growing. Efficiently programmable baseband processors are important for multi-standard radio platforms and software defined radio systems. In order to save development cost and silicon area for multi-standard systems, novel baseband processing and efficient VLSI architecture are worthy of developing.
In addition, mobile communications have evolved rapidly in recent years. The existing 3G standard, universal mobile telecommunication system (UMTS), is currently being upgraded with high speed packet access. The 3rd Generation Partnership Project (3GPP) has investigated the long term evolution (LTE) of UMTS to meet future demands. In LTE, it will introduce new access schemes on the air interface, e.g., orthogonal frequency-division multiple access (OFDMA) in downlink and single
carrier – frequency division multiple access (SC-FDMA) in uplink. LTE will also use MIMO to support peak data rates of 100 Mbps in downlink and 50 Mbps in uplink within a 20 MHz spectrum with two receive antennas and one transmit antenna at the user equipment. Hence, LTE is going to be a promising research issue.
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Appendix A
Derivation of (2.15)
The received frequency domain data Y in (2.15) is shown in (A.1). The first term of k the right hand side is the decayed original signal transmitted in the kth sub-carrier, and the second term denotes the inter-carrier interference (ICI) from others.
( )
The carrier frequency offset Δ is normalized to the sub-channel bandwidth f ( BWsub-carrier =1/NTs ), and the relative frequency offset is shown as ε , i.e.,
(1/ s) s
f NT NT f
ε = Δ = Δ . The detailed derivation of (A.1) is given below. The OFDM transmission symbol is given by the N point complex modulation sequence
2 / complex modulation values X . After passing through the band-pass channel, the k received sequence can be expressed as
2 ( )/
After the fast Fourier transform (FFT) in the receiver, the frequency domain data is given by
To simplify the notation, the noise term is ignored in the following derivation. Firstly, we consider the lth term (k = ) only, and the result is given by l
It is clear to see that (A.8) is the first term of the right hand side in (A.1). For the case k ≠ , i.e., ICI, the result is given by l
By the same technique described above, the following equation holds
2 ( )
Therefore, (A.9) is rewritten as
(
2 1 ( ) / 2 ( 1) ( ) /)
It is clear that (A.11) denotes the second term of the right hand side in (A.1).