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Abstract—This paper presents the GFDM system, a generalized
digital multi-carrier transceiver concept. GFDM
is based on traditional filter bank multi-branch multicarrier
concepts which are now implemented digitally.
Our GFDM approach exhibits some attractive features
which are of particular importance for scenarios exhibiting
high degrees of spectrum fragmentation. Spectrum
fragmentation is a typical technical challenge of digital
dividend use cases, exploiting spectrum white spaces in
the TV UHF bands which are located in close proximity
to allocated spectrum. Specifically, the GFDM features are
a lower PAPR compared to OFDM, a ultra-low out-ofband
radiation due adjustable Tx-filtering and last but
not least a block-based transmission using cyclic prefix
insertion and efficient FFT-based equalization. GFDM
enables frequency and time domain multi-user scheduling
comparable to OFDM and provides an efficient alternative
for white space aggregation even in heavily fragmented
spectrum regions.
I. INTRODUCTION
The presented GFDM scheme defines a transceiver
architecture and a PHY concept, allowing to opportunistically
exploit spectrum white spaces for wireless data
communications. For instance, spectrum holes in the
UHF TV bands (TVWS - TV white spaces) are a prominent
scenario due to the digital dividend [1] and serves as
motivating GFDM use case in this paper. The design of
such opportunistic devices is particularly difficult mainly
for two reasons. On the one hand, signal generation
should ensure ultra-low out of band radiation to strictly
avoid harmful interference to legacy TV signals. On the
other hand, the receivers should exhibit high sensitivity
in order to explore white spaces, i.e., to sense even very
weak TV signals. Apart from these fundamental requirements,
there are even more engineering challenges which
have to be addressed throughout this paper.
Typically, TV white spaces are not consecutively places in the spectrum, but the UHF TV spectrum exhibits
strong spectrum fragmentation. In order to efficiently
exploit all detected TV white spaces, a system is required
which can cope with strong spectrum fragmentation and
which is able to perform aggregation of several TVWS
by one single wide band signal. Hence, the GFDM
transceiver is wide band and addresses the following
demands:
• Low out of band radiation to avoid harmful interference
to legacy TV signals
• Simple equalization despite the wideband nature of
the transmit signal
• Frequency agile white space allocation, flexible
signal bandwidth
• Digital implementation to reduce the requirements
of the analogue front-end
Recent studies provided by the European FP6 project
ORACLE [2] indicate the usage of multi-carrier systems
to flexibly exploit vacant spectrum by switching subcarriers
on and off. However, the most prominent multicarrier
system OFDM is known to cause strong spectral
leakage even when using pulse shaping techniques or
guard carriers. Hence GFDM aims at combining the
flexibility and simplicity of OFDM with stronger interference
reduction mechanisms.
II. GFDM TRANSMITTER
GFDM is a multi-carrier system, which digitally
implements the classical filter band approach. Cyclic
prefix (CP) insertion is used to allow for low complex
equalization at the receiver side. According to Fig.1, a
tail biting technique is used to shorten the cyclic prefix in
order to enhance the spectral efficiency. Every subcarrier
is modulated individually, using some form of QAM
signalling. Let’s denote the QAM symbol stream on
CONCLUSION
In this paper we presented a multi-carrier system
architecture based on digitally implemented filter banks.
It is shown that GFDM significantly reduces the requirements
set on the analogue front end. In particular
GFDM combines both, the advantage of specific subcarrier
allocation and low PAPR. Low PAPR allows to
reduce the hardware cost and power consumption, which
is an important point of sale for future wireless systems.
Furthermore, each single subcarrier can be modulated
individually, which provides a high degree of flexibility
in the system design and allows for efficient multiuser
scheduling. Compared to conventional multi-carrier
system, we show that GFDM is a promising alternative
for heavily fragmented spectrum.