Transceiver
Design for Asymmetric Ultra-Wideband Links
Existing
ultra-wideband (UWB) techniques rely on symmetric transmitter and receiver
structures, which assume the same complexity level at all nodes throughout the
network. In single-band (SB-) UWB, this assumption implies high-rate
digital-to-analog (DA) and analog-to-digital (AD) converters at all nodes. In a
multi-band (MB-) UWB, this assumption means (multiple) local oscillators and
frequency synthesizers at all devices, which are very power consuming and prone
to carrier frequency and phase offsets. However, to establish physical
communication links between nodes with distinct complexity requirements,
asymmetric UWB transceivers need to be designed. This motivates our transceiver
designs for the asymmetric UWB links which allow the weak nodes to retain
low-complexity at both the Tx and Rx modes, and vice versa (see e.g., [1], [5], [10], [12]).
In
asymmetric UWB links, both high-complexity nodes and low-complexity nodes can
exist at the transmit end or the receive end. The low-complexity node (LCN)
only realizes the simplest single-band transmission with low A/D and D/A
conversion rates (see Fig. 1). The high-complexity node (HCN) can be a SB-UWB
transceiver with high A/D and D/A conversion rates or a multi-band (
Once
the conversion is achieved, the transceiver designs for multi-antenna
communications can be readily adopted. This is particularly attractive for UWB
communications where complexity is a major concern. In [1], [5]and [12], we show how these transceiver designs can be
integrated into our asymmetric link model. Especially, we deploy the geometric
mean decomposition (GMD) (see e.g., [11], [13]) approach to achieve optimality in terms of both
channel throughput and bit error rate (BER). Our analyses, together with the simulations,
confirm the feasibility and effectiveness of our asymmetric UWB links with MIMO
techniques.
Fig. 1. Low-complexity Node (a)
Transmitter diagram; and (b) Receiver diagram.
Fig. 2. High-complexity Node (a)
Transmitter diagram; and (b) Receiver diagram.
Orthogonal
Space-Time Block-Differential Modulation over Doubly-Selective Channels
The doubly-selective channel
provides double diversity gains. Multi-input multi-output (MIMO) schemes have
long been proved to provide improved capacity. When multiple transmit and/or
receive antennas are also employed, spatial diversity also becomes available.
If appropriately enabled at the transmitter and effectively collected at the
receiver, the 3-dimensional (space-multipath-Doppler) diversity gains can
considerably reduce the required SNR to achieve a prescribed error performance.
Most of the existing MIMO schemes only consider time invariant flat fading
channels. The others consider either time varying flat fading or time invariant
multipath channels.
Recently,
basis expansion model (BEM) is proposed to represent a block of time-varying
channel coefficients with parsimonious basis coefficients, which enables
pilot-assisted coherent and differential schemes for single-input single-output
(SISO) systems. In [2], we adopt the BEM channel model but consider a
MIMO setup. We develop an orthogonal space-time block-differential scheme over
doubly-selective channels. Since the channel is time varying, channel
estimation could become not only more complex but also imprecise. The differential scheme bypasses channel
estimation so that it reduces complexity and improves error performance. In
addition, the differential scheme saves the bandwidth by not using the pilots
for channel estimation. Both our analytical and simulation results show that
our proposed approach can collect full diversity gains in three dimensions:
space, multipath and Doppler.
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This material
is based upon work supported by the National Science Foundation under Grant No.
0621879. Any opinions, findings, and conclusions or recommendations expressed
in this material are those of the author(s) and do not necessarily reflect the
views of the National Science Foundation.