Signal designs for MIMO OFDM systems
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Multiple-input multiple-output (MIMO) is a promising wireless communication
technique to boost the wireless link capacity without requiring additional power or
bandwidth. Combined with orthogonal frequency division multiplexing (OFDM) the
MIMO OFDM system is considered an attractive candidate for high data rate wideband
wireless communications over frequency selective channels. 4G-LTE, WiFi (802.11) and
WiMax (802.16e) all adopt MIMO OFDM in their standards. ☐ In real world applications of MIMO OFDM system, diversified sources of interference
often exist and significantly influence the system performance of MIMO OFDM.
Firstly let's consider the single user point-to-point MIMO OFDM system. The interference
may be often caused by a long time delay spread of the channel. Typically
MIMO OFDM systems resort to cyclic prefix (CP) to protect the system from interference.
The simple one-tap equalization in OFDM also relies on the assumption that the
CP length is at least as large as the channel memory length. However, in a number of
applications, the sufficient CP length requirement is not practically satisfied for various
reasons. For example, in the next generation of WiFi standard, outdoor transmission is
much more involved. The time delay spread of the outdoor channel may be much longer
than that considered in the typical channel models in the current WiFi standards. If
the current OFDM symbol structure is inherited, we may face the insufficient CP problem
in the outdoor transmission scenario. Next consider the interference scenario in
MIMO OFDM cooperative communication system. In the emerging 5G cooperative
communication system, the relay node may adopt the amplify-and-forward (AF) relay
protocol and the in-band full-duplex (FD) mode. The simultaneous transmission and
reception at the relay node gives rise to severe self loopback interference. All the mentioned
types of interference have detrimental influence on the error rate performance
of the MIMO OFDM system. Therefore signal designs for MIMO OFDM system must
take interference cancellation or management into consideration. ☐ In the first topic, a new interference nulling based channel independent precoding
for MIMO OFDM systems of nt transmit and nr receive antennas with insufficient
CP is proposed. When the CP length is insufficient, intercarrier interference (ICI) from
the symbols in the current transmission block and interblock interference (IBI) from
the previous transmission block occur in the OFDM system. To cope with ICI and IBI
induced by the insufficient CP, we have proposed a design framework to completely
nulling the IBI. In our first channel independent precoding scheme, no channel state
information is needed. When MIMO OFDM system equips the same number of transmit
and receive antennas or the transmit antennas are more than the receive antennas,
we have shown that precoding at the transmitter combined with interference nulling
at the receiver can assist to either cancel interblock interference (IBI) or separate the
subspace occupied by IBI from the subspace used for information symbols. When the
transmit antennas are less than the receive antennas, it is demonstrated that information
symbols can be decoded without any precoding at the transmitter but just with
interference nulling and block linear equalization at the receiver. ☐ In the next topic, we consider a robust precoder design which combines statistical
CSI at the transmitter and the interference nulling based precoding structure
to gain a better performance compared to the channel independent precoding scheme.
Statistical channel state information at the transmitter in the form of covariance matrix
of the MIMO OFDM channel matrix is utilized to perform the robust precoding. Standard
optimization problem is formulated with regard to the largest mean square error
(MSE) of the minimum mean square error (MMSE) equalized symbol in the insufficient
CP signal model. By employing convex optimization technique, a closed-form solution
is derived for the optimization problem. Since the average MSEs are set equal using
this precoder, the BER performance is better than our previously proposed channel
independent precoder. ☐ In the third topic, we consider the OFDM-based transmission in the AF full
duplex relay system with residual self-interference. A delay diversity OFDM (DD
OFDM) transmission scheme in amplify-and-forward (AF) full-duplex relay systems
is investigated. One direct source-to-destination link, one relay forwarding link and
residual self-interference (RSI) are considered in the system. The necessary cyclic prefix
(CP) length is investigated and a suitable AF relay protocol in the full-duplex relay
OFDM system is proposed. This paper demonstrates that the AF relay link and the
direct source-to-destination link can be combined to provide spatial diversity. The key
is that the DD OFDM scheme is used to transform the spatial diversity into increased
channel frequency diversity that is further exploited by using the bit-interleaved coding.
The BER performance of the proposed system is verified by simulation results.