Abstract
In this article we study hardware-oriented versions of the recently appeared Layered ORthogonal lattice Detector (LORD) and Turbo LORD (T-LORD). LORD and T-LORD are attractive Multiple-Input Multiple-Output (MIMO) detection algorithms, that aim to approach the optimal Maximum-Likelihood (ML) and Maximum-A-Posteriori (MAP) performance, respectively, yet allowing a complexity quadratic in the number of transmitting antennas rather than exponential. LORD and T-LORD are also well suited to a hardware (e.g., ASIC or FPGA) implementation because of their regularity, parallelism, deterministic latency, and complexity. Nevertheless, their complexity is still high in case of high cardinality constellations, such as the 64-QAM foreseen by the 802.11n standard. We show that, when only global latency constraints exist, e.g., a fixed time to detect the whole OFDM symbol, the LORD and T-LORD complexity can be remarkably reduced, still approaching the ML and MAP performance, respectively. Notwithstanding the suboptimal low-complexity and hardware-oriented implementation, LORD and T-LORD approach the EXtrinsic Information Transfer of the ML and MAP detectors, respectively. To focus on a specific setting, we consider the indoor MIMO wireless LAN 802.11n standard, taking into account errors in channel estimation and a frequency selective, spatially correlated channel model.
Highlights
Because of the increasing demand of data rate and link robustness in wireless transmissions, Multiple-Input Multiple-Output (MIMO) technologies are nowadays an indispensable option in the wireless communications standards recently released or under definition, such as IEEE 802.11n [1], WiMax [2], and mobile long term evolution (LTE) [3]
MIMO is often combined with spacefrequency bit interleaved coded modulation (BICM) and orthogonal frequency-division multiplexing (OFDM) [1,2], which ensure that almost uncorrelated channels are experienced by different tones within an OFDM symbol
We show in particular that LORD and Turbo LORD (T-LORD) can perform very close to the ideal MAP detector for at least up to four antennas and for modulation orders of 3 bits per dimension, even in a realistic setting, with imperfect channel state information (CSI) and correlated channels
Summary
Because of the increasing demand of data rate and link robustness in wireless transmissions, Multiple-Input Multiple-Output (MIMO) technologies are nowadays an indispensable option in the wireless communications standards recently released or under definition, such as IEEE 802.11n [1], WiMax [2], and mobile long term evolution (LTE) [3]. If Nt > 2, this is not assured, due to possible error propagation in the decision feedback equalization (DFE) (8) This sub-optimal behavior can be mitigated crossing many sets Sj, i.e., letting a hyper-symbol x ∈ St be replaced by the candidate x’ ∈ Sj, if its ED is smaller and xt = xt = x: 3.2 LORD detector The LORD algorithm is composed of two stages.a The former is a pre-processing, common to several received sequences y, as long as the channel can be supposed constant for several OFDM symbols. Assume that Tmax = 6NdcTelem, i.e., on average 6 clock cycles are allowed to conclude the detection process for each antenna permutation, at each data carrier.b With Nt = 2, Ndc = 52 and a 64QAM constellation, Figure 3 plots the maximum number of fully-spanned tones (solid lines) as a function of the parallelism Plc and of the square subset side S.
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