KETONEN et al. : PERFORMANCE-COMPLEXITY COMPARISON OF RECEIVERS FOR A LTE MIMO-OFDM SYSTEM
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TABLE XII
The Receiver With the Best Goodput (QAM Constellation Ω)
suitable for vertically encoded MIMO communications. Soft
information feedback from the FEC decoder to the ∕√-bcsl
LSD stage was also considered as a strategy to improve the
performance. It provides up to 2-dB performance improvement.
The choice of the receiver algorithm is emphasized when the
number of antennas increases and the channel condition number
is high. There the nonlinear ML or MAP based receivers clearly
outperform the linear receivers, but the price is remarkably
increased computational complexity.
The considered receivers were synthesized to a Xilinx FPGA
to get a solid ground for implementation complexity compar-
ison. A modification on the tree search of the A-bcsl LSD was
presented to simplify its implementation with no compromise in
its error rate performance. Thus, it can achieve double detection
rate compared to the original ∕√-bcsl algorithm. On the selected
FPGA, the SIC receiver is fast enough to process the number
of subcarriers defined in the LTE standard for 5-MHz band-
width with all modulations and 2×2 and 4×4 antenna con-
figurations. ASIC implementation results were also provided.
The receivers were designed to have the same detection rate,
which would be enough for the LTE 20-MHz bandwidth. The
Æ-best LSD was found to be more than twice as complex as the
SIC receiver in the 2 × 2 antenna case but in the 4 × 4 case the
complexity difference was smaller. The latency of the SIC re-
ceiver does not depend on the used modulation and it can be used
with higher order modulations. The latency of the ∕√-bcsl LSD
increases with the modulation and the list size in both FPGA
and ASIC implementations. The maximum detection rates in
the ASIC implementations were 420 Mb/s with the SIC receiver
and 280 Mb/s with the Æ-best LSD.
The receivers with the highest goodput and the lowest
complexity on ASIC with correlated, moderately correlated
and uncorrelated channels with a given SNR are presented in
Table XII. It can be seen that the simpler LMMSE and SIC
receivers can be used in the uncorrelated channel but in the
correlated channel, the Æ-best LSD gives the best goodput. The
receiver and the modulation order could be changed adaptively
with the channel conditions and SNR in order to achieve the
best possible goodput with the least amount of receive power.
Some open research problems still remain. The system we
studied assumed that there is no channel state information at
the transmitter. With full or partial transmitter channel state
information, low mobility appropriate feedback schemes com-
bined with transmitter precoding could change the conclusions
and would be an interesting line of further work. Also adding
channel estimation to the performance and complexity eval-
uation would be an interesting topic. One further promising
topic would be the design and implementation of a reconfig-
urable overall architecture which would adaptively switch using
a simple or a more complex detector depending on the transmis-
sion requirements, available SNR, channel properties, etc. Some
preliminary results for the study of receivers for adaptive mod-
ulation and coding can be found in [35].
Acknowledgment
The authors would like to thank Mentor Graphics for the pos-
sibility to evaluate Catapult C Synthesis tool and M. Myllyla and
J. Ylioinas for their helpful comments.
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