17-06-2013, 03:16 PM
MMSE Receiver for Multiuser Interference Cancellation in Uplink OFDMA
[attachment=55560]
Abstract
In uplink orthogonal frequency division multiple access
(OFDMA) systems, multiuser interference (MUI) occurs due
to different carrier frequency offsets (CFO) of different users at
the receiver. In this paper, we present a minimum mean square
error (MMSE) based approach to MUI cancellation in uplink
OFDMA. We derive a recursion to approach the MMSE solution.
We present a structure-wise and performance-wise comparison
of this recursive MMSE solution with a linear PIC receiver as
well as other detectors recently proposed in the literature. We
show that the proposed recursive MMSE solution encompasses
several known detectors in the literature as special cases.
INTRODUCTION
Recent research has witnessed increased focus on orthogonal
frequency multiple access (OFDMA) on the uplink [1]-
[7]. This is because real-time mobile multimedia applications
demand high data rates on the uplink. The performance of
OFDM/OFDMA systems depend to a large extent on how well
the orthogonality among different subcarriers are maintained
at the receiver [9]. Factors including carrier frequency offsets
(CFO) between the transmitter and receiver induced by
Doppler effects and/or poor oscillator alignments, sampling
clock frequency discrepancies, and time delay caused by multipath
and non-ideal synchronization can destroy the orthogonality
among different subcarriers. Among the above factors,
the impact of CFO on the performance is the most crucial one
because the CFO values are large (typically of the order of
several KHz) due to carrier frequencies being of the order of
GHz. In uplink OFDMA, correction to one user's CFO would
misalign other initially aligned users. Thus, other user CFO
will result in significant multiuser interference (MUI) in uplink
OFDMA.
SYSTEM MODEL
We consider an uplink OFDMA system with K users, where
each user communicates with a base station (BS) through an
independent multipath channel as shown in Fig. 1. We assume
that there are N subcarriers in each OFDM symbol and
one subcarrier can be allocated to only one user.
Comparison with HLCC Scheme:
We point out that the PIC scheme presented in [7]1 is similar
to the one in Eqns. (18), (19), and (20). The difference is
that, in [7], masked versions ofR and Y(') vectors are used in
the cancellation instead of using unmasked versions as in (18)
and (20). In other words, the HLCC scheme can be viewed as
a special case of the linear PIC in Eqns. (18), (19), and (20)
by i) replacing R with M(')R in (18), and ii) replacing y(2
with MM)Y$() in (20). The effects of the above masking of
vectors in HLCC are highlighted in the following section.
CONCLUSIONS
We presented a MMSE based approach to multiuser interference
cancellation in uplink OFDMA. We derived a recursion
to approach the MMSE solution. We present a structure-wise
and performance-wise comparison of this recursive MMSE
solution with a linear PIC receiver as well as other detectors
recently proposed in the literature. We showed that the proposed
recursive MMSE solution encompasses several known
detectors in the literature as special cases.
[attachment=55560]
Abstract
In uplink orthogonal frequency division multiple access
(OFDMA) systems, multiuser interference (MUI) occurs due
to different carrier frequency offsets (CFO) of different users at
the receiver. In this paper, we present a minimum mean square
error (MMSE) based approach to MUI cancellation in uplink
OFDMA. We derive a recursion to approach the MMSE solution.
We present a structure-wise and performance-wise comparison
of this recursive MMSE solution with a linear PIC receiver as
well as other detectors recently proposed in the literature. We
show that the proposed recursive MMSE solution encompasses
several known detectors in the literature as special cases.
INTRODUCTION
Recent research has witnessed increased focus on orthogonal
frequency multiple access (OFDMA) on the uplink [1]-
[7]. This is because real-time mobile multimedia applications
demand high data rates on the uplink. The performance of
OFDM/OFDMA systems depend to a large extent on how well
the orthogonality among different subcarriers are maintained
at the receiver [9]. Factors including carrier frequency offsets
(CFO) between the transmitter and receiver induced by
Doppler effects and/or poor oscillator alignments, sampling
clock frequency discrepancies, and time delay caused by multipath
and non-ideal synchronization can destroy the orthogonality
among different subcarriers. Among the above factors,
the impact of CFO on the performance is the most crucial one
because the CFO values are large (typically of the order of
several KHz) due to carrier frequencies being of the order of
GHz. In uplink OFDMA, correction to one user's CFO would
misalign other initially aligned users. Thus, other user CFO
will result in significant multiuser interference (MUI) in uplink
OFDMA.
SYSTEM MODEL
We consider an uplink OFDMA system with K users, where
each user communicates with a base station (BS) through an
independent multipath channel as shown in Fig. 1. We assume
that there are N subcarriers in each OFDM symbol and
one subcarrier can be allocated to only one user.
Comparison with HLCC Scheme:
We point out that the PIC scheme presented in [7]1 is similar
to the one in Eqns. (18), (19), and (20). The difference is
that, in [7], masked versions ofR and Y(') vectors are used in
the cancellation instead of using unmasked versions as in (18)
and (20). In other words, the HLCC scheme can be viewed as
a special case of the linear PIC in Eqns. (18), (19), and (20)
by i) replacing R with M(')R in (18), and ii) replacing y(2
with MM)Y$() in (20). The effects of the above masking of
vectors in HLCC are highlighted in the following section.
CONCLUSIONS
We presented a MMSE based approach to multiuser interference
cancellation in uplink OFDMA. We derived a recursion
to approach the MMSE solution. We present a structure-wise
and performance-wise comparison of this recursive MMSE
solution with a linear PIC receiver as well as other detectors
recently proposed in the literature. We showed that the proposed
recursive MMSE solution encompasses several known
detectors in the literature as special cases.