An Integrated Subchannel Scheduling Algorit hm for Adaptive Modulation and Coding (AMC) MIMO-OFDM Wireless Systems Lei Li , Zhisheng Niu Department of Electronic Engineering Tsinghua University , 100084, Beijing, P . R. China [email protected]Abstract— In this pape r , we develo p an inte grate d subc hanne l scheduling algorithm to maximize the system throughput while guarantee minimum data rate requirements for multimedia users in multiuser MIMO-OFDM systems downlink transmission uti- lizing adaptive modulation and coding (AMC) with limited chan- nel state inf ormat ion feedback. By integrat ed subc hannel sche dul- ing, we app ly the mathematic al equi vale nce between antennas and subcarriers in the analysis, getting multiple parallel transmit sub- channels, and then evaluate the channel state from the viewpoint of receiver s. Joint space-frequen cy diversity as well as multiuser diversity is exploited simultaneously by the subchannel allocation algor ithm. A modifi ed propor tiona l fair sched uling is prop osed and a fas t algori thm fo r mor e pra ct ica l imp lement ati on is als o pro- pose d. By numer ical example s, syst em throughp ut and fairness superiority of the our scheduling scheme are verified. I. I NTRODUCTION In recent years, two powerful technologies in physical layer design: OFDM (Orthogonal Frequency Division Multiplexing) and MIMO (Mu lti ple Inp ut Mul tip le Out put ) pro vid e add iti ona l dimension of freedom for radio resource management in spec- tral resource and spatial resource. We call it multi-dimensional radio resource management. OFDM has been emerging as a promising technology due to its ability in comba ting freque ncy selec tiv e fadi ng. In OFDM systems, a broadband signal is divided and modulated on mul- tiple narrowband subcarriers. Since the frequency-domain fad- ing deteriorates the SNR of certain subcarriers, but improves oth ers’ abo ve the avera ge SNR value, the pot ential los s ofthroughput due to the exclusion of faded subcarriers can be mitigated by employing higher order modulation and coding mod es on the sub car rie rs exh ibi ting high SNR va lue s [1] , whi ch is called Adaptive Modulation and Coding (AMC). In a mul- tiuser OFDM system, multiuser diversity and frequency diver- sity may be exploited by assigning subcarriers to the users with best channel gain [2]. Ref. [3] proposed joint subcarrier and bit allocation algorithm with minimum total transmission power for real-time services with fixed data rate. In [4], Gener alize d Processor Sharing (GPS) scheduling is integrated in power and subcarrier allocation to achieve maximum system throughput and provide fairness to heterogeneous traffic as well. MIMO have als o bee n att rac ting muc h attention bec aus e the y have the potential of providing enormous increase in spectral efficiency of wireless systems [5], [6], [7]. By employing mul- tiple trans mit and/o r recei ve anten nas, multi ple spati al subc han- nels are created, and it is unlikely all the subchannels fade si- multaneously, thus providing space diversity over fading envi- ronments. In multiuser environments, channel state dependent scheduling schemes are examined in [8] to increase the system capacity by multiuser diversity. For fairness consideration, ref. [9] applies round robin s cheduling in the selection of scheduled user group, then maps the selected users to the spatial chan- nels one-by-one. In MIMO environments, however , scheduling alone is hard to satisfy user’s diverse QoS requirements. From the persp ecti ve of radio resourc e manag ement , the combined MIMO-OFDM system is more desirable to allocate the channel’s degrees of freedom in space and frequency in a fle xible wa y . Cur rent studie s on rad io res ource manage- ment in MIMO-OFDM systems mainly focus on OFDM sub- carrier management under MIMO transmission environments [10], instead of joint space and frequency resource optimiza- tion. It is subop timum bec ause the inherent spac e diversit y ofthe MIMO chan nels is not exploit ed. In [11], the authors es- tablished a basic mathematical analogy between antennas and subcarriers and explained how this similarity can be used for space-time-frequency (STF) coding to exploit the inherent di- versity among both the required subcarriers and antennas si- multaneously . Howev er, their analogy between antennas and subcarriers is only valid from the viewpoint of transmit diver- sity and without concerning multiuser diversity prope rty. In the latter environment, users have to evaluate the channel state and feedback the evaluation information to the transmitter. The ev- ident difference between MIMO channels and OFDM channels is that for diff erent anten nas cross talk always exis ts, which con- stitu tes recei ve div ersit y contri but ing great ly to the perfo rmanc e in MIMO systems, while there is no crosstalk across OFDM subcarriers. In this paper, we apply the mathematical analogy between ante nnas and subca rriers to radio resourc e sche dulin g and get multiple parallel transmit subchannels. We evaluate these sub- channels from the viewpoint of receivers by taking into con- side ration of the receive div ersity combin ation . Based on this parallel subchannels model, we propose an optimal resource scheduling problem across all the subcarriers and antennas by GPS to guarantee minimum tolerant data rate for QoS users.
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8/3/2019 An Integrated Sub Channel Scheduling Algorithm for Adaptive Modulation and AMC in MIMO OFDM Wirelss Systems
Abstract— In this paper, we develop an integrated subchannelscheduling algorithm to maximize the system throughput whileguarantee minimum data rate requirements for multimedia usersin multiuser MIMO-OFDM systems downlink transmission uti-lizing adaptive modulation and coding (AMC) with limited chan-nel state information feedback. By integrated subchannel schedul-
ing, we apply the mathematical equivalence between antennas andsubcarriers in the analysis, getting multiple parallel transmit sub-channels, and then evaluate the channel state from the viewpointof receivers. Joint space-frequency diversity as well as multiuserdiversity is exploited simultaneously by the subchannel allocationalgorithm. A modified proportional fair scheduling is proposedand a fast algorithm for more practical implementation is also pro-posed. By numerical examples, system throughput and fairnesssuperiority of the our scheduling scheme are verified.
I. INTRODUCTION
In recent years, two powerful technologies in physical layer
design: OFDM (Orthogonal Frequency Division Multiplexing)
and MIMO (Multiple Input Multiple Output) provide additional
dimension of freedom for radio resource management in spec-
tral resource and spatial resource. We call it multi-dimensional
radio resource management.
OFDM has been emerging as a promising technology due to
its ability in combating frequency selective fading. In OFDM
systems, a broadband signal is divided and modulated on mul-
tiple narrowband subcarriers. Since the frequency-domain fad-
ing deteriorates the SNR of certain subcarriers, but improves
others’ above the average SNR value, the potential loss of
throughput due to the exclusion of faded subcarriers can be
mitigated by employing higher order modulation and coding
modes on the subcarriers exhibiting high SNR values [1], whichis called Adaptive Modulation and Coding (AMC). In a mul-
tiuser OFDM system, multiuser diversity and frequency diver-
sity may be exploited by assigning subcarriers to the users with
best channel gain [2]. Ref. [3] proposed joint subcarrier and bit
allocation algorithm with minimum total transmission power
for real-time services with fixed data rate. In [4], Generalized
Processor Sharing (GPS) scheduling is integrated in power and
subcarrier allocation to achieve maximum system throughput
and provide fairness to heterogeneous traffic as well.
MIMO have also been attracting much attention because they
have the potential of providing enormous increase in spectral
efficiency of wireless systems [5], [6], [7]. By employing mul-
Fig.3 shows fairness comparison, which demonstrates the
fairness property of the WPF with late update algorithm in
terms of the throughput for each user over a scheduling slot.
We set four persistently backlogged users with weights
½
¾
½
and
¿
¾
. The average throughput for each
user over a past window with the length of Ø
scheduling slots
are calculated (here Ø
is set to 200). As the figure depicts,the throughput for each user is proportional to its weight, ex-
cept small fluctuations due to the channel station fluctuating.
Therefore, the our WPF algorithm can achieve the fairness per-
formance as that under GPS scheduling.
5 10 15 20 25
7
8
9
10
11
12
13
14
15
16
S y s t e m t h
r o u g h p u t
( b p s / H z )
User number
Optimal multiuser allocation
WPF-late updateWPF-immediate update
AA-RRS
Fixed allocation
Fig. 2. Average throughput versus number of users with ZF receiver
5 10 15 20 25
9
10
11
12
13
14
15
16
S y s t e m t h
r o u g h p u t ( b p s / H z )
User number
Optimal multiuser allocation
WPF-immediate updateWPF-late update
Fixed allocation
Fig. 3. Average throughput versus number of users with MMSE receiver
VII. CONCLUSIONS
In this paper, we have developed a resource allocation
method to maximize the system throughput for multiuser
MIMO-OFDM systems downlink transmission with limited
CSI feedback. It has been shown that the proposed GPS-type
schemecanguaranteeminimum data rate requirements for mul-
timedia users, and at the same time make ef ficient resource uti-
lization by exploiting joint space-frequency diversity as well
10 20 30 40 50
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
T h r o u g h p u t a l l o c a t e
d ( b p s )
Time (tc
scheduling slots)
User 1User 2
User 3User 4
Fig. 4. Throughput allocation comparison
as multiuser diversity simultaneously. We have also proposed
a modified proportional fair scheduling algorithm to avoid thecomputational burden. By numerical examples, we have veri-
fied the superiority of the proposed scheme in system through-
put and fairness for QoS users.
REFERENCES
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