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WSEAS TRANSACTIONS on COMMUNICATIONS

Guocai Li, Yaohuan Gong

Study on MIMO Schemes for 3G-LTE DownlinkGuocai Li and Yaohuan Gong School of Electronic Engineering University of Electronic Science and Technology of China No.4, Section 2, North Jianshe Road, 610054, Chengdu, CHINA liguocai@uestc.edu.cn,http://www.uestc.edu.cn/Abstract: - The combination of MIMO and OFDM becomes a key standardization work for the downlink channel in 3G Long-Term Evolution (3G-LTE). This paper studies one of the core techniques of 3G-LTE downlink: the coding and decoding schemes in a time-variable multipath environment. It discusses the MIMO schemes for 3G-LTE downlink, the STBC/SFBC, SFTC/STTC and Group layered SFC coding-decoding schemes. This paper also discusses channel estimation scheme based on pilot symbol assistance (PSA), including the pilot pattern and the channel tracking algorithms. The computation complexity, frequency efficiency, BER performance and the effects of channel estimation are analyzed under multi-path fading channel environments. Then we give computer simulation results for the coding-decoding schemes. We believe that results will provide beneficial information to design the 3G-LTE downlink channels. Key-Words: 3G long-term Evolution(3G-LTE), MIMO-OFDM- STBC/SFBC, SFTC/STTC, Group layered SFC

1 IntroductionMultipath fading channel and spectrum efficiency are the two severe challenges for 3G Long-Term Evolution (3G-LTE). 3G-LTE Data transmission in downlink is based on OFDMA. OFDM has been widely studied and it appears as the preferred multiple access schemes for 3G-LTE downlink. OFDM converts the wideband frequency-selective fading channel into multiple flat fading one. It is a popular modulation choice for many applications for its intrinsic ability to handle the most common distortions encountered in a wireless environment, without requiring complex reception algorithms. For independent fading of the users who occupied by the different subcarriers, OFDMA exploits multiuser diversity to meet users QoS requirement. Many OFDM like schemes have been proposed for 3G-LTE, such as MC-WCDMA, MC-TD-SCDMA, OFDMA, SC-FDMA, etc. Furthermore, MIMO (multiple-inputmultipleoutput) smart antenna systems have the ability to turn multipath propagation, traditionally a pitfall of wireless transmission, into a beneficial factor for wireless communications. MIMO improves spectral efficiency by exploiting the rich scattering of RF signals which is typical for indoor and urban environments. There are mainly two ways to handle the wideband MIMO: MIMO wideband equalization and combination of MIMO and OFDM. MIMO wideband equalization is quite complex to

implement. MIMO combined with OFDM substantially reduces the complexity of spatialtemporal processing. Then, MIMO-OFDM is a more effective solution for 3G-LTE. Data transmission is based on MIMO-OFDM has been widely studied. However, the situation in 3GLTE downlink is less clear considering the tradeoff among coding-decoding, channel estimation, the tracking of time varying and performance. In this paper, we discussed the 3G-LTE physical layer in the downlink direction, including the STBC/SFBC, SFTC/STTC, Group layered SFC coding-decoding schemes and channel estimation based on pilot symbol assistance (PSA). Their basic performance and computation complexity in time varying channel are simulated and compared. The rest of the paper is structured as follows. Section 2 gives a briefly description of the LTE downlink physical (PHY) layer with MIMO. The MIMO coding-decoding schemes, the PSA pilot pattern and channel tracking schemes are presented and analyzed in Section 3. In Section 4, simulation results are presented. Finally, Section 5 gathers the conclusions and future work.

2 3GPP-LTE Downlink ModelThe MIMO-OFDM system model for Space Time/Space Frequency (ST/SF) Coding in 3G-LTE downlink is shown in Fig 1 and Fig 2.

ISSN: 1109-2742

883

Issue 8, Volume 8, August 2009

WSEAS TRANSACTIONS on COMMUNICATIONS

Guocai Li, Yaohuan Gong

Pilot SymbolS1[n, k ]Frame Forming

other user data

s1[n, m]

P/S

IFFT

S/P

ST/SF Coding

sM T [n, m]S M T [ n, k ]

P/S

IFFT

S/P

Frame Forming

Pilot Symbol

Other user data

Fig 1 ST/SF Coding MIMO scheme for LTE downlink (transmitter)

y1[n, m]Remove guard symbol Pilot symbol recover

Y1[n, k ]

FFT

S/P

P/S

ST/SF Decoding

y M R [ n, m ]Remove guard symbol

Pilot symbol recover

YM R [n, k ]

FFT

Fig 2 ST/SF Coding MIMO scheme for LTE downlink (Receiver) Suppose that the system is equipped with M T transmitting (Tx) antennas and M R receiving (Rx) antennas. The output signal from IFFT can be described as sq [ n, m] in time domain For the nth OFDM symbol, the channel response between the qth transmitter antenna and the ith receiver antenna can be described as

sq [n, m] =

1 N

Sq [n, k ]ek =0

N 1

j

2 km N

The signal with guard interval ( N g ) is

s [n, m + N ] m = N g , , 1 sq [n, m] = q m = 0 N 1 (2) sq [n, m]

S/P

P/S(1)

Channel Estimate Pilot Symbol

h i , q [n] = [hi ,q [n, 0], hi ,q [n,1],

, hi , q [n, L 1]]T

(3)

where i = 1, 2, , M R q = 1, 2, , M T and L is the channel order. The received signal after matched filtering and the guard symbol removed are

ISSN: 1109-2742

884

Issue 8, Volume 8, August 2009

WSEAS TRANSACTIONS on COMMUNICATIONS

Guocai Li, Yaohuan Gong

yi [n, m] = hi , q [n, l ]sq [n, m l ] + vi [n, m]2 k ( m l ) j 1 hi , q [n, l ]S q [n, k ]e N (4) N l =0 q =1 k =0 +vi [n, m]

L 1 M T

=

l = 0 q =1 L 1 M T N 1

Many STC-STD algorithms have been presented to apply for a wideband system, especially for the 3G-LTE.

3.1 STBC SchemeAll of the STBC schemes could be applied to MIMO-OFDM system. Assuming that the number of OFDM sub carriers is N , the interval of input signal is Ts , then the interval of data block is NTs . Suppose that two successive data vectors are S1 and

where vi [ n, m] is the AWGN noise with zero mean and variance of V .2

When the guard interval exceeds the maximum time delay of the channel ( N g > L ), the InterSymbol-Interference (ISI) can be neglected and the received signal in frequency domain can be described as

S 2 , respectively, where S1 = [ S1 (0),

, S1 ( N 1)]T ,

Yi [n, k ] = yi [n, m]em=0

N 1

j

2 km N

S 2 = [ S2 (0), , S 2 ( N 1)]T and the length of the vector is N . Based on Alamoutis design [5],during the first symbol transmission interval, two vectors of signals S1 and S 2 are transmitted from antenna 1 and 2, respectively. In the second interval * S* and S1 are transmitted. 2 The received signals at the kth subcarrier of ith antenna at the first symbol period and next period are

2 kl MT j L 1 = hi ,q [n, l ]e N S q [n, k ] + Vi [n, k ] (5) q =1 l = 0

= H i , q [n, k ]Sq [n, k ] + Vi [n, k ] (i = 1, 2,whereq =1

MT

, M R , k = 0,1,L 1 l =0 j

, N 1)2 kl N

Yi ,1[k ] =(6)

H i ,q [n, k ] = hi , q [n, l ]eSuppose input

H i ,1[k ]S1[k ] + H i ,2 [k ]S2 [k ] + i ,1[k ], Yi ,2 [k ] = H i ,1[k ]S [k ] + H i ,2 [k ]S [k ] + i ,2 [k ],* 2 * 1

S I = [ S I [0], S I [1],

information vector is , S I [Q 1]] and N > M T L ;

(11)

The output of the encoder is S = [S 0 , S1 ,Yk = Es H (ej 2 k K

, S N 1 ] ,(7)

(i {1, 2,

, M R }, )

The received signal vector of the kth carrier is)S k + n k

The receiver constructs two decision statistics based on the linear combination of the received signals

The maximum likelihood decoding algorithm is employed:

S1[k ] = Hi =1 MR

MR

* i ,1

[k ]Yi ,1[k ] + H i ,2 [k ]Y *i ,2 [k ] (12)i =1 MR

MR

S = arg min || Yk Es H (eSl =0

N 1

j 2

k N

)S k ||

2

(8)

S 2 [k ] = H *i ,2 [k ]Yi ,1[k ] H i ,1[k ]Y *i ,2 [k ] (13)i =1 i =1

3 MIMO Coding Schemes for LTEThe typical applications of MIMO are spatial multiplex and spatial diversity. The multiplex system (such as BLAST) increases the transmit rate, while the diversity system could increase the physical link reliability. In all cases of MIMO system, there are given multiplex gain and diversity gain. Increasing the number of Tx antennas provides a significant performance improvement. However, the decoding complexity becomes very high for a large number of Tx antenna.

These are the input of the maximum likelihood decoder. Substituting (11) into (12) and (13):

S1[k ] = H i , q [k ] S1[k ]2 i =1 q =1 * i ,1

M R MT

+Hi =1

MR

[k ] i ,1[k ] + H i ,2 [k ]i =1 2

MR

(14)* i ,2

[k ]

S 2 [k ] = H i , q [k ] S2 [k ]i =1 q =1

M R MT

+Hi =1

MR

* i ,2

[k ] i ,1[k ] H i ,1[k ]i =1

MR

(15)* i ,2

[k ]

ISSN: 1109-2742

885

Issue 8, Volume 8, August 2009

WSEAS TRANSACTIONS on COMMUNICATIONS

Guocai Li, Yaohuan Gong

It can be seen that, for transmit signal pairs of

S1[k ] S 2 [k ] , the decision statistics input is the linear combinations of M R M T signals. When thefading of the channel between any Tx and Rx antenna pair is mutually independent, the scheme can achieve a full transmit diversity of M R M T . The average symbol error probability (SER) can be expressed as

Yn ,i = H n ,i ,1S n ,1 + H n,i ,2S n,2 + Z n,i

(21)

where