MIMO-OFDM技术：空间时频编码与无线通信

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"Space-time/frequecy coding for MIMO-OFDM" 在即将来临的第四代（4G）宽带无线通信系统中，多输入多输出（MIMO）无线技术和正交频分复用（OFDM）的结合被认为是支持高数据速率和高性能的关键技术之一。这种结合充分利用了MIMO-OFDM在空间、时间和频率域上的编码能力，极大地提高了在恶劣无线环境中的可靠性和鲁棒性。 MIMO技术允许在同一时间利用多个天线发送和接收数据，通过空间多样性的利用，可以显著提高系统的容量和性能。而OFDM则是一种将高速数据流分解成多个较低速率的子载波的技术，每个子载波在频域上是正交的，这有助于对抗多径衰落和频率选择性衰落，从而改善无线通信的性能。在MIMO-OFDM系统中，空间时频编码（Space-Time/Frequency Coding）进一步增强了系统的性能。这种编码策略结合了空间、时间及频率三个维度的信息处理，通过在这些域上精心设计编码方案，可以提供额外的错误纠正能力和抵抗干扰的能力。例如，它可以采用分集技术来增强信号的可靠性，同时使用编码技术来提高纠错性能。文章中提到，未来的宽带通信系统需具备低复杂度的数据处理、更高的数据速率以及更强的性能。MIMO-OFDM技术正是为了解决无线信道中的非视距传播、时间选择性衰落和频率选择性衰落等挑战。通过使用MIMO天线阵列，可以在空间中创建多个独立的传输路径，即使单个路径受到干扰或衰落，其他路径仍能保持有效的通信。 OFDM技术通过将信号划分为许多小的子载波，降低了每个子载波的带宽，使得它们更容易抵抗频率选择性衰落。同时，MIMO与OFDM的结合可以通过空间和频率的多工操作，实现更高的频谱效率，从而在相同的频谱资源上传输更多的数据。 "Space-time/frequecy coding for MIMO-OFDM" 是一种高级的无线通信技术策略，它结合了MIMO的天线多样性和OFDM的频率多样性，通过在空间、时间和频率三个维度进行编码，以实现更高效、更稳健的数据传输，是4G及未来无线通信系统中的关键技术。

IEEE Wireless Communications • June 2007

OFDM

OFDM is essentially a discrete implementation

of multicarrier modulation, which divides the

transmitted bitstream into many different sub-

streams and sends them over many different

subchannels. Typically, the subchannels are

orthogonal and the number of subchannels are

chosen such that each subchannel has a band-

width much less than the coherence bandwidth

of the channel. Thus, intersymbol interference

(ISI) on each subchannel is very small. For this

reason, OFDM is widely used in many high data

rate wireless systems.

Figure 1a shows a simplified block diagram of

N-tone OFDM system. First, the incoming

bits are mapped to data symbols according to

some modulation scheme such as QPSK or

QAM. Then the serial data stream is converted

into a number of parallel blocks, and each of

them has length-

N. Then, each block of symbols

(including pilot symbols, which are used for

channel estimation or synchronization) will be

forwarded to the IFFT and transformed into an

OFDM signal. After that, the OFDM signal will

be appended with a cyclic prefix by copying the

last

samples to the top of the current OFDM

block. By choosing the length of the cyclic prefix

larger than the maximum path delay of the chan

nel, ISI can be eliminated [11]. Afterward, the

OFDM blocks will be converted to serial signals

and sent out. At the receiver, assuming a perfect

timing and carrier frequency synchronization,

the received signals will be first converted to

parallel signals and then the cyclic prefix will be

removed. After going through the DFT block,

the data symbols are detected with the estimated

channel information. After demodulation, the

transmitted bit stream is recovered.

In broadband time-varying multipath fading

channels, OFDM has some of its own challeng-

ing issues, such as peak-to-average-power ratio

(PAPR) and carrier frequency offset (CFO).

However, much more unique merits of OFDM

make it widely used in wireless applications and

standards. The merits can be generally summa-

rized as follows [11]:

•

OFDM is easy to implement in the digital

domain thanks to the use of DFT.

• OFDM is bandwidth efficient, since the paral-

lel subcarriers are overlapping but orthogonal

to each other without causing interference.

• OFDM is robust to multipath fading thanks to

the use of a cyclic prefix.

• OFDM is insusceptible to most forms of

impulse noise thanks to the parallel transmis

sion.

•

OFDM provides a high flexibility in resource

allocation since it splits the broadband chan-

nel into a number of parallel subchannels.

Thus, various resources (data rate and power)

can be dynamically allocated to different sub-

channels.

MIMO-OFDM SYSTEM MODEL

Future broadband wireless systems should pro-

vide high data rate and high performance over

very challenging channels that may be time-

selective and frequency-selective. The combina-

tion of MIMO and OFDM has the potential of

meeting this stringent requirement since MIMO

can boost the capacity and the diversity and

OFDM can mitigate the detrimental effects due

to multipath fading.

A general MIMO-OFDM system is shown in

Fig. 1b, where

transmit antennas, M

receive

antennas, and

N-tone OFDM are used. First, the

incoming bit stream is mapped into a number of

data symbols via some modulation type such as

QAM. Then a block of

data symbols S = [s

, ⋅⋅⋅, s

] are encoded into a codeword matrix

C of size NT × M

, which will then be sent

through

antennas in T OFDM blocks, each

block consisting of

N subchannels. Specifically,

, c

, ⋅⋅⋅, c

will be transmitted from the jth

transmit antenna in OFDM blocks 1, 2, · · ·,

respectively, where

denotes a vector of length-

N, for all j = 1, 2, · · ·, M

and n = 1, 2, · · ·, T.

The codeword matrix

C can be expressed as

(1)

After appending the cyclic prefix on each

OFDM block,

will be transmitted from the jth

transmit antenna in the

nth OFDM block.

After passing through the MIMO channels,

the received signals will be first sent to the

reverse OFDM block (cyclic prefix removal and

DFT) and then sent to the decoder. If the chan-

nel state information (CSI) is available at the

receiver, the optimal ML detection can be per-

formed.

CODE RATE

Since in Eq. 1 the total number of N

informa

tion symbols are sent over

NT channels where N

channels are used in T times, we can get the













MO M

■ Figure 1. A simplified block diagram of MIMO-OFDM system, where S =

, s

, ⋅⋅⋅, s

] denotes a block of Ns data symbols.

OFDM

MOD

P/S

Fad

ing

channel

Pilot

insertion

Cyclic

prefix

S/P IFF

OFDM

MOD

Coding

(a) An OFDM transceiver

(b) A MIMO-O

FDM transceiver

Digital

modulation

Raw

data

Decoding

S/P

Channel

estimation

Remove

cyclic

prefix

P/S FF

Digital

demodulation

Output

OFDM

DEMOD

OFDM

DEMOD

LETAIEF LAYOUT 6/6/07 12:19 PM Page 34

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