Sunday, May 27, 2007

[WiMAX] Fundamentals of WiMAX - (3)

Today we are talking about one of the two core technologies of WiMAX: Multiple Antenna Techniques (let's abbreviate in MAT). Another important one is OFDM, which will be deferred to next topic. Like all 3G systems picked CDMA, we can anticipate that perhaps almost all upcoming 4G systems will employ OFDM and MIMO (Multiple Input Multiple Output) techniques. (Recent an interesting news is that Qualcomm just gave a tough talk on the patents of 4G. )

What can MAT provide? Generally speaking, MAT supports
1) spatial diversity,
2) beamforming,
3) and spatial multiplexing.

In old times (1970s), people only use multiple antenna in receiver owing to the cost and size consideration. In this case, with selection combining or maximal ratio combining (MRC), the performance under fading environment can be improved significantly due to receive diversity.

Later, people noticed that using MAT with beamforming can concentrate the power of the desired signal in a specific direction and lower down the interference to other links away from this direction. This is especially useful for multi user network. They call it smart antenna technique. One example is the TD-SCDMA system in 3G area.

In the late of 1990s, a key breakthrough was made for MAT. Transmit diversity schemes were invented, such as the space-time block code (STBC). Now the promising area of MAT is the spatial multiplexing.

For the receive diversity, you can find it from most of the wireless communication books. So we will focus our concentration on the transmit diversity and beamforming.

1) Transmit diversity
There are two types of transmit diversity, one is open-loop, another one is closed-loop. Open-loop means at the transmitter you don't need a feedback for the channel state from the receiver. On the contrary, closed-loop requires it. Thus you should guarantee the channel is with reciprocity. In some cases, you can assume the channel to be reciprocal, although it isn't.

Open-loop transmit diversity
The most popular open-loop transmit diversity scheme is STBC, where a special coding scheme (called Alamouti code, name of the inventor, who is currently the CTO of Intel's Mobility Wireless Group) is used in the transmitter. In short, a Nt*Nr STBC achieves the same diversity order and data rate as a Nr*Nt receive diversity system with MRC, but with a 10*log_10(Nt) transmit power penalty because of Nt transmit antennas.

For WiMAX or other OFDM-based system, the space/time coding can be coded over adjacent subcarriers rather than time slots.

Closed-loop transmit diversity
In this case, channel state information(CSI) is required at the transmitter. Because channel changes quickly in a highly mobile scenario, it is difficult to track the channel state in time and feedback it to the transmitter. Therefore, closed-loop transmit diversity tends to be suitable for fixed or low-mobility scenarios. Two important closed-loop schemes are: transmit selection diversity (TSD) and linear diversity precoding (LDP).

In TSD, only a subset of Nt transmit antennas are selected with good CSI, thereby comparing with STBC, TSD has less transmit power penalty because fewer transmit signals are sent.

In LDP, a linear precoder is used at the transmitter and a linear postcoder is applied at the receiver. They are used to improve the link reliability and can achieve higher SNR than the open-loop schemes.

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