Diversity Techniques

Space-Time Trellis Codes

In chapter 1, we have shown the capacity a MIMO channel, but no system implementation has been mentioned. In this chapter, we will introduce the Space-Time Codes which is one approach to take advantages of MIMO channel and provide high data rate or high link quality. Space-time codes have a variety of different structures, each of them has their one advantages and disadvantages. In this chapter, we will first introduce briefly the three diversities commonly used in communication systems. Then we move on to space-time codes which may combine several of the three diversities. The famous space-time block codes will be briefly introduced, and then we focus on the space-time trellis codes which are the channel codes we used in our proposed system in chapter 4.

2.1 Diversity Techniques

Many channels, especially wireless channels, suffer from attenuation due to destructive addition of multipaths in the propagation media and due to the interference from other users. Severe attenuation makes it impossible for the receiver to determine the transmitted signal unless some less-attenuated replica of the transmitted signal is provided to the receiver. This resource is called diversity and it is an

Chapter 2 Space-Time Trellis Codes

important contributor to reliable wireless communications. According to the domain where diversity is introduced, there are three categories of diversity: time diversity, frequency diversity, space diversity.

2.1.1 Time Diversity

The replicas of transmitted signals are provided to the receiver in the form of redundancy in time domain. Identical messages are transmitted in different time slots, and the receiver would receive several uncorrelated fading signals. To be uncorrelated, the time separation between identical messages must be at least the coherent time of the channel. The definition of the coherent time is the period over which the channel fading process is correlated. In many systems, redundancy in time domain is introduced by the error control coding (ECC), and an interleaver is placed after error control coding to provide time separation greater than the coherent time. However, in the receiver, deinterleaving process introduces message delay.

For slow fading channels, a larger interleaver is required to exceed the coherent time, and therefore, a larger message delay is introduced. This drawback may be vital to some delay-sensitive applications, especially voice applications. Another drawback of this scheme is that there will be a certain bandwidth efficiency loss due to the redundancy in time domain.

2.1.2 Frequency Diversity

The replicas of transmitted signals are provided to the receiver in the form of redundancy in frequency domain. The frequency separation is required to be at least the coherent bandwidth to obtain uncorrelated fading replicas in the receiver. The definition of the coherent bandwidth is similar to the coherent time: the frequency span over which the channel fading property is uncorrelated. Several mature

Chapter 2 Space-Time Trellis Codes

communication systems introduce the frequency diversity to increase the data rate or improve the link quality. Spread spectrum is one example, this technique includes direct sequence spread spectrum (DSSS), frequency hopping, multicarrier modulation, and CDMA systems. A combination of error-control coding and OFDM can also be considered as frequency diversity, because the time diversity provided by ECC has been transferred into frequency domain by OFDM modulator. These techniques use bandwidths that are far more than enough just to provide frequency diversity, thus, like time diversity, it induces a loss in bandwidth efficiency due to the redundancy introduced in frequency domain.

2.1.3 Space Diversity

The replicas of transmitted signals are provided to the receiver in the form of redundancy in spatial domain. It is typically implement using multiple antennas or antenna arrays arranged in space in a certain manner. Therefore, space diversity is also called antenna diversity. The space separation between antennas is required to be at least the coherent distance. Usually, a few wavelengths are enough for the antennas to experience different fadings. One advantage of this technique is that unlike time and frequency diversity, it doesn’t suffer from the loss in bandwidth efficiency. This advantage makes it very attractive to high data rate wireless communications.

2.1.4 Transmit and Receive Diversity

We can further classify space diversity into receive diversity and transmit diversity depending on where the multiple antennas are applied. The receive diversity is adapted in a variety of mobile communication systems with the aim to both suppress co-channel interference and minimize the fading effects. It is reasonable to apply

Chapter 2 Space-Time Trellis Codes

receive diversity at the base station for uplink (from mobiles to base stations) communication because the power requirement and the dimension requirement are easier to meet compared with mobile devices. For example, in GSM systems, multiple antennas are used at the base station to create uplink receive diversity, compensating for the relatively low transmission power from the mobile. For downlink (from base station to mobiles), it is much more difficult to apply receive diversity at the mobiles. Firstly, placing multiple antennas in a portable mobile device is against the public favor in a smaller device. Secondly, multiple antennas mean more power consumption which is also against the public favor in a power-saving device. Therefore, transmit diversity is more adequate for the downlink communication.

However, in contrast to receive diversity which is widely applied in mobile systems, transmit diversity has gained little attention and is less understood. The reason is that it is more difficult to exploit transmit diversity, and the difficulty is because the transmitted signals are mixed up before they arrive the receiver.

Therefore, the receiver requires extra signal processing to separate the transmitted signals before the transmit diversity can be exploited. This signal processing is not always perfect, and may suffer from performance loss. In the next session, we will introduce a transmit diversity technique called “Space-Time Codes”, and in Chapter 4, we will propose a signal processing to separate the transmitted signals.

2.1.5 Combine Different Diversity

Not all forms of diversity can be available at all times. To obtain diversity, the resources providing redundancies must be uncorrelated. Otherwise, you simply obtain the same information twice. For example, in slow fading channels, time diversity is not an option due to large coherent time. When delay spread is small,

Chapter 2 Space-Time Trellis Codes

frequency diversity is not an option due to large coherent bandwidth. When the platform is a small mobile device, space diversity is not suitable due to limited dimensions. Nevertheless, combining different diversities will improve the data rate or link quality if they are available. Transmit diversity and receive diversity can also be combined to provide more advantages.