Description Of MIMO And OFDM Computer Science

Essay add: 30-03-2016, 19:46   /   Views: 3

In recent years, the demands of communication systems have increased to increase network capacity, data rate and to provide better quality of service. Generally modern communication systems were designed to provide the user high-speed and high-quality access to information, without restriction of time and location [1]. Both wired and wireless solutions are being possible to achieve this goal. Wired technology is more reliable and cheaper than the wireless communication systems, but it confines the user to a fixed point. So the ultimate solution for achieving mobility with ease is wireless communication systems. The cellular mobile, wireless networks and personal communications systems are emerging wireless communication systems based on the radio transmission [1]. These days wireless communication systems are usually used for high speed applications such as video streaming, file uploading and downloading. Now the major problem for these types of systems is that they require a reliable connection so that the requirement can be fulfilled, considering the complex space-time varying wireless environments and limited spectrum of radio frequency [2].

Multiple Input Multiple Output (MIMO) Systems are one of the best technologies which have fulfilled the demanding requirements of important advances. MIMO can be used to increase the transmission data rates, without using extra bandwidth or transmission power and can also define the links ranges. It combats the effect of fading and interference by exploiting antenna diversity and spectral multiplexing gain [3-4]. Formidable technology is formed my combining MIMO with orthogonal frequency division multiplexing (OFDM). A vast research is focused to extract the advantages of a system created by these two technologies [5].

Background and description of MIMO and OFDM

The main reason behind the rapid advancement of the wireless communication field is due to increasing demand of high data rates and better quality of service (QoS) [1]. This escalating demand is fulfilled to some extent using the conventional Single-Input and Single-Output (SISO) system, though it is limited by fading and interference. SISO achieve these targets by increasing the fundamental communication resources such as transmitter power, bandwidth, space and time. Numbers of problem arise due to this and these are:

If the power at the transmitter is increased above the specific level, then the biological threat will be posed [6], and it also reduces the battery life.

It is difficult to construct and afford the linear receiver with sensitivity beyond 30-35db [6].

It is very expensive and difficult to increase channel bandwidth due to costly and limited nature of the frequency spectrum [6].

Due to the reasons discussed above, there is a need of biologically safe and economically efficient techniques, which fulfils the modern wireless systems demands. The "Space diversity technique" was introduced in the early 1960s [7]. In this technique, multiple antennas were deployed on the receiving side in order to receive the multiple copies of the transmitted signal. The output signals of receiving antennas were combined using three different methods, maximal ratio combining, selection combining and equal gain combining to get better performance [8]. The idea of using multiple antennas on the transmitting end was introduced in 1990s, and it is known as transmitter diversity [8]. Until the 1990s, multiple antennas were used either on transmitter end or receiver end to gain the advantage of beamforming and diversity. Beamforming improves the link signal-to-noise ratio (SNR) by focusing the energy in the required direction [9].

In multiple-input multiple-output (MIMO) systems, multiple antennas are used at both transmitter and receiver side. Since the initial work, of Telatar [10], Tarokh [11], Foschini and Gans [12], proposed multiple antenna transmission and reception for achieving improved spectral and power efficiency. According to their research, MIMO systems channel capacity is significantly better than estimated Shannon limit for SISO systems.

In 1993, MIMO was proposed by Paluraj and Kailath for increasing link capacity by taking advantages of random fading and delay spread [13]. The idea of improving the performance of wireless communication system without additional usage of the spectrum was supported by this work. This made avenues beyond diversity, which only mitigate the effect of multipath propagation. MIMO can be seen as an initial step which converted the multipath propagation into a benefit for the communication system. Recently, due to the success of MIMO it is adopted in various standards of wireless communications [9].

MIMO systems are now commonly used in several standards of wireless communication systems. Their use is found in various IEEE standards (802.11 & 802.16) for wireless local area networks and 3rd Group Partnership Project (3GPP) for cellular networks. The IEEE standard 802.11 was introduced for Wi-Fi (wireless fidelity). The IEEE 802.11a was its very first version, and it supports data rate up 11 Mega bits per second (Mbps). The second version (IEEE 802.11b) supports data rate up to 54 Mbps. The newest version (IEEE 802.11n) uses MIMO technology, and due to this it can achieve data rate up to 150 Mbps. MIMO also incorporates in IEEE standard 802.16e for worldwide interoperability for Microwave Access (WiMAX) as well as 3GPP release 7 and 8 standard for long term evolution (LTE) [14].

To transmit data over wireless channel MIMO is used with the number of different modulation techniques such as orthogonal Frequency Division Multiplexing (OFDM), Time Division Multiple Access (TDMA), Time Division Multiple Access (TDMA), or Code Division Multiple Access (CDMA). Research shows at high transmission rate OFDM technique perform best than the rest of modulation technique, due of its processing and implementation is relatively simple.

The idea of parallel data transmission was introduced by Doelz et al. back in 1957 [15]. In classic parallel data transmission scheme, bandwidth was divided into the number of non-overlapping subchannels [16]. Later on in 1960s the concept which lay down, the foundation for OFDM was proposed by Chang [17] and Saltzber [18], they introduce the idea of parallel but partly overlapping subchannels. In the mid-1990s, Alard and Lassalle [19] used OFDM for digital broadcasting systems. These days OFDM is adopted as a core technology for a number of broadband wireless systems such as, WLAN standard (IEEE 802.11a & IEEE 802.11g) [20-21], WMAN standard IEEE 802.16d-2004 [22], and Mobile cellular standard 3GPP LTE [23].

The basic principle behind the OFDM is to split the single high-rate carrier into multiple low-rate carriers [1]. The problem of frequency selective fading is solved using this process because it converts the bandwidth of signal into the number of subchannels and each subchannel is considered as flat fading. Another advantage of multi-carrier system is that it mitigates the complexity of equalisation as compared to single-carrier system [24].

To increase the data rate MIMO system is equipped with multi carrier system which makes it well-suited with OFDM technique that is also associated with narrowband channels for subcarriers. So the combination of both these technologies is used to boost the data rate with the well-organized use of spectrum.

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