Orthogonal Frequency Division Multiplexing

The Release 10 of LTE is the version that is approved by the ITU-R as an IMT Advanced technology and so is called LTE-Advanced. Release 10 of the LTE that came out in March 2011 included carrier aggregation, enhanced downlink MIMO, uplink MIMO, enhanced ICIC and advanced relays. Following the Release 10 came the Release 11 that incorporated enhanced carrier aggregation and additional intra-band carrier aggregation. In 2008 the work associated with IMT-Advanced within the ITU-R circulated the detailed requirements and process via a circular letter. For 3GPP, LTE advanced is not a new technology but is an evolutionary step in the ongoing development of LTE. To reach the IMT Advanced standards, wider bandwidth by aggregation of various multiple carriers and evolved use of advanced antenna techniques in both downlink and uplink are the two main phenomenon added in the Release 10 of LTE. The transmission scheme used for this technology is called OFDM (Orthogonal Frequency Division Multiplexing) which is also prevalent in other radio-access technologies like WiMax and DVB broadcasting technologies.

There are four major techniques that allocate capacity for wireless WAN communications. These techniques are Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and Orthogonal Frequency Division Multiplexing (OFDM). (Goleniewski, L. 2007) Each technique has pros and cons when compared amongst each other. The following is a summary of why each technique has a need, and some of the challenges that come with that technique.

Standards need to be set in order to handle current capabilities and handle the demands of what is to come. Standards that come with radio communication have been around for a while and one of those standards includes Public safety radio services. This standard was created for a given scenario were the government would have to send out a message to inform the public. Any agencies that deal with public safety are required to meet this standard. You may here these messages come across the radio in states of emergency and see tests on your television. This standard is known as SO-04-005 for communications involving radio. The standard has to meet all of the ANSI/TIA/EIA-102 standards, also has to have the capability of using the existing 800 MHz bandwidth, and has to work with interface devices that are present know and to come in the future.

TDM advantage over FDM is that it offers bandwidth saving and there is low interference between the signals that are being multiplexed.

The applications include wireless local area networks, wireless personal area networks and wireless metropolitan networks. Currently, OFDMA is being investigated as one of the most promising radio transmission techniques for LTE of the 3rd Generation Partnership Project (3GPP), International Mobile Telecommunications-Advanced

According to Weldon (2014), it also provides 4G coverage using different technologies and when connected with the company’s enhanced backhaul, the carrier has the ability to enhance its network capabilities and provide superior mobile broadband speeds for data and video services. Further, AT&T’s wireless network also works with other GSM digital transmission technologies for 3G and 2G data communications (AT&T Inc. Annual Report, 2014). The communication’s service provider continues to expand on the number of locations including airports and restaurants where customers can access broadband Internet connections using wireless fidelity or the radio frequency that is referred to as Wi-Fi

One of the major disadvantages of OFDM is the high Peak – to – Average power ratio of OFDM signal. These multi – carrier systems have a problem that Peak – to – Average power ratio increases with the increase of number of subcarrier, which causes poor efficiency or serious performance degradation to transmit power amplifier. Many PAPR reduction techniques have been proposed. These techniques can be mainly categorized into signal scrambling techniques and signal distortion techniques. In this topic, there is study OFDM system and short view of different PAPR reducing techniques like Clipping, Companding, Peak Windowing, Scrambling, Coding and Selective Mapping (SLM).

In order to maintain the competitiveness of the 3GPP cellular system, Long Term Evolution (LTE), is developed. For downlink transmission the technique selected is Orthogonal frequency division multiple access (OFDMA). In this time and frequency resources are reused in adjacent cells, inter cell interference becomes the crucial limiting factor. This problem can be overcome by using interference mitigation techniques. The

Abstract multiple carriers are simultaneously modulated using Orthogonal frequency-division multiplexing (OFDM) which is a transmission technique. Since the OFDM-based system has high spectral efficiency and is robust against intersymbol interference and frequency-selective fading channels. The major problem of this system is high peak-to-average power ratio (PAPR) of transmitted signal which causes distortion of signal.

Abstract- A paradigm for multiple-input multiple-output (MIMO) channel modeling demands diversity technique to compensate the effects of fading and shadowing. In this paper, error performance of MIMO system is analyzed with orthogonal space time block code (OSTBC) and maximal ratio combining (MRC) techniques over Weibull-gamma (WG) fading channel. WG distribution forms a composite channel model which is used to determine the effects of multipath as well as shadowing. Closed form expressions for symbol error rate (SER) are computed for MIMO systems using OSTBC and MRC in this scenario. Consequently, the effects of fading severity by varying shape parameters are also shown by simulation and analytical results.

Keywords— Orthogonal frequency division multiplexing (OFDM), Generalized K-fading, binary phase shift keying (BPSK), 16-qardature amplitude modulation (16-QAM), symbol error rate (SER).

Code division multiple access (CDMA) is a technology which is used for radio communication purpose. Multiple access technique is a broad area for communication purpose , CDMA is an example of multiple access technology, through which several number of transmitter can send information in a single communication channel. In case of CDMA a particular bandwidth can shared between several number of users. To permit this technique to achieve without undue interference between users, spread spectrum technology and a special coding scheme are used by CDMA.

Orthogonal frequency division multiplexing is considered as highly successful communication model compares to conventional communication models because of low sensitivity to multipath propagation and eminent spectral efficiency. OFDM too

Both wimax and LTE uses OFDM (Orthogonal Frequency Division Multiplexing). OFDM uses orthogonal frequencies as carrier frequencies so that there will be no interference between the signals i.e. it uses the principle of orthogonality. This in turn improves the spectral efficiency.

The main objective of this project was to successfully implement transceiver that would help in developing 802.11 n modem. The architecture required the realization of 802.16 (Wi-max) and LTE transceivers. It intended to study various alternatives for implementation of the OFDM which resulted in reduced chip size. It included developing a complete system of indigenously developed 802.11n modem which then offered maximum throughput, robust and highly scalable WLAN connections/networks.