OFDM technology is a powerful modulation technique that can be used for many applications. It is suitable for a wide range of wireless communication systems including cellular networks, wireless local area networks, digital audio broadcasting, and wireless multimedia networks. The future scope for OFDM technology is very promising. It can be used in emerging technologies such as 5G, the Internet of Things (IoT), and smart grids. It can also be used to improve the performance of existing systems such as Wi-Fi, WiMAX, and 3G/4G cellular networks by allowing higher data rates and better spectral efficiency.
In recent years, the use of OFDM (Orthogonal Frequency Division Multiplexing) has come to the fore with its acceptance in each area of communications standards such as Wireless Local Area Networks (WLAN) standards, Worldwide Interoperability for Microwave Access (WiMAX), Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB). It is acknowledged in Wireless Communications due to its multi-fold benefits, such as high bandwidth efficiency and resistance to multi-path fading. In wireless communication systems, the signal energy experiences scattering and reflection from the objects present in the communications environment, thus, giving birth to the challenge of receiving reliable data links with low power levels at the receiver side. The signal also experiences channel fading owing to the multi-path reflections caused by the interfering objects. The solution to this problem is OFDM based communications systems.
What is Orthogonal Frequency Division Multiplexing (OFDM)?
OFDM or Orthogonal Frequency Division Multiplexing technique is the widespread choice to reduce multipath fading effects and to provide massive data rates. This, in turn, decreases the multipath distortion and frequency interference. While classical communications divided the allocated bandwidth into multiple applications or channels, OFDM is a form of ‘non-trivial’ frequency-division multiplexing, that splits the bandwidth assigned to a single application. Therefore, OFDM can be referred to as a multicarrier transmission technique, where the splitting of bits takes place at the input stream and the input stream is split into various parallel streams that are then utilized to modulate different carriers. The underlying idea behind the OFDM is to divide the allocated spectrum or bandwidth into several narrowband subcarriers. Although the spectra of the modulated signals overlap in the frequency domain, they remain orthogonal to each other.
Over the years, OFDM has become the choice of the physical layer in many of the wireless standards we use today. This is mostly due to the optimized hardware implementations using Fast Fourier Transform (FFT) that make it an ideal choice for practical implementations. OFDM has been adopted for terrestrial broadcasting, and cellular and broadband wireless standards. It is shown in the figure below. The signal is divided into both the time and the frequency domains.
Frequency division increases the channel's capacity, while time division ensures a reduction in Inter Symbol Interference (ISI) or provides less interference to the adjacent symbols. In accordance with this, the symbols in the time domain have a guard interval between them.
In a classical multipath communication system, many copies of the same symbol are received at the receiver after experiencing various delays, which results in ISI. To combat this effect, OFDM systems make use of the concept of a Cyclic Prefix (CP) or guard interval, where a defined number of samples from the end of the data stream are added to the start of the OFDM symbol in the time domain. In addition to the guard interval, a guard band is also realized by setting a few narrowband subcarriers at the beginning and towards the end of the allocated spectrum to zero. The guard band not only simplifies spectrum shaping but also serves the role of preventing aliasing on the outer subcarriers.
Coded Orthogonal Frequency Division Multiplexing (COFDM)
Coded OFDM is an important variant of OFDM where the information bits are first error protected, then OFDM modulated and then transmitted. This type of scheme is specifically used for Digital Video Broadcasting (DVB-T2) standards. The DVB-T2 standard specifically offers two types of Forward Error Coding (FEC) schemes to ensure robustness against the effects of the channel. These are the Low-Density Parity-Check (LDPC) inner code scheme and the Bose–Chaudhuri–Hocquenghem (BCH) outer code scheme. They are used to ensure superior performance in an environment with high noise levels and interference. The inner LDPC code supports the generation of long codes that have a powerful and robust error correction capability. On the other hand, outer BCH codes reduce the error floor of the LDPC code.
The block diagram of COFDM is shown in the figure above. The information bits are first encoded using FEC schemes. The encoded bits are modulated and then transformed to the time domain. Finally, a cyclic prefix is added to every COFDM signal before transmission.
Characteristics of OFDM
Orthogonality: OFDM is a special form of Frequency-division multiplexing with all the subcarrier signals within a communication channel being orthogonal to each other. In OFDM, the crosstalk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly contributes to the simplification of the transmitter and receiver. A separate filter for each sub-channel is not required in OFDM systems.
Guard Interval: In wireless communication systems, the primary issues are dealing with self-interference (or Inter symbol interference or ISI) and multi-path fading, which occur when the same signal arrives at a receiver via different paths. The solution to prevent multipath errors is to transmit a short block of data (a symbol) and to determine the time when all the multipath echoes fade and then, send another symbol. This waiting time is called the guard interval. In the environment of multi-path effects, the time period of the guard interval is directly proportional to the robustness of the system. However, during the guard interval, the system gets no benefit of the available spectrum. So, the effective channel capacity is reduced. There is no denying the fact that some amount of guard interval is necessary for any wireless system, but the overall aim of the system shall be to minimize that interval and maximize the symbol transmission time. This challenge is met in OFDM systems by dividing transmissions among multiple subcarriers. The same guard interval can then be applied to each subcarrier, while the symbol transmission time is multiplied by the number of subcarriers.
Adaptive Transmission: The effectiveness of the channel capacity can be further improved if information about the channel is sent over a return channel. Based on this feedback information, it can be determined if a particular range of frequencies suffers from interference or attenuation; the carriers within that range can then be disabled or made to run slower by applying more robust modulation or error coding to those subcarriers. To cope with this situation, Discrete Multitone Modulation (DMT) is used in OFDM-based communication systems that adapt the transmission to the channel conditions individually for each subcarrier, using ‘bit-loading.’ In this technique, the upstream and downstream speeds can be varied by allocating either more or fewer carriers for each purpose.
OFDM Extension with Multiple Access: OFDM can be combined with multiple access using time, frequency, or coding separation of the users.In orthogonal frequency-division multiple access (OFDMA), frequency-division multiple access is achieved by assigning different OFDM sub-channels to different users. In multi-carrier code-division multiple access (MC-CDMA), also known as OFDM-CDMA, OFDM is combined with CDMA spread spectrum communication for coding separation of the users.
Space Diversity: In OFDM-based communication systems, receivers can take advantage by receiving signals from several spatially dispersed transmitters simultaneously, since only destructive interference is done between the transmitters on a limited number of subcarriers. In reality, the coverage area is increased, and the outage probability is decreased. This is due to the increased received signal strength averaged over all subcarriers. OFDM may also be combined with other forms of space diversity, for example, antenna arrays and MIMO channels.
Advantages of OFDM
Classical wireless communication systems adhere to strict spectral masks. However, the use of OFDM makes spectral shaping easier and more efficient to implement.
OFDM technique increases the symbol duration and makes the system more resilient to the effects of multipath fading. With the use of a cyclic prefix, Inter Symbol Interference (ISI) can be reduced.
A separate filter is not needed for the subcarriers, so a simplified transmitter and receiver design is aided.
OFDM systems make the equalization process easier.
OFDM is a perfect choice for battery power consumer devices due to the efficient implementation of the FFT algorithm.
Disadvantages of OFDM
OFDM signals lie on the idea of orthogonality between the narrowband subcarriers. However, in the presence of Doppler shifts, the subcarriers are no longer orthogonal to each other, which can result in Inter Carrier Interference (ICI) and can affect the performance of an OFDM system.
OFDM signals have a variation in amplitude because of the linear combination of the individually modulating subcarriers. Therefore, in some instances, this could result in the combination of subcarriers either in-phase or out-of-phase, which results in a high Peak to Average Power Ratio (PAPR). This means that a highly linear amplifier would be needed at the transmitter in order to avoid distortion. This can pose a problem because amplifiers with a large (or linear) working range consume more power.
Applications of OFDM
Digital radio, digital audio broadcasting, and satellite radio.
Digital television standards, Digital Video Broadcasting-Terrestrial/Handheld (DVB-T/H), DVB-Cable 2 (DVB-C2).
Wired data transmission, Asymmetric Digital Subscriber Line (ADSL), and cable internet providers. Fiber optic transmission may use either OFDM signals or several distinct frequencies as FDM.
Wireless LAN (WLAN) data transmission. All Wi-Fi systems use OFDM, including IEEE 802.11a/b/g/n/ac/ax. OFDM has also used in Metropolitan Area Network (MAN) IEEE 802.16 WiMAX networks.
OFDM is also used in cellular data, Long-Term Evolution (LTE) and 4G cellphone networks. It is also an integral part of 5G NR cellular deployments.
US Patent Analysis
The patent data in this article shows information related to OFDM based communication systems, including the top-rated assignees, and patent filing trends across the globe and in recent years.
It has been noticed that the bigger players in this field are Qualcomm, followed by LG Electronics and Samsung Electronics. Other top companies which contribute to this technology are Ericsson, Huawei, Intel, and Apple. Qualcomm has been at the forefront of OFDM wireless communications and has been actively involved in the development of the technology since the late 1990s and has invested significant resources into the development of OFDM-based products and solutions. As a result, Qualcomm has a large portfolio of patent filings in the area of OFDM wireless communications. This includes patents covering the design of OFDM-based systems, components, methods, and technologies, as well as patents covering specific implementations of OFDM-based systems and components.
The graph shown below represents the number of applications related to the OFDM-based communication systems filed in the last ten years. It is noticed that the number of patent filings has increased significantly in 2019. This is due to the growing demand for high-speed wireless communications in the same year. With the advent of 5G networks and other wireless technologies, the need for more efficient wireless communication had increased, leading to an increase in research and development in this area. Furthermore, the introduction of new standards for OFDM wireless communications such as LTE-Advanced and Wi-Fi 6 had also led to an increase in the number of patent filings. However, the dip in the number of patent filings in OFDM wireless communications after 2019 may be due to several reasons. The emergence of software-defined radios (SDRs) and cognitive radios (CRs) have also affected the demand for OFDM technologies. Furthermore, the rapid development of IoT and smart devices had led to a shift in the focus from OFDM-based communication systems to other low-power, low-cost communication systems. Finally, the cost of developing and patenting OFDM-based technologies may have become too high for some companies, leading to a decrease in the number of patent filings.
Conclusions
OFDM technology is a powerful modulation technique that can be used for many applications. It is suitable for a wide range of wireless communication systems including cellular networks, wireless local area networks, digital audio broadcasting, and wireless multimedia networks. The future scope for OFDM technology is very promising. It can be used in emerging technologies such as 5G, the Internet of Things (IoT), and smart grids. It can also be used to improve the performance of existing systems such as Wi-Fi, WiMAX, and 3G/4G cellular networks by allowing higher data rates and better spectral efficiency. Additionally, its robustness against various types of fading can be further improved by introducing advanced signal processing techniques. With the increasing demand for higher data rates, better spectral efficiency, and improved reliability, OFDM is expected to play a significant role in future wireless communication systems.
