Orthogonal Frequency Division Multiplexing By Sirisha Nagabhairava

Different Modulation Techniques: FDMA --- a multiplexing technique that uses different frequencies to combine multiple streams of data for transmission over a communications medium. FDM assigns a discrete carrier frequency to each data stream and then combines many modulated carrier frequencies for transmission. For example, television transmitters use FDM to broadcast several channels at once.multiplexingcarriermodulatedbroadcastchannels TDMA --- a type of multiplexing that combines data streams by assigning each stream a different time slot in a set. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel. Within T-Carrier systems, such as T-1 and T-3, TDM combines Pulse Code Modulated (PCM) streams created for each conversation or data streammultiplexingT-1T-3Pulse Code Modulated (PCM) CDMA --- a digital cellular technology that uses spread-spectrum techniques. Unlike competing systems, such as GSM, that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence.cellularGSMTDMAchannel

OFDM OFDM is similar to FDMA in that the multiple user access is achieved by subdividing the available bandwidth into multiple channels, which are then allocated to users. However, OFDM uses the spectrum much more efficiently by spacing the channels much closer together. This is achieved by making all the carriers orthogonal to one another, preventing interference between the closely spaced carriers. Coded Orthogonal Frequency Division Multiplexing (COFDM) is the same as OFDM except that forward error correction is applied to the signal before transmission. This is to overcome errors in the transmission due to lost carriers from frequency selective fading, channel noise and other propagation effects.

The orthogonality of the carriers means that each carrier has an integer number of cycles over a symbol period. Due to this, the spectrum of each carrier has a null at the centre frequency of each of the other carriers in the system. This results in no interference between the carriers, allowing then to be spaced as close as theoretically possible. This overcomes the problem of overhead carrier spacing required in FDMA. Each carrier in an OFDM signal has a very narrow bandwidth (i.e. 1 kHz), thus the resulting symbol rate is low. This results in the signal having a high tolerance to multipath delay spread, as the delay spread must be very long to cause significant inter-symbol interference (e.g. > 100 ms).

OFDM time domain waveforms are chosen such that even though they overlap, mutual orthogonality is maintained. The noise performance of OFDM was found to depend solely on the modulation technique used for modulating each carrier of the signal. The performance of the OFDM signal was found to be the same as for a single carrier system, using the same modulation technique. The minimum signal to noise ratio (SNR) required for BPSK was ~7 dB, where as it was ~12 dB for QPSK and ~25 dB for 16PSK. The only main weak point that was found with using OFDM, was that it is very sensitive to frequency, and phase errors between the transmitter and receiver. The main sources of these errors are frequency stability problems; phase noise of the transmitter; and any frequency offset errors between the transmitter and receiver. This problem can be mostly overcome by synchronizing the clocks between the transmitter and receiver, by designing the system appropriately, or by reducing the number of carriers used.

OFDM Vs CDMA: OFDM performs extremely well compared with CDMA, providing a very high tolerance to multipath delay spread, peak power clipping, and channel noise. In addition to this it provides a high spectral efficiency. CDMA was found to perform poorly in a single cellular system, with each cell only allowing 7-16 simultaneous users in a cell, compared with 128 for OFDM. This was for a 1.25 MHz bandwidth and 19.5 kbps user data rate. This low cell capacity of CDMA was attributed to the use of non- orthogonal codes used in the reverse transmission link, leading to a high level of inter-user interference.

OFDM transmits data as a set of parallel low bandwidth (100 Hz - 50 kHz) carriers. The frequency spacing between the carriers is made to be the reciprocal of the useful symbol period. The resulting carriers are orthogonal to each other provided correct time windowing at the receiver is used. The carriers are independent of each other even though their spectra overlap. OFDM allows for a high spectral efficiency as the carrier power, and modulation scheme can be individually controlled for each carrier. Adaptive modulation Most OFDM systems use a fixed modulation scheme over all carriers for simplicity. However each carrier in a multiuser OFDM system can potentially have a different modulation scheme depending on the channel conditions. Any coherent or differential, phase or amplitude modulation scheme can be used including BPSK, QPSK, 8PSK, 16QAM, 64QAM, etc. Each modulation scheme provides a trade off between spectral efficiency and the bit error rate. The spectral efficiency can be maximised by choosing the highest modulation scheme that will give an acceptable Bit Error Rate (BER).

User allocation Grouped carriers The simplest scheme is to group the carriers allocated to each user. Grouping carriers minimises inter-user interference due to distortion, power level variation and frequency errors. However, grouping the carrier makes the transmission susceptible to fading, as the whole group of carriers can be lost in a null in the spectrum. Adaptive Frequency Hopping A new adaptive hopping technique is proposed such that carrier block hopping is based on the channel conditions. After the radio channel has been characterised each user is allocated carriers which have the best SNR ratio for that user. Since each user will be in a different location the fading pattern will be different for each remote station. The strongest carriers for one user are likely to be different from other users. Thus most users can be allocated the best carriers for them with minimal clashes.

Comb Spread Carriers Carriers can be allocated in a comb pattern, spreading them over the entire system bandwidth. This improves the frequency diversity, preventing all the carriers used by a user being lost in a null in the spectrum. However, this allocation scheme may be susceptible to inter- user interference. This type of user allocation is useful in applications that can not use adaptive hopping. Multiple Transmitter Cells Multiple transmitter cells are particularly suitable for wireless LAN applications. Shadowing makes it difficult to achieve good coverage of a building. Repeaters are often required. In a multiuser OFDM system repeaters could be added where needed, with no additional problems. A multiple transmitter cell could be as simple as a coax running the length of a building corridor with multiple tap off points.

Maximizing Signal Strength for OFDM Inside Buildings Propagation inside buildings suffer from large shadowing and high multipath effects. This is a serious problem for Wireless Local Area Network (WLAN) systems. Shadowing and path loss can be minimized by exploiting the multipath tolerance of Orthogonal Frequency Division Multiplexing (OFDM). This can be achieved by using multiple transmission antennas spread over the area of a WLAN cell. These antennas act as repeaters, transmitting and receiving the same signal at the same time. This decreases the average path loss, but increases the multipath delay spread. The reduced path loss allows an increased system capacity, quality of service or a decrease in intercellular interference in a cellular WLAN.

Adding a Guard Period to OFDM One of the most important properties of OFDM transmissions is its high level of robustness against multipath delay spread. This is a result of the long symbol period used, which minimises the inter-symbol interference. The level of multipath robustness can be further increased by the addition of a guard period between transmitted symbols. The guard period allows time for multipath signals from the pervious symbol to die away before the information from the current symbol is gathered.

By moving to the 5 GHz frequency band and by using OFDM modulation, the a standard provides two key benefits over b. It increases the maximum speed per channel (from 11Mbps to 54 Mbps) and increases the number of non-overlapping channels. These benefits come, however, with some tradeoffs in terms of compatibility and range. Because they operate in different frequency bands, a and b products are not compatible.

A 2.4 GHz b access point, for example, won’t work with a 5 GHz a network card. However, both standards can certainly co-exist. For example, an a user and an b user, using separate access points and clients for each, connected to the same LAN, can operate in the same physical space and share network resources including broadband and internet access. The higher operating frequency of a equates to a relatively shorter range. You will need a larger number of a access points to cover the same area. Hiperlan/2 adaptively changes the forward error correcting coding rate and the carrier modulation scheme (BPSK, QPSK, 16 quadrature amplitude modulation (QAM), 64 QAM) allowing the data rate to be maximized based on the current radio channel characteristics.

OFDM Fast Facts OFDM was invented more than 40 years ago OFDM has been adopted by several standards: Asymmetric Digital Subscriber Line (ADSL) services Digital Audio Broadcast (DAB) IEEE a/g, a, Power Line Networking (HomePlug) VDSL Because OFDM is suitable for high data-rate systems, it is also being considered for the following standards: Fourth generation (4G) wireless services. IEEE n and IEEE

Benefits of OFDM High spectral efficiency Resiliency to RF interference Lower multi-path distortion Spectral efficiency Each of the sub-carriers transporting information are just far enough apart to theoretically avoid interference with each other. This parallel- form of transmission over multiple sub-carriers enables OFDM-based wireless LANs to operate at higher aggregate data rates, such as up to 54 Mbps with IEEE a and g implementations. Lower multi-path distortion (delay spread) OFDM signals typically have a time guard of 800 ns, however, which provides very good performance on channels having delay spreads up to 250 ns. This is good enough for all but the most harsh environments. Delay spread due to multi-path propagation is generally less than 50 ns in homes, 100 ns in offices, and 300 ns in industrial environments. The higher data rates and robust communications of OFDM enable the implementation of wireless LANs and MANs supporting higher-speed applications operating over wider areas, where the environment is somewhat hostile toward radio transmissions.

Applications of OFDM OFDM is the basis for the global standard for asymmetric digital subscriber line (ADSL) and for digital audio broadcasting (DAB) in the European market. In the wireless network space, OFDM is the heart IEEE a, g, and HiperLAN/2. An ideal application for OFDM is to satisfy needs for outdoor wireless point-to-point and point-to-multipoint configurations. In fact, most initial products based on OFDM are providing this capability. OFDM operates well in the typically RF hostile outdoor environment.

An Example of OFDM: IEEE a The IEEE a standard specifies an OFDM Physical Layer that splits an information signal across 52 separate sub-carriers to provide transmission of data at a rate of 6, 9, 12, 18, 24, 36, 48, or 54 Mbps. 6, 12, and 24 Mbps are mandatory for all products. Four of the sub- carriers are pilot sub-carriers that the system uses as a reference to disregard frequency or phase shifts of the signal during transmission. A pseudo binary sequence is sent through the pilot sub-channels to prevent the generation of spectral lines. The remaining 48 sub-carriers provide separate wireless “pathways” for sending the information in a parallel fashion. The resulting sub-carrier frequency spacing is MHz (20MHz/64). The a version of OFDM uses a combination of BPSK (binary phase shift keying), QPSK (quadrature phase shift keying), and QAM (quadrature amplitude modulation), depending on the chosen data rate. With a OFDM modulation, the binary serial signal is divided into groups (symbols) of 1, 2, 4, or 6 bits, depending on data rate chosen, and converted into complex numbers representing applicable constellation points.

Conclusion OFDM’s high degree of spectral efficiency, resiliency to interference and multi-path distortion, and existing inclusions in the leading higher rate wireless LAN standards, provides a strong based for the development of newer broadband wireless networks. OFDM’s New Users. Even in WLANs, OFDM isn't just IEEE a and IEEE g; it's also used in the European Telecommunications Standards Institute (ETSI)'s HiperLAN/2 (high-performance radio LAN 2) standard. In addition, Japan's Mobile Multimedia Access Communications (MMAC) WLAN broadband mobile technology uses OFDM.