The use of millimeter-wave (mmWave) spectrum up to 52.6 gigahertz (GHz) is the key to enabling higher data throughput. At these higher frequencies, more contiguous spectrum is available to send more data through the channel.   A maximum carrier bandwidth of up to 400 megahertz (MHz) and up to 16 component carriers (CCs), and allows multiple carriers to be combined to create up to 800 MHz of channel bandwidth.

For higher capacities, faster speeds, lower latency and more system capacity to be realized, additional cell sites need to be installed. The additional cell sites will be in the form of enhanced Macro Cells, Small Cells and in-building Distributed Antenna Systems (DAS) networks.

One of the key elements of any radio communications system is the way in which radio communications are maintained in both directions. Terms including simplex, duplex, frequency division duplex, FDD, and time division duplex, TDD, are all methods that can be used.

For cellular systems it is necessary that it is possible to talk or send data in both directions simultaneously, and this places a number of constraints on the schemes that may be used to control the transmission flow. As it is such a key element of the system, it is necessary to settle on the scheme that will be used from outset. As a result the duplex scheme to be used forms a very basic part of the overall specification for the cellular (or any radio communications system) that is to be used.

The different schemes for controlling the transmission range from simplex through half duplex to full duplex. Furthermore, schemes such as TDD and FDD need to be defined for the system depending upon its application and the traffic it is likely to carry. Many aspects of the performance will be governed by aspects such as whether FDD or TDD is used.

Transmission control schemes

There are a variety of different ways of controlling the two way passage of information using two transmitters. They range in complexity from the simplest systems requiring the least complex circuitry and providing more basic performance, to more complex systems providing higher levels of performance. However each scheme has its own advantages and disadvantages.

• Simplex:   Although the definition of simplex is not always clear the ANSI (American National Standards Institute) definition for a simplex transmission, is one that can only occur in one direction. One example of this may be a broadcast system. Occasionally simplex may refer to a half duplex scheme as described below.
• Half duplex:   This is a duplex scheme whereby communication is possible in two directions, but communication is only possible in one direction at a time. If one transmitter is transmitting, the other one must wait until the first stops before transmitting. This form of communication is used for walkie-talkies, CB, etc. It may also be referred to as Simplex, in some circumstances although exact definitions can be contradictory at times.
• Full duplex:   Full duplex, which is sometimes referred to simply as duplex is a scheme whereby transmissions may be sent in both directions simultaneously. However it sis till necessary for the transmissions to be separated in some way to enable the receivers to receive signals at the same time as transmissions are being made. There are two ways of achieving this. One is to use frequency separation (frequency division duplex, FDD, and the other is to use time, time division duplex, TDD.

The two schemes are both widely used. Some cellular systems use TDD while others use FDD. Some standards also allow for the use of either as both FDD and TDD have their own advantages and disadvantages.

For the FDD scheme to operate satisfactorily, it is necessary that the frequency, i.e. channel separation between the transmission and reception frequencies must be sufficient to enable the receiver not to be unduly affected by the transmitter signal. This is known as the guard band.

Receiver blocking is an important issue with FDD schemes, and often highly selective filters may be required. For cellular systems using FDD, filters are required within the base station and also the handset to ensure sufficient isolation of the transmitter signal without desensitising the receiver. While cost is not such a significant driver for the base stations, placing a filter into the handsets is more of an issue.

The use of an FDD system does enable true simultaneous transmission and reception of signals. However two channels are required and this may not always use the available spectrum efficiently.

The spectrum used for FDD systems is allocated by the regulatory authorities. As there is a frequency separation between the uplink and downlink directions, it is not normally possible to reallocate spectrum to change the balance between the capacity of the uplink and downlink directions if there are changing capacity requirements for each direction.Time division duplex, TDD

The other system uses only a single frequency and it shares the channel between transmission and reception, spacing them apart by multiplexing the two signals on a time basis. Time Division Duplex, TDD, is used with data transmissions (data or digitised voice), transmitting a short burst of data in each direction. As the transmission periods are relatively short no time delay is noticed on voice transmissions resulting from the time delays introduced by using TDD.

TDD - time division duplex

While FDD transmissions require a guard band between the transmitter and receiver frequencies, TDD schemes require a guard time or guard interval between transmission and reception. This must be sufficient to allow the signals travelling from the remote transmitter to arrive before a transmission is started and the receiver inhibited. Although this delay is relatively short, when changing between transmission and reception many times a second, even a small guard time can reduce the efficiency of the system as a percentage of the time must be used for the guard interval. For systems communicating over short distances, e.g. up to a mile or so the guard interval is normally small and acceptable. For greater distances it may become an issue.

It is often found that traffic in both directions is not balanced. Typically there is more data travelling in the downlink direction of a cellular telecommunications system. This means that, ideally, the capacity should be greater in the downlink direction. Using a TDD system it is possible to change the capacity in either direction relatively easily by changing the number of time slots allocated to each direction. Often this is dynamically configurable so it can be altered to match the demand.

A further aspect to be noted with TDD transmissions is the aspect of latency. As data may not be able to be routed immediately onto a transmission as a result of the time multiplexing between transmit and receive, there will be a small delay between the data being generated and it being actually transmitted. Typically this may be a few milliseconds dependent upon the frame times, but in some applications it may be of interest, although for normal digitised speak, there would be no noticeable delay.

Transmission directions

It is often necessary to distinguish between the link from the mobile to the base station, and the link from the base station to the mobile. This is often necessary not only when talking of the duplex schemes in use.

There are obviously two links:

• Downlink, or forward link:   This is the link from the base station to the mobile or user equipment. The words may be abbreviated to DL or D/L.
• Uplink, or reverse link:   This is the link from the mobile or user equipment to the base station. The words uplink may be abbreviated to UL or U/L.

Although the definitions of transmission direction in this format are generally sued for cellular communications, they may also be used in other areas where a base station and mobile or remote equipments are in use.

TDD FDD Comparison

Both TDD and FDD have their advantages and each can be used to advantage in different circumstances. Before deciding on a particular type of duplex scheme, it is necessary to analyse the advantages and disadvantages of each. An FDD TDD comparison will then determine the best option.

Comparison of TDD and FDD duplex schemes Attribute TDD FDD Spectrum usage Uses only a single frequency for transmission and reception Requires one channel for transmission and another for reception. Spectral efficiency may not be as good. Unbalanced traffic It is possible to easily adjust the capacity in either direction by changing the number of slots dedicated to either direction. This can be achieved dynamically within the protocols of the system. Capacity in either direction can only be made by re-allocating channels. This is not normally easy to achieve as allocations are made by regulators specifically for uplink or downlink with sufficient spacing, and are normally balanced providing the same capacity in either direction. Distance Is normally suited to small distances as guard time increases with distance as signal propagation time increases and this needs to be accommodated. Signals take 3.3 µs to travel a kilometre and 5.4 µs to travel 1 mile. Does not have a problem with small or large distances. Latency A small degree of additional latency may be added as a result of the TDD multiplexing. FDD introduces no additional time delays and latency as channels are always "open". Equipment costs No major additional equipment costs required as transmit-receive switching is cheap to effect. Filters are normally required to prevent the transmitter block and de-sensitising the receiver. These costs can be a cost driver in items such as cellular handsets where volumes are high.

In view of this TDD FDD comparison, TDD systems are often used in scenarios where short distances are required, with the possibility of unbalanced data traffic. FDD schemes are better over greater distances and where the traffic is balanced, i.e. similar in both directions.

Both TDD and FDD duplex schemes have their own advantages and disadvantages. Accordingly they are used in different applications, or in different areas where the advantages of TDD and FDD can be used to the greatest advantage. In view of the advantages of unbalanced uplinks and downlinks in short range cellular and wireless applications, TDD solutions are finding an increasing number of applications, while FDD systems are still in widespread use where the there are different requirements.

Transmission control schemes

There are a variety of different ways of controlling the two way passage of information using two transmitters. They range in complexity from the simplest systems requiring the least complex circuitry and providing more basic performance, to more complex systems providing higher levels of performance. However each scheme has its own advantages and disadvantages.

• Simplex:   Although the definition of simplex is not always clear the ANSI (American National Standards Institute) definition for a simplex transmission, is one that can only occur in one direction. One example of this may be a broadcast system. Occasionally simplex may refer to a half duplex scheme as described below.
• Half duplex:   This is a duplex scheme whereby communication is possible in two directions, but communication is only possible in one direction at a time. If one transmitter is transmitting, the other one must wait until the first stops before transmitting. This form of communication is used for walkie-talkies, CB, etc. It may also be referred to as Simplex, in some circumstances although exact definitions can be contradictory at times.
• Full duplex:   Full duplex, which is sometimes referred to simply as duplex is a scheme whereby transmissions may be sent in both directions simultaneously. However it sis till necessary for the transmissions to be separated in some way to enable the receivers to receive signals at the same time as transmissions are being made. There are two ways of achieving this. One is to use frequency separation (frequency division duplex, FDD, and the other is to use time, time division duplex, TDD.

The two schemes are both widely used. Some cellular systems use TDD while others use FDD. Some standards also allow for the use of either as both FDD and TDD have their own advantages and disadvantages.

Frequency division duplex, FDD

Frequency division duplex, FDD, uses the idea that the transmission and reception of signals are achieved simultaneously using two different frequencies. Using FDD it is possible to transmit and receive signals simultaneously as the receiver is not tuned to the same frequency as the transmitter.

FDD - frequency division duplex

For the FDD scheme to operate satisfactorily, it is necessary that the frequency, i.e. channel separation between the transmission and reception frequencies must be sufficient to enable the receiver not to be unduly affected by the transmitter signal. This is known as the guard band.

Receiver blocking is an important issue with FDD schemes, and often highly selective filters may be required. For cellular systems using FDD, filters are required within the base station and also the handset to ensure sufficient isolation of the transmitter signal without desensitising the receiver. While cost is not such a significant driver for the base stations, placing a filter into the handsets is more of an issue.

The use of an FDD system does enable true simultaneous transmission and reception of signals. However two channels are required and this may not always use the available spectrum efficiently.

The spectrum used for FDD systems is allocated by the regulatory authorities. As there is a frequency separation between the uplink and downlink directions, it is not normally possible to reallocate spectrum to change the balance between the capacity of the uplink and downlink directions if there are changing capacity requirements for each direction.Time division duplex, TDD

The other system uses only a single frequency and it shares the channel between transmission and reception, spacing them apart by multiplexing the two signals on a time basis. Time Division Duplex, TDD, is used with data transmissions (data or digitised voice), transmitting a short burst of data in each direction. As the transmission periods are relatively short no time delay is noticed on voice transmissions resulting from the time delays introduced by using TDD.

TDD - time division duplex

While FDD transmissions require a guard band between the transmitter and receiver frequencies, TDD schemes require a guard time or guard interval between transmission and reception. This must be sufficient to allow the signals travelling from the remote transmitter to arrive before a transmission is started and the receiver inhibited. Although this delay is relatively short, when changing between transmission and reception many times a second, even a small guard time can reduce the efficiency of the system as a percentage of the time must be used for the guard interval. For systems communicating over short distances, e.g. up to a mile or so the guard interval is normally small and acceptable. For greater distances it may become an issue.

The guard interval required for TDD will comprise two main elements:

• A time allowance for the propagation delay for any transmissions from the remote transmitter to arrive at the receiver. This will depend upon the distances involved, but it takes 3.3 microseconds to travel a kilometre, 5.4 microseconds to travel 1 mile.
• A time allowance for the transmitter receiver to change from receive to transmit. Switching speeds can vary considerably between equipments but can take a few microseconds.

As a result, TDD is not normally suitable for use over long distances as the guard time increases and the channel efficiency falls. Also transmit receive switching must be fast.

It is often found that traffic in both directions is not balanced. Typically there is more data travelling in the downlink direction of a cellular telecommunications system. This means that, ideally, the capacity should be greater in the downlink direction. Using a TDD system it is possible to change the capacity in either direction relatively easily by changing the number of time slots allocated to each direction. Often this is dynamically configurable so it can be altered to match the demand.

A further aspect to be noted with TDD transmissions is the aspect of latency. As data may not be able to be routed immediately onto a transmission as a result of the time multiplexing between transmit and receive, there will be a small delay between the data being generated and it being actually transmitted. Typically this may be a few milliseconds dependent upon the frame times, but in some applications it may be of interest, although for normal digitised speak, there would be no noticeable delay.

Transmission directions

It is often necessary to distinguish between the link from the mobile to the base station, and the link from the base station to the mobile. This is often necessary not only when talking of the duplex schemes in use.

There are obviously two links:

• Downlink, or forward link:   This is the link from the base station to the mobile or user equipment. The words may be abbreviated to DL or D/L.
• Uplink, or reverse link:   This is the link from the mobile or user equipment to the base station. The words uplink may be abbreviated to UL or U/L.

Although the definitions of transmission direction in this format are generally sued for cellular communications, they may also be used in other areas where a base station and mobile or remote equipments are in use.

TDD FDD Comparison

Both TDD and FDD have their advantages and each can be used to advantage in different circumstances. Before deciding on a particular type of duplex scheme, it is necessary to analyse the advantages and disadvantages of each. An FDD TDD comparison will then determine the best option.

Comparison of TDD and FDD duplex schemes Attribute TDD FDD Spectrum usage Uses only a single frequency for transmission and reception Requires one channel for transmission and another for reception. Spectral efficiency may not be as good. Unbalanced traffic It is possible to easily adjust the capacity in either direction by changing the number of slots dedicated to either direction. This can be achieved dynamically within the protocols of the system. Capacity in either direction can only be made by re-allocating channels. This is not normally easy to achieve as allocations are made by regulators specifically for uplink or downlink with sufficient spacing, and are normally balanced providing the same capacity in either direction. Distance Is normally suited to small distances as guard time increases with distance as signal propagation time increases and this needs to be accommodated. Signals take 3.3 µs to travel a kilometre and 5.4 µs to travel 1 mile. Does not have a problem with small or large distances. Latency A small degree of additional latency may be added as a result of the TDD multiplexing. FDD introduces no additional time delays and latency as channels are always "open". Equipment costs No major additional equipment costs required as transmit-receive switching is cheap to effect. Filters are normally required to prevent the transmitter block and de-sensitising the receiver. These costs can be a cost driver in items such as cellular handsets where volumes are high.

In view of this TDD FDD comparison, TDD systems are often used in scenarios where short distances are required, with the possibility of unbalanced data traffic. FDD schemes are better over greater distances and where the traffic is balanced, i.e. similar in both directions.

Both TDD and FDD duplex schemes have their own advantages and disadvantages. Accordingly they are used in different applications, or in different areas where the advantages of TDD and FDD can be used to the greatest advantage. In view of the advantages of unbalanced uplinks and downlinks in short range cellular and wireless applications, TDD solutions are finding an increasing number of applications, while FDD systems are still in widespread use where the there are different requirements.

The Mobile Phone

The mobile phone or cell phone as it is often called is equally important to the network in the operation of the complete cellular telecommunications network. Despite the huge numbers that are made, they still cost a significant amount to manufacture, discounts being offered to users as incentives to use a particular network. Their cost is a reflection of the complexity of the mobile phone electronics. They comprise several different areas of electronics, from radio frequency (RF) to signal processing, and general processing.

The design of a cell phone is particularly challenging. They need to offer high levels of performance, while being able to fit into a very small space, and in addition tot his the electronics circuitry needs to consume very little power so that the life between charges can be maintained.

Radio frequency elements

The radio frequency section of the mobile phone is one of the crucial areas of the cell phone design. This area of the mobile phone contains all the transmitter and receiver circuits. Normally direct conversion techniques are generally used in the design for the mobile phone receiver.

The signal output from the receiver is applied to what is termed an IQ demodulator. Here the data in the form of "In-phase" and "Quadrature" components is applied to the IQ demodulator and the raw data extracted for further processing by the phone.

On the transmit side one of the key elements of the circuit design is to keep the battery consumption to a minimum. For GSM this is not too much of a problem. The modulation used is Gaussian Minimum Shift Keying. This form of signal does not incorporate amplitude variations and accordingly it does not need linear amplifiers. This is a distinct advantage because non linear RF amplifiers are more efficient than linear RF amplifiers.

Unfortunately EDGE uses eight point phase shift keying (8PSK) and this requires a linear RF amplifier. As linear amplifiers consume considerably more current this is a distinct disadvantage. To overcome this problem the design for the mobile phone is organised so that phase information is added to the signal at an early stage of the transmitter chain, and the amplitude information is added at the final amplifier.

Analogue to Digital Conversion

Another crucial area of any mobile phone design is the circuitry that converts the signals between analogue and digital formats that are used in different areas. The radio frequency sections of the design use analogue techniques, whereas the processing is all digital.

The digital / analogue conversion circuitry enables the voice to be converted either from analogue or to digital a digital format for the send path, but also between digital and analogue for the receive path. It also provides functions such as providing analogue voltages to steer the VCO in the synthesizer as well as monitoring of the battery voltage, especially during charging. It also provides the conversion for the audio signals to and from the microphone and earpiece so that they can interface with the digital signal processing functions.

Another function that may sometimes be included in this area of the mobile phone design or within the DSP is that of the voice codecs. As the voice data needs to be compressed to enable it to be contained within the maximum allowable data rate, the signal needs to be digitally compressed. This is undertaken using what is termed a codec.

There are a number of codec schemes that can be used, all of which are generally supported by the base stations. The first one to be used in GSM was known as LPC-RPE (Linear Prediction Coding - Regular Pulse Excitation). However another scheme known as AMR (Adaptive Multi-Rate) is now widely used as it enables the data rate to be further reduced when conditions permit without impairing the speech quality too much. By reducing the speech data rate, further capacity is freed up on the network.

Digital Signal Processing

The DSP components of the mobile phone design undertake all the signal processing. Processes such as the radio frequency filtering and signal conditioning at the lower frequencies are undertaken by this circuitry. In addition to this, equalisation and correction for multipath effects is undertaken in this area of the design.

Although these processors are traditionally current hungry, the current processors enable the signal processing to be undertaken in a far more power effective manner than if analogue circuits are used.

Control processor

The control processor is at the heart of the design of the phone. It controls all the processes occurring in the phone from the MMI (Man machine interface) which monitors the keypad presses and arranging for the information to be displayed on the screen. It also looks after all the other elements of the MMI including all the menus that can be found on the phone.

Another function of the control processor is to manage the interface with the mobile network base station. The software required for this is known as the protocol stack and it enables the phone to register, make and receive calls, terminate them and also handle the handovers that are needed when the phone moves from one cell to the next. Additionally the software formats the data to be transmitted into the correct format with error correction codes included. Accordingly the load on this processor can be quite high, especially when there are interactions with the network.

The protocols used to interact with the network are becoming increasingly complicated with the progression from 2G to 3G. Along with the increasing number of handset applications the load on the processor is increasing. To combat this, the design for this area of the phone circuitry often uses ARM processors. This enables high levels of processing to be achieved for relatively low levels of current drain.

A further application handled by this area of the design of the mobile phone is the monitoring the state pf the battery and control of the charging. In view of the sophisticated monitoring and control required to ensure that the battery is properly charged and the user can be informed about the level of charge left, this is an important area of the design.

Battery

Battery design and technology has moved on considerably in the last few years. This has enabled mobile phones to operate for much longer. Initially nickel cadmium cells were used, but these migrated to nickel-metal-hydride cells and then to lithium ion cells. With phones becoming smaller and requiring to operate for longer from a single charge, the capacity of the battery is very important, and all the time the performance of these cells is being improved.

Although mobile phones are one of the most commonplace pieces of electronics equipment these days, they are nevertheless complicated inside. An understanding of the mobile phone basics can often be useful when looking at the way a cellular network and cellular technology in general works.

Connectivity in both wired and wireless forms is part of everyday life. From wired and fibre broadband to mobile communications - 2G, 3G, 4G and 5G, Wi-Fi, Bluetooth and many other wireless technologies through to standards like Ethernet, USB and many others. Wi-Fi is particularly important as demonstrated by the number of Wi-Fi routers, Wi-Fi repeaters and the like that are available for sale.

Mobile phones / cellular telecommunications

• Cellular / mobile communication basics
• What is 3GPP?
• 2G GSM
  • 2G GPRS
  • 2G GSM EDGE
• IS95 / cdmaOne

• 3G UMTS
  • 3G HSPA
• 4G LTE
  • LTE Advanced
• 5G technology
• 6G - next generation concepts

Wireless Connectivity

This addresses a variety of topics associated with wireless conenctivity. Everything from Wi-Fi, Wi-FI routers and repeaters, etc through to other forms of wireless conenctivity including Bluetooth, LoRa, NFC and many more. With the technology for Smart homes and cities becoming more commonplace, these technologies are being used increasingly.

• WiFi - IEEE 802.11
• Bluetooth
• RFID: radio frequency identification
• NFC: near field communication
• IEEE 802.15.4
• WiMAX

• DECT
• SIGFOX
• LoRa
• Zigbee
• Z-Wave
• Wireless M bus
• WHDI

Wired Connectivity

Although wireless technologies like Wi-Fi are widely used, wired connectivity is important. Ethernet is once such example as it is used for many computer conenctions. Items like Ethernet cables and many more can be found, although with other wired connectivity areas like USB, serial communications and networking solutions like NFV and SDN.

• Erlang
• E carrier
• Optical fibre communications
• VoIP
• Networking fundamentals
• What is the Cloud
• Network, functions virtualization, NFV
• Software defined networking, SDN
• SD-WAN

• ISDN
• Ethernet IEEE 802.3
• USB, Universal Serial Bus
• Serial data standards
  • RS232
  • RS422
  • RS449
  • 20 mA current loop
  • RS485

Radio systems

• Licence-free radio systems
• Land / private mobile radio summary
• TETRA
• DMR

• dPMR
• APCO P25
• NXDN

Orthogonal Frequency Division Multiplex (OFDM)

Orthogonal Frequency Division Multiplex, OFDM is a form of signal format that uses a large number of close spaced carriers that are each modulated with low rate data stream. The close spaced signals would normally be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is shared across all the carriers and this provides resilience against selective fading from multi-path effects.

Read more about OFDM, Orthogonal Frequency Division Multiplexing.

OFDM, Orthogonal Frequency Division Multiplexing is a form of signal waveform or modulation that provides some significant advantages for data links.

Accordingly, OFDM, Orthogonal Frequency Division Multiplexing is used for many of the latest wide bandwidth and high data rate wireless systems including Wi-Fi, cellular telecommunications and many more.

The fact that OFDM uses a large number of carriers, each carrying low bit rate data, means that it is very resilient to selective fading, interference, and multipath effects, as well providing a high degree of spectral efficiency.

Early systems using OFDM found the processing required for the signal format was relatively high, but with advances in technology, OFDM presents few problems in terms of the processing required.

Development of OFDM

The use of OFDM and multicarrier modulation in general has come to the fore in recent years as it provides an ideal platform for wireless data communications transmissions.

However the concept of OFDM technology was first investigated in the 1960s and 1970s during research into methods for reducing interference between closely spaced channels. IN addition to this other requirements needed to achieve error free data transmission in the presence of interference and selective propagation conditions.

Initially the use of OFDM required large levels of processing and accordingly it was not viable for general use.

Some of the first systems to adopt OFDM were digital broadcasting - here OFDM was able to provide a highly reliable form of data transport over a variety of signal path conditions. Once example was DAB digital radio that was introduced in Europe and other countries. It was Norwegian Broadcasting Corporation NRK that launched the first service on 1st June 1995. OFDM was also used for digital television.

Later processing power increased as a result of rising integration levels enabling OFDM to be considered for the 4G mobile communications systems which started to be deployed from around 2009. Also OFDM was adopted for Wi-Fi and a variety of other wireless data systems.

Basic concept of OFDM, Orthogonal Frequency Division Multiplexing

One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will cause interference between the carriers as a result of inter-modulation distortion. This will introduce unwanted signals that would cause interference and impair the orthogonality of the transmission.

In terms of the equipment to be used the high peak to average ratio of multi-carrier systems such as OFDM requires the RF final amplifier on the output of the transmitter to be able to handle the peaks whilst the average power is much lower and this leads to inefficiency. In some systems the peaks are limited. Although this introduces distortion that results in a higher level of data errors, the system can rely on the error correction to remove them.

Guard interval on OFDM signals

The distribution of the data across a large number of carriers in the OFDM signal has some further advantages. Nulls caused by multi-path effects or interference on a given frequency only affect a small number of the carriers, the remaining ones being received correctly. By using error-coding techniques, which does mean adding further data to the transmitted signal, it enables many or all of the corrupted data to be reconstructed within the receiver. This can be done because the error correction code is transmitted in a different part of the signal.

Key features of OFDM

The OFDM scheme differs from traditional FDM in the following interrelated ways:

• Multiple carriers (called subcarriers) carry the information stream
• The subcarriers are orthogonal to each other.
• A guard interval is added to each symbol to minimize the channel delay spread and intersymbol interference.

OFDM, orthogonal frequency division multiplexing has gained a significant presence in the wireless market place. The combination of high data capacity, high spectral efficiency, and its resilience to interference as a result of multi-path effects means that it is ideal for the high data applications that have become a major factor in today's communications scene.

Use of cyclic prefix is a key element of enabling the OFDM signal to operate reliably.

The cyclic prefix acts as a buffer region or guard interval to protect the OFDM signals from intersymbol interference. This can be an issue in some circumstances even with the much lower data rates that are transmitted in the multicarrier OFDM signal.

Frequency offset OFDM synchronization

It is particularly important that the demodulator in an OFDM receiver is able to synchronize accurately with the carriers within the OFDM signal. Offsets may arise for a number of reasons including any frequency errors between the transmitter and the receiver and also as a result of Doppler shifts if there is movement between the transmitter and receiver.

If the frequency synchronisation is impaired, then the orthogonality of the carriers is reduced within the demodulation process and error rates increase. Accordingly it is essential to maintain orthogonality to reduce errors and maintain the performance of the link.

Radio signals and modulation

There are many different forms of radio signal that can be used. From simple forms of modulation like amplitude, frequency and phase modulation, through to complex waveforms using techniques like direct sequence spread spectrum and orthogonal frequency division multiplexing, there are many different techniques and technologies used in radio frequency engineering.

The descriptions of the different types of modulation and waveform are in many cases accompanied by the electronic circuits used for their modulation and demodulation. These are useful for the RF design of circuits that need to process these forms of modulation.