ANTENNA TECHNOLOGIES IN WIMAX

Advanced antenna technologies specified in the WiMAX system to mitigate the non-LOS propagation problems and ensure high quality signal receptions include Diversity and multiple-input multiple-output (MIMO) systems, adaptive antenna systems (AAS), as well as beamforming systems.

1 Diversity Systems

Diversity technique provides the receiver with multiple copies of the transmitted signal, each of them received over independently fading wireless channel. The notion of diversity relies on the fact that with M independently fading replicas of the transmitted signals available at the receiver, the probability of an error detection is improved to p^M, where p is the probability that each signal will fade below a usable level. The link error probability is therefore improved without increasing the transmitted power. Recently, the use of diversity technique at the transmitter side also gained wide attentions, and has resulted in the consideration of the more general case of multiple transmit–multiple receiving antennas or MIMO systems.

2 MIMO Systems

The two options for MIMO transmissions in the WiMAX standard are space-time codes and multiplexing. For space-time codes, both space-time trellis codes and Alamouti space-time block codes are specified. However, it is the Alamouti space-time block codes that has yet been implemented by vendors due to its reduced complexity (eventhough space-time trellis code has better link performance improvements). In the Alamouti scheme designed for two transmitting antennas, a pair of symbol is transmitted at a time instant, and a transformed version of the symbols are transmitted in the next time instant. At the receiver, the decoder detects the four symbols transmitted over two time slots and processes them to obtain 2-branch diversity gain. Thus the Alamouti scheme achieves full diversity, with a rate-1 code. For the multiplexing option, the multiple antennas are used for capacity increase. In this option, original high-rate stream is partitioned into N low-rate substreams and each substream is transmitted in parallel over the same channel, using different antennas. If there are enough scatterers between the transmitter and the receiver, adequate MIMO detection algorithms like zero-forcing, minimum mean-square error (MMSE), or vertical Bell labs Layered Architecture for space-time codes (V-BLAST), etc., can be designed to separate the substreams. Thus the link capacity (theoretic upper-bound on the throughput) is increased linearly with min(N,M), where N is the number of transmit and M is the number of receiving antennas.

3 MIMO Systems with Antenna Selection

For MIMO systems to be deployed on mobile WiMAX devices, the concept of antenna selection is very essential. Because RF chain dominates the link budget in wireless systems, mobile devices are unable to implement large numbers of RF chains to incorporate high order MIMO systems. For such systems therefore, a reduced numbers of RF chains are implemented and antennas with the best received energies are adaptively selected and switched on to the implemented RF chains for MIMO signal processings, as illustrated in Figure 1. The performance of such system has been studied quite elaborately in the literature, in comparison to the full complexity system that utilizes all available antennas. It was shown that the diversity gain performance is maintained in the reduced-complexity system despite the use of antenna selection, while the coding gain deteriorates proportional to the ratio of the selected antennas to the total available antennas.

 
Figure 1: MIMO subset antenna selection.

4 MIMO Technologies in IEEE 802.16m Standard

The IEEE 802.16 standards committee has recently initiated the process of extending the existing IEEE 802.16e standard (mobile WiMAX) for high capacity, high-QoS mobile application. The new standard was dubbed IEEE 802.16m at the IEEE January session in London, 2008. The working group tasked with the responsibility of producing the working documents for the new standard was named task group m (TGm). The group hopes to complete the specification for the new standard by the end of 2009. When completed, the standard will be backward compatible with IEEE 802.16e, and interoperable with 4G cellular standards supporting the IMT-advanced technologies. Although the details of the IEEE 802.16m standard is not available at the moment, the most important features being touted for the standard include

• Target downstream speed of 100 Mbps in highly mobile mode, and upto 1 Gbps in normadic mode (upstream rate are not yet known, but would be at least at par with 802.16e).
• Channel sizes upto 40 MHz (802.16e currently supports upto 20 MHz channel size).
• Use of TDD and FDD.
• Backward-compatibility with 802.16e.
• OFDMA radio (same as in 802.16e).
• Mandatory MIMO antenna technology of size 4 × 4 (four transmitting, and four receiving antennas).

In contrast to the IEEE 802.16e, which supports mandatory MIMO antenna technology of size 2 × 2 (two transmitting, and two receiving antennas), the use of mandatory higher-capacity MIMO technology in 802.16m will provide extra capacity to support the targeted high-speed in the downstream. Since downstream has been the bottleneck in wireless services, this improvement will provide significant boost in system capacity, to enable the system support wide range of multimedia services expected in 4-G compatible technologies.

PRESENT–FUTURE TECHNOLOGIES

Over the last two years IEEE 802.16-2004 products have become more widely used and with several deployments in the field, basic experience with WiMAX technology has been increased. The new, upcoming IEEE 802.16e, which is an amendment to the IEEE 802.16-2004 standard, promises significant improvements. The major enhancements in WiMAX technology for the upcoming version of the standard are outlined in Table 1.

Table 1: Evolution of IEEE 802.16 Technology 

Technology	IEEE 802.16d	IEEE 802.16e	IEEE 802.16j
Range (km)	0.7	1.8	2 (per hop)
AAS	–	STC/MRC/SM/BF	Cooperation
Air-interface	OFDM	OFDMA	Relay-based
QoS	Basic	E-rtPS	Enhanced
Profile	IP Padios	ASN	ASN
Scenarios	Nomadic	Mobile	Mobile relaying

The most important enhancement concerns the system gain, which is roughly increased by 15–25 dB, and hence the cell range is also increased. For fixed/nomadic terminals it is evident that 802.16e is much more efficient and is also capable of catering for mobile terminals. Another decisive improvement is the introduction of AAS, which increases robustness through STC, MRC, and spectral efficiency through spatial multiplexing (SM) and BF. The transition to OFDMA clearly boosts the radio resource management efficiency and improves QoS particularly in the presence of VoIP service. It is evident that the evolution of WiMAX targets three objectives: to increase the system gain and reach customers inside their homes/offices and on the move, to boost capacity so as to reduce service costs and be competitive with other access technologies, and finally to coexist in a seamless manner in the upcoming all IP networks.

This is clearly the case with the development IEEE 802.16j which introduces the concept of relays. A relay-based network can in principle extend the range boundlessly, however, in practice 2-hop links are more likely to be implemented (for delay and throughput issues). From the designer’s perspective if the first link (BS-relay) is line of sight then the system range can be several times higher than conventional systems, and hence for coverage-limited networks the dimensioning would result in much reduced costs. Furthermore, if the BS employs BF toward the relays, concurrent communication with several of them may be established in the form of spatial-division multiple access (SDMA), therefore boosting cell capacity.

While the all IP architecture is on the way (access service network (ASN) gateway), with major manufacturers of network products supporting this direction, the next WiMAX standard, IEEE 802.16m is also under consideration. IEEE 802.16m will revise the air-interface in the scope of international telecommunications union–radiocommunication sector (ITU-R) requirements for IMT-2000 and IMT-Advanced.

COMPARISON WITH COMPETING TECHNOLOGIES

At some point current 2G and 3G network operators will migrate to a 4G network technology. Mobile WiMAX is likely to face competition from 3G and 4G technology enhancements. They include the code division multiple access (CDMA) variants CDMA2000 and wideband-CDMA (WCDMA) and their enhancements which are 1x evolution data optimized (1xEVDO) and HSDPA, respectively. Unlike in the early days of the CDMA vs. GSM competition, this higher generation competition will be quite different and fruitful because for these new generations networks; the applications are separated and do not depend on each other. 4G networks will go far beyond 2G and 3G by mainly improving three parameters:

• Interface technology: 4G standards will make a radical change and will use OFDM [9]. The new modulation itself will not automatically bring an increase in speed but very much simplifies the following two enhancements:
• Channel bandwidth: 4G systems will use a bandwidth of up to 20 MHz, i.e., the channel offers four times more bandwidth than channels of current systems. As 20 MHz channels might not be available everywhere, most 4G systems will be scalable, e.g., in steps of 1.25 MHz. It can therefore be expected that 4G channel sizes will range from 5 to 20 MHz.
• MIMO: The idea of MIMO is to use the multipath phenomena. Although this behavior is often not desired, MIMO makes active use of it by using several antennas at the sender and receiver side, which allows the exchange of multiple data streams, each over a single individual wave front. Two or even four antennas are foreseen to be used in a device. How well this works is still to be determined in practice but it is likely that MIMO can increase throughput by a factor of two in urban environments.

Increasing channel size and using MIMO will increase throughput by about 8–10 times. Thus speeds of 40 Mbps per sector of a cell are thus possible. Using a commonly accepted evaluation methodology for 3G systems, mobile WiMAX has been simulated against the 3G enhancements [23]. These simulations have shown that

• Mobile WiMAX peak data rates are up to 5x better than 3G+ technologies.
• Mobile WiMAX spectral efficiency is 3x better than any 3G+ technology.
• Lower equipment cost for WiMAX due to certified products (compare with WiFi).
• WiMAX requires new infrastructure while high-speed packet access (HSPA) rides on UMTS.
• Roughly the same coverage (average ~5 km).
• Roughly the same performance (average ~2 Mbps per user).
• HSDPA launched in 2006 while HSUPA will come in 2008.
• WiMAX standard set end of 2005 and first products in 2006.
• HSPA has a higher acceptance with mobile operator.

TECHNOLOGIES EMPLOYED BY WiMAX

Mobile WiMAX operates in licensed frequency bands in the range of 2 to 6 MHz. The technologies employed by mobile WiMAX include the following:

• Scalable orthogonal frequency division multiple access (SOFDMA) on the physical layer
• MIMO
• IP
• Adaptive antenna systems (AAS)
• Adaptive modulation schemes (AMS)
• Advanced encryption standard (AES) encryption
  
  

PHYSICAL LAYER

Mobile WiMAX will initially operate in the 2.3, 2.5, 3.3, and 3.4–3.8 GHz spectrum bands using SOFDMA. OFDMA is perhaps the most important technology associated with WiMAX. SOFDMA is based on OFDMA which in turn is based on OFDM. OFDM is a form of frequency division multiplexing, but it has higher spectral efficiency and resistance to multipath fading and path loss compared to other multiplexing methods. It divides the allocated frequency spectrum into subcarriers which are at right angles to each other. This reduces the possibility of cross-channel interference thereby allowing the subcarriers to overlap. This reduces the amount of frequency spectrum required, hence the high spectral efficiency. The reduced data rate of each stream reduces the possibility of intersymbol interference because there is more time between the arrival of symbols from different paths. This feature of OFDM makes it resistant to multipath fading and ideal for nonline of sight (NLOS) applications. In OFDMA each frequency subcarrier is divided into subchannels which can be accessed by multiple users hence increasing the capacity of OFDM.

Scalable OFDMA is a form of OFDMA which allows variable channel bandwidth allocation from 1.25 to 20 MHz. SOFDMA has capabilities which make it ideal for the implementation of IP and hybrid automatic repeat request (HARQ). WiMAX also uses other features to enhance the performance of OFDMA. They include dynamic frequency shifting, MIMO, AAS, and software-defined radios. Dynamic frequency shifting monitors the signal and changes frequencies to avoid interference. Software-defined radios are controlled by changing software settings and this gives the equipment more flexibility when switching frequencies.

MIMO is a technology that has already found use in WiFi (IEEE 802.11n). MIMO multiplies the point-to-point spectral efficiency by using multiple antennas and RF chains at both the BS and the MS. MIMO achieves a multiplicative increase in throughput compared to single-input, single-output (SISO) architecture by carefully coding the transmitted signal across antennas, OFDM symbols, and frequency tones. These gains are achieved at no cost in bandwidth or transmit power.

AAS are spatial processing systems which combine antenna arrays with sophisticated signal processing. They reduce the effects of interference from multiple signal paths thereby also contributing to high capacity of the system and the use of mobile WiMAX in NLOS environments.

MAC SUBLAYER

The 802.16 MAC sublayer uses a scheduling algorithm for which the subscriber station only needs to compete for initial entry into the network. The scheduling algorithm also allows the BS to control QoS parameters by balancing the time-slot assignments among the application needs of the subscriber stations.

WiMAX supports QoS differentiation for different types of applications. The 802.16 standard defines the following types of services:

• Unsolicited grant services (UGS): UGS is designed to support constant bit rate (CBR) services, such as T1/E1 emulation, and Voice-over-IP (VoIP) without silence suppression.
• Real-time polling services (rtPS): rtPS is designed to support real-time services that generate variable size data packets on a periodic basis, such as MPEG video or VoIP with silence suppression.
• Nonreal-time polling services (nrtPS): nrtPS is designed to support nonreal-time services that require variable size data grant burst types on a regular basis.
• Best effort (BE) services: BE services are typically provided by the Internet today for Web surfing.

TECHNOLOGICAL STRENGTHS OF WiMAX TO ADDRESS QoS

WiMAX is designed with QoS in mind and it has some underlying technological strengths that help it offer improved QoS. Some of these strengths are outlined in this section.

1 WIMAX PHY LAYER

In WiMAX, the upstream PHY layer consists of time division multiple access (TDMA) and demand assigned multiple access (DAMA). For TDMA, the channel for upstream communication is divided into multiple time slots and the access of time slots for various clients is governed by the MAC layer at the receiver end. The time slots allocated for various clients can be varied depending on demands. The downstream traffic can be continuous time division multiplexing (TDM) or burst mode transfer. In continuous TDM, data for various clients is multiplexed onto the same stream and is received by all clients at the same coverage sector. For bursty data, bursts are sent to the receiver in a similar fashion to the TDMA upstream burst. With time slots-based communication, overheads due to contentions and collisions can be reduced significantly, which can improve the QoS.

The modulation used in WiMAX is the orthogonal frequency division multiplexing (OFDM). WiMAX OFDM features multiple subcarriers ranging from a minimum of 256 up to 2048, each modulated with either BPSK, QPSK, 16 QAM, or 64 QAM modulation. The advantage of orthogonality is that it minimizes self-interference, a major source of error in received signals in wireless communications. WiMAX supports different signal bandwidths ranging from 1.25 to 20 MHz to facilitate transmission over longer ranges in different multipath environments. Multipath signals, another limiting factor for higher sustained throughput in wireless communications, specially when the terminal nodes have the mobility, are caused by reflections between a transmitter and receiver whereby the reflections arrive at the receiver at different times. 

Interference caused by multipath tends to be highly problematic when the delay spread, the time span separating the reflection, is on the order of the transmitted symbol time. For WiMAX, due to its OFDMA, symbol times tend to be in the order of 100 µs, which makes multipath less of a problem. Moreover, in WiMAX, a guardband of about 10 µs, called the cyclic prefix, is inserted after each symbol to mitigate the effect of multipath. Another feature of WiMAX PHY is the use of advanced multiantenna signal processing techniques, mainly in the form of multiple input multiple output (MIMO) processing and beamforming. For MIMO, the received signal from one transmitting antenna can be quite different to the received signal from a second antenna, a common scenario in indoor or dense metropolitan areas where there are many reflections and multipaths between the transmitter and the receiver. In such cases, a different signal can be transmitted from each antenna at the same frequency and still be recovered at the receiver by signal processing.

Beamforming, on the other hand, attempts to form a coherent construction of the multiple transmitters at the receiver, which can ultimately offer a higher SNR at the receiver resulting in higher bandwidth or longer range communication. In WiMAX, it is also possible to combine both MIMO and beamforming in cases like 4-antenna systems.

All these features in the WiMAX PHY layer contribute to higher throughput and stability at the receiver end, which makes WiMAX an excellent platform to deliver a predefined level of QoS. With improved throughput and stability, management of QoS is considerably easier in WiMAX compared to other similar wireless standards. Increased throughput, however, does not ensure guaranteed QoS, and bandwidth management is another crucial part that plays a big role for maintaining QoS. This is where WiMAX MAC comes into action.

2 WIMAX MAC

WiMAX MAC is designed for the point-to-multipoint wireless communication with the capability to support higher-layer protocols including ATM, IP, and other future protocols. One of the design considerations of WiMAX MAC is to accommodate very high bit rates of the broadband PHY layer, while delivering ATM-compatible QoS at the same time. A connection oriented MAC architecture in WiMAX provides a platform for strong QoS control. MAC uses a scheduling algorithm that enables the subscriber station (SS) to only compete once for initial entry into the network and upon successful entry, the SS is allocated a time slot by the BS. The time slot can increase or decrease according to the needs and it remains assigned to the SS for the whole communication period. The time slot assigned to an SS cannot be used by other subscribers, which makes WiMAX MAC increasingly stable under overload and over-subscriptions. It also works as a key tool for the BS to control QoS by adjusting the time-slot assignments according to the applications’ needs of the SSs.

Adaptive Antenna System (AAS) | Technologies

An adaptive antenna system allows a transmitter to focus radio beams to increase the transmission range, reduce interference and increase signal quality. When an AAS system is used to allow multiple users to communicate with the same transceiver (multiple beams), it is called spatial division multiple access (SDMA). SDMA technology has been successfully used in satellite communications for several years. In some SDMA systems, radio beams may dynamically change with the location of the mobile radio.

The WiMAX system is designed with AAS capability. To support AAS, it is necessary to supplement the medium access control (MAC) protocol with additional commands so that base stations can better monitor subscriber stations which may be operating in a narrow focused beam area. If the subscriber station were to move out of the focused beam area, the system could loose control of the subscriber station.

Figure 1 shows an example of a WiMAX adaptive antenna system (AAS). The cell site can focus radio signals using the same frequency to multiple devices within the same cell site. Focusing of the radio signal allows for an increase in the distance that a cell site can have when communicating with devices. Using AAS technology, the system can adapt the direction of the focused beam to a specific device as it moves throughout the coverage area.

Figure 1: WiMax Adaptive Antenna System

Diversity Transmission

Diversity transmission is the process of using two or more signals to carry the same information source between a transmitter and a receiver. Diversity transmission can use the physical separation of antenna elements (spatial diversity), the use of multiple wavelengths (frequency diversity) and the shifting of time (time diversity).

Protocols on the WiMAX system are designed to take advantage of diversity transmission options and to allow for the use of multiple input multiple output (MIMO) antenna systems. MIMO is the combining or use of two or more radio or telecom transport channels for a communication channel through the user of multiple antenna elements. The use of MIMO to combine alternate transport links provides for higher data transmission rates (inverse multiplexing) and increased reliability (interference control).

Transmission Diversity

Transmission diversity is the process of sending two or more signals from the same information source so a receiver can select or combine the signals to produce a received signal of better quality than a single transmitted signal.

Receive Diversity

Receive diversity is used to select or combine a received signal to yield a stronger signal quality level. Receive diversity uses two antennas that are physically separated vertically or horizontally.

Receiver diversity can compensate for radio signal fading that may occur on a single antenna, and may be performed by maximum ratio combining (MRC) or selection diversity. Maximal ratio combining is the process of combining the signals from two or more antenna elements to increase the level and quality of a received signal. Selection diversity is the process of selecting one antenna from a set of receiving antennas to increase the level or quality of a received signal.

Frequency Diversity

Frequency diversity is the process of receiving a radio signal or components of a radio signal on multiple channels (different frequencies) or over a wide radio channel (wide frequency band) to reduce the effects of radio signal distortions (such as signal fading) that occur on one frequency component but do not occur (or are not as severe) on another frequency component.

Temporal (Time) Diversity

Time diversity is the process of sending the same signal or components of a signal through a communication channel where the same signal is transmitted or received at different times. The reception of two or more of the same signal with time diversity may be used to compare, recover, or add to the overall quality of the received signal.

Spatial Diversity

Spatial diversity is a method of transmission or reception employed to minimize the effects of fading by the simultaneous use of two or more antennas spaced a number of wavelengths apart.

Antenna diversity is a form of spatial diversity that improves the reception of a radio signal by using the signals from two (or more) antennas to minimize the effects of radio signal fading or distortion. Antenna diversity typically requires the antennas to be spaced a number of wavelengths apart.

Space time coding is the adding of time information to transmission carriers to allow diversity operation by identifying and processing multiple carriers of the same signal that may arrive at different times and or from different locations.

Figure 1 shows different types of diversity transmission and reception. The antenna (spatial) diversity utilizes the distance between antennas to improve signal performance. Frequency (spectral) diversity transmits the same or related information on multiple frequency signals to reduce frequency selective fading. Time (temporal) diversity overcomes the challenges of burst distortion by allowing the same information signal to be received at different times.

Figure 1: Diversity Transmission

Modulation | Technologies

Modulation is the process of changing the amplitude, frequency, or phase of a radio frequency carrier signal with the information signal (such as voice or data). The 802.16 system uses different types of digital modulation depending on a variety of transmission factors. The modulation types used in 802.16 systems include binary phase shift keying (BPSK), quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM).

Binary Phase Shift Keying (BPSK)

Binary phase shift keying (BPSK) is a modulation process that converts binary bits into phase shifts of the radio carrier without substantially changing the frequency of the carrier waveform. The phase of a carrier is the relative time of the peaks and valleys of the sine wave relative to the time of an unmodulated “clock” sine wave of the same frequency. BPSK uses only twophase angles, corresponding to a phase shift of zero or a half cycle (zero or 180 degrees of angle). WiMAX uses BPSK modulation when a very robust signal is required.

Quadrature Phase Shift Keying (QPSK)

Quadrature phase shift keying (QPSK) is a type of modulation that uses 4 different phase shifts of a radio carrier signal to represent the digital information signal. These shifts are typically +/- 45 and +/- 135 degrees.

Quadrature Amplitude Modulation (QAM)

QAM is a combination of amplitude modulation (changing the amplitude or voltage of a sine wave to convey information) together with phase modulation. There are several ways to build a QAM modulator. In one process, two modulating signals are derived by special pre-processing from the information bit stream. Two replicas of the carrier frequency sine wave are generated; one is a direct replica and the other is delayed by a quarter of a cycle (90 degrees). Each of the two different derived modulating signals are then used to amplitude modulate one of the two replica carrier sinewaves respectively. The resultant two modulated signals can be added together. The result is a sine wave having a constant unchanging frequency while having an amplitude and phase that both vary to convey the information. At the detector or decoder the original information bit stream can be reconstructed. QAM conveys a higher information bit rate (bits per second) than a BPSK or QPSK signal of the same bandwidth, but is also more affected by interference and noise.

Figure 1 shows that amplitude and phase modulation (QAM) can be combined to form an efficient modulation system. One digital signal changes the phase and another digital signal changes the amplitude.

Figure 1: Quadrature Amplitude Modulation (QAM)

Adaptive Modulation

Adaptive modulation is the process of dynamically adjusting the modulation type of a communication channel based on specific criteria (e.g. interference or data transmission rate). WiMAX systems use adaptive modulation to ensure the modulation type matches the channel characteristics (signal quality level).

In general, the more efficient (data transmission capacity) the modulation type, the more complex or precise the modulation process is. The more precise the modulation process (smaller changes represent digital bits), the more sensitive the modulation is to distortion or interference. This usually means that as the data transmission rate increases, the sensitivity to interference intensifies. To help manage this process and ensure the maximum data transmission rate possible, 802.16 systems automatically change their data modulation types and data transmission rates (Autorate) based on the ability of the channel to transfer data. The 802.16 systems will usually try to send information at the highest data transmission rate possible. If the data transmission rate cannot be maintained, the 802.16 systems will attempt to transmit at the next lower data transmission rate. Lower data transmission rates generally use a less complex (more robust) modulation type.

Frequency Reuse | Technologies

Frequency reuse is the process of using the same radio frequencies on radio transmitter sites within a geographic area that are separated by sufficient distance to cause minimal interference with each other. Frequency reuse allows for a dramatic increase in the number of customers that can be served (capacity) within a geographic area on a limited amount of radio spectrum (limited number of radio channels). Frequency reuse allows WiMAX system operators to reuse the same frequency at different cell sites within their system operating area.

The number of times a frequency can be reused is determined by the amount of interference a radio channel can tolerate from nearby transmitters that are operating on the same frequency (carrier to interference ratio).

Carrier to interference (C/I) level is the amount of interference level from all unwanted interfering signals in comparison to the desired carrier signal. The C/I ratio is commonly expressed in dB. Different types of systems can tolerate different levels of interference dependent on the modulation type and error protection systems. The typical C/I ratio for narrowband mobile radio systems ranges from 9 dB (GSM) to 20 dB (analog cellular). WiMAX systems can be much more tolerant to interference levels (possibly less than 3 dB C/I) when OFDM and adaptive antenna systems are used.

WiMAX systems may also reuse frequencies through the use of cell sectoring. Sectoring is a process of dividing a geographic region (such as a radio coverage area) where the initial geographic area (e.g. cell site coverage area) is divided into smaller coverage areas (sectors) by using focusing equipment (e.g. directional antennas).

Figure 1.20 shows how radio channels (frequencies) in a WiMAX communication system can be reused in towers that have enough distance between them.

Figure 1.20: WiMax Frequency Reuse

The radio channel signal strength decreases exponentially with distance. As a result, mobile radios that are far enough apart can use the same radio channel frequency with minimal interference.

Orthogonal Frequency Division Multiplexing (OFDM)

Some of the key technologies used in WiMAX systems include orthogonal frequency division multiplexing, frequency reuse, adaptive modulation, diversity transmission and adaptive antennas.

Orthogonal Frequency Division Multiplexing (OFDM)

OFDM is a process of transmitting several high speed communication channels through a single communication channel using separate sub-carriers ( frequencies) for each radio channel. The use of OFDM reduces the effects of multi-path and delay spread, which is especially important for lower frequencies and near line of sight (NLOS) transmission.

Multi-path propagation is the transmission of a radio signal which travels over two or more paths from a transmitter to a receiver. Multi-path transmission can cause changes in the received signal level as delayed signals can either add or subtract from the received signal level. Multi-path is not usually a challenge on systems that use higher frequencies as these systems tend to use highly directional (high-gain) antennas for direct line of sight transmission.

Multi-path propagation is frequency dependent meaning that the multiple paths radio signals travel will vary depending on its’ frequency.

Figure 1illustrates how a transmitted signal may travel through multiple paths before reaching its destination. In this example, the same signal is reflected off an office building where it is received by the subscriber device. The reflected signal is delayed (travels a longer path) and subtracts from the direct signal resulting in a dead spot (fade) at the receiver. Furthermore, mutli-path propagation is sensitive to frequency and that distortion occurs at different points when other frequencies are used. When a different frequency is used, the reflected signal is redirected and it does not subtract from the direct signal.

Figure 1: Multi-path Propagation

For a wide radio channel that is divided into several sub-carriers, each subcarrier channel operates at a different frequency and can have different transmission characteristics than other sub-carriers. Because multiple sub-carriers are typically combined for a single subscriber, this can reduce the effects of multi-path fading.

The use of multiple sub-carriers also has the effect of reducing the symbol rate, which can reduce the effects of delay spread. Delay spread is a product of multi-path propagation where symbols become distorted and eventually overlap due to the same signal being received at a different time. It becomes a significant problem in mountainous areas where signals are reflected at great distances. Delay spread can be minimized by either using an equalizer to adjust for the multi-path distortions or to divide a communication channel into sub-carriers (e.g. OFDM) where each sub-carrier transfers data at a much slower data transmission rate thereby reducing the effects of delay spread.

Figure 2 demonstrates how OFDM divides a single radio channel into multiple coded sub-channels. A high-speed digital signal is divided into multiple lower-speed sub channels that are independently from each other and can be individually controlled. The OFDM process allows bits to be sent on multiple sub channels. The channels selected can be varied based on the quality of the sub channel. In this figure, a portion of a sub channel is lost due to a frequency fade. As a result of the OFDM encoding process, the missing bits from one channel can be transmitted on other channels.

Figure 2: Orthogonal Frequency Division Multiplexing (OFDM)