Broadband Wireless Networks

Mobile communication facilities and the installation of Internet Protocol (IP) are involved in the development of the main applications that are provided in the so-called Information Society. These applications based on mobility and Internet Protocol (IP) date back to several decades. For example, Internet Protocol dates back to the sixties and mobile communications and their first conceptualizations hark back to the forties.

Mobile communications have evolved over three generations:

a) The first generation, based on the many different analog standards. b) The second generation, involving the introduction of digital standards such as GSM, TDMA, and CDMA, with the subsequent introduction of low-capacity short-message applications. c) The third generation, which is in the process of being developed and whose main standards are WCDMA and CDMA2000.

Nevertheless, the appearance of new technologies and the resulting new architectures and devices associated with them, as well as the powerful influence of the Internet Protocol, have led to new developments in the field of wireless communications, especially with respect to broadband data services. At present, in some textbooks the concept of fourth generation (4G) is being mentioned as the integration of wireless networks under the paradigm of supporting the IP protocol and the combination of solutions based on the IEEE 802.11 and IEEE 802.16 standards.

In this context, new challenges are emerging such as the evolution of mobile networks toward next-generation networks, known as NGN.

As for the rest of the article, it focuses on aspects of interest of broadband wireless access technologies, referred to as BWA.

1.2     Broadband Wireless Access Technology

The term broadband wireless access encompasses all wireless applications and technologies, whether mobile or fixed. In this framework, the following technologies have been identified:

• IEEE 802.15a, or UWB - IEEE 802.11, referred to as Wi-Fi, as well as its corresponding extensions IEEE 802.11a, IEEE 802.11b, IEEE 802.11, and IEEE 802.11h.

• IEEE 802.16, referred to as WiMax, as well as its corresponding extensions IEEE 802.16d, IEEE 802.16e.

UWB: Better known as Ultra Wide Band, it is personal type of technology, capable of delivering effective speeds of up to 480 Mb/s at distances that are less than or equal to 10 meters.

This technology is used for the interconnection of peripherals to a central computer and makes it possible to administer multiple high-definition video signals simultaneously.

Wi-Fi: It is based on the IEEE 802.11 standard and its corresponding extensions. This standard has been widely developed by government and business sectors, airports, private local area networks, among others. Its installation is based on the use of picocells, or cells with a smaller range.

A breakdown of the principal extensions of this technology and its principal characteristic is provided below:

One of the principal constraints is the number of customers who can be served simultaneously, the coverage distances from the terminal equipment, with respect to the RF access port, as well as the availability of a pure electromagnetic spectrum, in the case of unlicensed bands.

Wi-Max: It is an emerging technology that will make it possible to provide a last-mile wireless solution with radio ranges on the order of 19 km under near line of sight (NLOS) conditions and up to 50 km under line of sight (LOS) conditions.

It is expected that this technology shall be developed in three phases. The first phase should support the IEEE 802.16d-2004 standard, by the use of an outdoor antenna and aimed at customers with a fixed location. Phase two is sustained by the same specification but is based on terminal user equipment with indoor antennas, making expeditious supply and activation possible.

The third phase is supported by the IEEE-802.16e specification. This variant shall permit the end user’s mobility within the platform’s coverage area.

MBWA: Referred to as Mobile Broadband Wireless Access, it is a standard parallel to Wi-Max, which appears as a competitor of IEEE 802.16e. This standard is aimed at being positioned as a broadband technology, with ranges of up to 12 km and transmission speeds on the order of 1.5 Mbit/s to 128 Kbit/s.

A summary table, containing the principal technological characteristics, is presented below:

2.0     IEEE 802.16 Standard

The principal technical aspects of the IEEE 802.16 standard, especially with reference to its propagation characteristics shall be addressed below.

2.1     Background to the IEEE 802.16 Standard

The principal precursors related to the development of the IEEE 802.16 standard are briefly described below:

1. In August 1988, the IEEE 802.16 standard was presented as the outcome of the meeting of N-West, known as the U.S. National Institute of Standard and Technology, resulting in the establishment of the IEEE 802.16 Working Group.

2. The aspects of specifying the radio interface is delegated in the Subgroup of the Broadband Wireless Access, known as BWA.

3. In October 2001, the first version of IEEE16 was approved, defining the over-the-air interface and the protocol controlling access to the medium for wireless metropolitan access networks (WMAN).

4. The new version of the IEEE 802.16a standard is known as IEEE 802.16d, which was adopted in June 2004.

5. At present, the IEEE 802.16e standard is being specified. It is estimated that it will support mobile access at speeds on the order of 100 to 120 km/h.

6. An industrial forum of the standard called Wi-Max has been created. This forum has identified various frequency bands for these products, namely: 2.5 to 2.69 GHz, 3.4 to 2.5 GHz and unlicensed segments of 5.725 to 5.850 GHz.

2.2      Reference Model

The reference model associated with the IEEE 802.16d standard, which is comprised of a physical layer and a MAC layer, is shown below:

Fig.1. Reference model

As indicated in Figure 1, the MAC layer is subdivided into three layers, namely:

Convergence sublayer (CS), which makes it possible to map or transform the data coming from external networks by means of the CS service access point, called the CS SAP, to MAC-type service data units (MAC-SDUs).

The data service units (SDU) are received by the common part sublayer (CPS) through the MAC service access point (SAP). In this layer, each SDU flow is classified and is associated with a service flow identifier known as SFID and a connection identifier (CID); in addition, the head of the useful load can be suppressed from this layer.

The MAC common part sublayer (CPS) brings together the functionalities of access to the system, broadband administration, establishment of connections, and maintenance of the connection.

The MAC layer also contains a separate security layer that makes it possible to provide authentication, exchange of keys and ciphering.

The MAC layer exchanges information that should be sent to the physical medium by the physical access point (PHY SAP), which depends on implementation in particular.

2.2.1      Physical layer (PHY)

The physical layer was conceived to optimize the operation of broadband wireless systems that use the frequency field of 2 to 11 GHz and that operate under the next line of sight (NLOS) condition. The IEEE 802.16 a/d standard envisages three possibilities for the physical layer, which are:

• Wireless MAN-SCa: this specification considers an over-the-air interface supported by a single modulated carrier.

• Wireless MAN-OFDM: This specification uses an orthogonal frequency division multiplexing (OFDM) scheme that is comprised of 256 carriers.

• Wireless MAN-OFDMA: It uses the OFDM scheme of 2,048 carriers, the access of multiple users is made effective assigning a subset of carriers to each individual receiver, and therefore this standard is referred to as OFD multiple access (OFDMA).

The OFDM-based systems showed better performance when using NLOS, and their mathematical analysis is complex because it is based on the fast Fourier transform (FFT).

2.2.2     OFDM Technology

OFDM technology makes it possible to eliminate the interference between symbols called ISI and reduces the complexity of adaptive techniques, which is combined with the orthogonal characteristic of the carriers. Use of orthogonal carriers makes it possible, on the one hand, to achieve identification and selective fading and, on the other hand, to obtain greater spectral efficiency.

An orthogonal frequency division multiplexing scheme is shown in Figure 2 below:

In OFDM, 256 subcarriers are used; of these 192 are used for data, 56 are fallback subcarriers, -28 in the lower part and 28 in the upper part, which perform the role of guard bands, and 8 are used for permanent pilot signals.

The transmission scheme for standard 802.16 is shown below in the block diagram:

Processing is comprised of:

1. Randomizer block permitting the distribution of data energy on the available spectrum.

2. Forward error correction (FEC) block for error correction, comprised of a concatenated Reed Salomon convolutional coding block.

3. Interleaving block for temporal diversity supplies and to minimize the effect of error bursts without adding any extra heading.

4. A block to map the data of information to the symbols of the modulation scheme used.

5. A block to map the symbols depending on the OFDM symbols modulation scheme.

6. A block for the transformation of the OFDM symbol from the frequency domain to the time domain.

7. Block to insert the cyclical prefix required to optimize transmission in the multi-path environment.

8. Block to undertake the establishment of the signal.

9. Block to accommodate the signal for its radiation.

Two aspects associated inherently with each functional block differentiating the Wi-Max technology are:

• The capacity to adapt the modulation scheme to channel conditions.

• The capacity to correct errors achieved through the concatenation of the Salomon Network block code.

Some improvements that are being proposed to the standard IEEE 802.16 are indicated below:

1. Spatial multiplexing.

2. The introduction of the automatic repeat request (ARQ) for the purpose of guaranteeing reliable PDU data transmission.

3. Improvements in canceling the interferences, especially to apply the Wi-Max to mobile systems.

3      Propagation Model

When basing the IEEE 802.16 standard on an NLOS system, the aspects of diffraction, polarization changes, signal scattering and reflection must be considered as part of the propagation model.

A proposal that is well adapted to the Wi-Max standard has been identified, and it is based on the model proposed by ATT Wireless, called IEEE 802.16.3c-01/29v4 “Channel Models for Fixed Wireless Applications by Erge et al.”  As a rule, the model considers an antenna height of 15 to 40 mts, cell radiuses on the order of 10 mts and coverage requirements ranging from 80 to 90%.  In view of the intensity of the subject, some basic aspects of it shall be dealt with.

This model considers that the radio channel should be characterized by:

• Path losses

• Multipath delay spread

• Fading

• Doppler effect

• Effects of interference of the adjacent channel and co-channel.

As for path losses, two types are considered:

• Suburban environment

• Urban environment

For the suburban environment, the forecasting model focuses on Okumura-Hata, which is valid for the range of 0.5 to 1.5 GHz and antenna heights of over 30 mts.

ATT adjusted the model for operation on the 1.96GHz band, with propagation losses defined as follows:

P.L= A+10 g Log₁₀ [d/do]+s  "d>do

From

             A=20log₁₀ [4pdo/l], l wavelength in mts

g = a-bhb+c/Ho, for base stations in the range of 10 £ hb £ 10m

hb = height of radio antenna base

do = 100 mts

a, b and c are constants that depend on the type of field.

The value of s is associated with the effect of the density of the trees and represents a standard deviation of losses, its range is as follows:

8.2 £ s £ 10.6dB   

For the path in urban environments, the model used as the point of departure is COST231 WalFish-Ikegami. For this case, ATT made minimum changes in its adjustment parameters.

4       Conclusions

Wireless applications are acquiring a very important place in the development of wireless communications.

The IEEE 802.16 standard stands out as having considerable potential for providing multimedia applications.

The IEEE 802.16 standard tends to be introduced as a standard with mobility applications, which points to it as the 4G prototype.

The IEEE 802.16 standard is positioned as an important element for the development of the next generation of mobile services called 4G.

Eng. Guillermo Rivero Gónzalez
ICE -  Costa Rica