Physical Layer (PHY)

The goal of this layer is to provide excellent, yet simple, performance. The PHY layer must be able to adapt to different conditions and also needs to be flexible for jumping from channel to channel without errors in transmission or losing clients (CPEs). This flexibility is also required for being able to dynamically adjust the bandwidth, modulation and coding schemes. Orthogonal Frequency Division Multiple Access (OFDMA) will be the modulation scheme for transmission in up and downlinks. With OFDMA it will be possible to achieve this fast adaptation needed for the BS's and CPEs. OFDMA is a multi-user version of the Orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of sub-carrier frequencies to individual CPEs as shown in the illustration below. This allows simultaneous low data rate transmission from several users.

By using just one TV channel the approximate maximum bit rate is 19 Mbit/s at a 30 km distance. The speed and distance achieved is not enough to fulfill the requirements of the standard. The feature Channel Bonding deals with this problem. Channel Bonding enables to use more than one channels for transmission and reception. This allows the system to have higher bandwidth which will be reflected in a better system performance. Failure of one or more of the channels does not lead to data loss, only a longer transmission time. Failure of one or more channels leads to the automatic rerouting of missed packets, without data loss.

Topology

The initial drafts of the 802.22 standard specify that the network should operate in a point to multipoint basis (PMP). The system will be formed by base stations (BS) supporting multiple subscriber stations called customer-premises equipment (CPE). The CPEs will be attached to a BS via a wireless link. The BSs will control the medium access for all the CPEs attached to it. The BS is responsible for the media access control, managing distributed sensing and deciding what to do next based on the data collected from the CPEs and neighboring BSs.


For the distributed sensing the CPEs help the base station obtain current radio environment information by scanning the spectrum in their vicinity and reporting the results. The BS, with the information gathered, will evaluate whether a change is necessary in the channel used, or on the contrary, if it should stay transmitting and receiving in the same channel.

802.22 (WRAN) vs 802.16 (WiMAX)

Finally, it is important to understand the core differences between 802.22 and 802.16 (WiMAX) as confusion often arises when discussing these two IEEE projects. Since 802.22 is mostly targeted at rural and remote areas, its coverage range is considerably larger than 802.16 (see Figure 1) to allow for a good business case, and this is why 802.22 is the first standard ever for WRANs. Also, 802.16 does not include incumbent protection techniques necessary to operate in licensed bands, while it has an ongoing project (802.16h) currently concentrating on coexistence among 802.16 systems only.

Figure 1. 802.22 wireless RAN classification as compared to other
popular wireless standards

A little story on IEEE 802.22 WRAN

IEEE 802.22 is a standard for Wireless Regional Area Network (WRAN) using white spaces in the TV frequency spectrum. The development of the IEEE 802.22 WRAN standard is aimed at using cognitive radio (CR) techniques to allow sharing of geographically unused spectrum allocated to the Television Broadcast Service, on a non-interfering basis, to bring broadband access to hard-to-reach, low population density areas, typical of rural environments, and is therefore timely and has the potential for a wide applicability worldwide. It is the first world wide effort to define a standardized air interface based on CR techniques for the opportunistic use of TV television bands on a non-interfering basis.

IEEE 802.22 WRANs are designed to operate in the TV broadcast bands while assuring that no harmful interference is caused to the incumbent operation, i.e., digital TV and analog TV broadcasting, and low power licensed devices such as wireless microphones. The standard was expected to be finalized in the first quarter of 2010. IEEE 802.22 Draft D1 is available and comment resolution is underway. IEEE P802.22.1 is a standard being developed to enhance harmful interference protection for low power licensed devices operating in TV Broadcast Bands. IEEE P802.22.2 is a recommended practice for the installation and deployment of IEEE 802.22 Systems. IEEE 802.22 WG is a working group of IEEE 802 LAN/MAN standards committee which is chartered to write the 802.22 standard. The two 802.22 task groups (TG1 and TG2) are writing 802.22.1 and 802.22.2 respectively.

In response to a Notice of proposed rulemaking (NPRM) issued by the U.S. Federal Communications Commission (FCC) in May 2004, the IEEE 802.22 working group on Wireless Regional Area Networks was formed in October 2004. Its project, formally called as Standard for Wireless Regional Area Networks (WRAN) - Specific requirements - Part 22: Cognitive Wireless RAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Policies and procedures for operation in the TV Bands focuses on constructing a consistent, national fixed point-to-multipoint WRAN that will use UHF/VHF TV bands between 54 and 862 MHz. Specific TV channels as well as the guard bands of these channels are planned to be used for communication in IEEE 802.22.

The IEEE 802.22 Work Group kicked off in November 2004 and approved the functional requirement document for WRAN systems in September 2005. Ten initial proposals were merged into a single one in March 2006, and the draft standard (D0.1) was developed in May 2006.  The complete 802.22 standard is expected to be approved by May 2007.  For 802.22 WRAN systems, the primary users (PU), those with priority rights, mainly include incumbent analog and digital TV stations, TV translators, TV boosters, TV receivers, and wireless microphones.  The secondary user (SU) are the CPEs and have a requirement to avoid generating harmful interference to the PUs.


The Wi-Fi Alliance has started working on a new standard called IEEE 802.11 (White-Fi) which will be deployed for use in the TV white spaces. The IEEE 802.11af is similar to the current IEEE 802.11 Wi-Fi with an added cognitive radio technology in order to be used in the white space spectrum.

Cognitive Radio

Cognitive Radio is an intelligent wireless communication system that is aware of its surrounding environment and uses the methodology of understanding-by-building to learn from the environment and adapt its internal states to statistical variations in the incoming RF stimuli by making corresponding changes in certain operating parameters in real-time. [1]

Cognitive radio is a paradigm for wireless communication in which either network or wireless node itself changes particular transmission or reception parameters to execute its tasks efficiently. This parameter alteration is based on observations of several factors from external and internal cognitive radio environment, such as radio frequency spectrum, user behavior, and network state.[2]

The driving force behind cognitive radio technology is attributed to the lack of enough radio spectrum to accommodate for the increasing need of spectrum for high data rate communication. Imagine what happens as more devices go wireless -- not just laptops, or cell phones, but sensor networks that monitor everything from temperature in office buildings to moisture in cornfields, radio frequency ID tags that track merchandise at the local supermarket, devices that monitor nursing-home patients. 

All these gadgets have to share a finite and increasingly crowded amount of radio spectrum. But as anyone who has twirled a radio dial knows, not every channel in every band is always in use. In fact, the Federal Communications Commission (FCC) – the spectrum controlling body of the US – has determined that, in some locations or at some times of day, 70 percent of the allocated spectrum may be sitting idle.

Fig. 1. TV band occupancy over time and frequency at some particular instance

There are two major subsystems in a cognitive radio; a cognitive unit that makes decisions based on various inputs and a flexible software defined radio (SDR) unit whose operating software provides a range of possible operating modes. A separate spectrum sensing subsystem is also often included in the architectural of a cognitive radio to measure the signal environment to determine the presence of other services or users. It is important to note that these subsystems do not necessarily define a single piece of equipment, but may instead incorporate components that are spread across an entire network. As a result, cognitive radio is often referred to as a cognitive radio system or a cognitive network.

The cognitive unit is further separated into two parts as shown in the block diagram below. The first labeled the “cognitive engine” tries to find a solution or optimize a performance goal based on inputs received defining the radio’s current internal state and operating environment. The second engine is the “policy engine” and is used to ensure that the solution provided by the “cognitive engine” is in compliance with regulatory rules and other policies external to the radio.

Fig. 1. Cognitive radio concept architecture

References: 1. S. Haykin, “Cognitive Radio,”JSAC 2005

       2. http://en.wikipedia.org/wiki/Cognitive_radio.

What is 'White Space"?

White space, in a communications context, refers to underutilized portions of the radio frequency (RF) spectrum. Large portions of the spectrum are currently unused, in particular the frequencies allocated for analog television and  those used as buffers to prevent interference between channels. In the United States, frequency allocations in the RF spectrum are made by the Federal Communications Commission (FCC). In November 2008, the FCC voted unanimously to make  unlicensed portions of the spectrum available for use.  At that time, at least three-quarters of the spectrum allocated for analog television was unused. These frequencies will become available once the changeover to digital television is complete in February 2009.

White space allocation is expected to stimulate development of wireless technologies and services.  According to Google co-founder Larry Page, white space operation will be like "Wi-Fi on steroids," because the signals in that portion of the spectrum have much longer ranges than those currently used for Wi-Fi. The increase in range means that fewer base stations will be required to give better coverage; that increased efficiency, in turn, should yield better service at lower costs. Signals in the white space range can also penetrate through solid objects better, which should yield more reliable service.

Opponents of white space allocation have argued that it could lead to unexpected instances of disruptive and potentially dangerous interference between different services using the same frequencies at the same time. The FCC is testing white space devices designed to operate in the newly available frequencies to ensure that they will not cause interference. According to the FCC, wireless microphones and other low-power auxiliary stations will be able to continue to operate in bands below 700MHz.

Introduction

I am currently working on my Masters degree program in Broadband Telecommunications Technology at Eindhoven University of Technology (TU/e). As part of the program, I am following a course called Regulations and Standards for Wireless communications (0EL70). Every student have to create a blog on a topic of his/her choice in the field of wireless communications and I have chosen to write about Wireless Regional Area Network (WRAN). 


I will post information on the developments in wireless communication with respect to wireless regional area networks that are based on cognitive technologies which will be employed in the unused analog TV spectrum. I will start with introduction to the terms and technologies used, the relationships among them and the standards and regulation in place so far. As this is my first blog and I am learning bloging, I wouldn't expect it to be perfect. Everybody is welcome to comment on and contribute to the posts.