Mobile WiMAX : Meet 802.16e

We are about to experience yet another revolution. Worldwide interoperability for microwave access (WiMAX) has the potential to do to the internet exactly what mobiles did to telephones.

The world was revolutionized with the advent of mobile phones. Along came the concepts of text messaging and missed calls. These ensured we always stayed connected to our contacts.

We are about to experience yet another revolution. Worldwide interoperability for microwave access (WiMAX) has the potential to do to the internet exactly what mobiles did to telephones - to make it wireless on the large scale. Additionally, it can also be used to help mobile telecommunications.

What was wrong with WiFi?

A popular misconception is that WiMAX is aimed at replacing WiFi. This isn’t entirely true. WiFi is only concerned with creating a wireless pocket of connectivity around the router, instead of being strangled with a wire.

WiMAX, on the other hand, takes wireless to the maximum (although that isn’t what it stands for), by facilitating wireless broadband connectivity of up to 3 Mbps spread over large areas. WiFi can be used to create wireless local area networks and provide internet connectivity to computers in the range of a router however it doesn’t provide a viable means to connect computers over a wide range, say that of a city. WiFi can thus be thought of as a wireless alternative to Ethernet networks, while WiMAX is an alternative to technologies such as DSL.

WiMAX

WiMAX is an acronym for Worldwide Interoperability for Microwave Access. It is a relatively new technology, having been first introduced in 2004 as 802.16, more popularly known now as ’fixed WiMAX’. It was created as a technology to provide the last-mile connectivity for broadband access. This means that it can potentially be used as a replacement for current broadband network systems such as DSL, landline or cable connectivity.

Where WiMAX wins out here is due to the fact that it is wireless, and as such huge savings can be expected from implementing this is place of wired technologies. Just like cellular-phone towers, setting up one WiMAX tower has the potential to cover a large area, and as such, can also be used to provide connectivity to some of the more remote areas where wired connectivity is currently unavailable, infeasible or expensive.

In a revision accepted in 2005 named 802.16e, many improvements were made to the standard, some of which were to enable the use of WiMAX in mobile installations.

IEEE 802.16e-2005

This is indeed the full standard name for “Mobile WiMAX”. This amendment of the WiMAX standard is perhaps most popular for the addition of features enabling its use in mobile installations. The standard describes itself as “Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands”, changing the title of the 802.16 standard to “Air Interface for Fixed and Mobile Broadband Wireless Access Systems”. WiMAX, by itself, promises wonders, then why all the hype about this new amendment?

Consider this – you have full speed broadband access on the go; surfing wirelessly at home, you could check your email on the way to work, and continue working once there without ever needing to connect a wire! What if this connectivity existed everywhere, you would never have to worry about searching for hotspots. The whole city would be a hotspot. But then, since this is India, you’ll have to imagine waiting for another three decades before this becomes a reality.

One of the key technologies for facilitating mobile access of WiMAX networks is a scheme called SOFDMA (Scalable Orthogonal Frequency Division Multiple Access) which is a scalable form of the system used in 802.16-2004 OFDMA (Orthogonal Frequency Division Multiple Access).

SOFDMA Deconstructed

Scalable Orthogonal Frequency Division Multiple Access, quite a mouthful! However if we break it down into its components it begins to sound much more sensible.

Scalable: Scalability is important when considering any technology. Especially for WiMAX which can be used to provide connectivity for as far as 50 miles, and maybe used for bandwidths up to 70Mbps by a large number of people simultaneously.

Orthogonal: This means mutually perpendicular. Here it applies to signals, that the signals are perpendicular to each other so that when one is at its peak, all others are zero.

Frequency Division: This is a means of transmitting multiple signals via a single medium, by using carriers of different frequencies. DSL is an example of how voice signals and internet connectivity signals are both carried in the same line.

Multiple access: This bit is obvious, a single access internet connection would be amazing surely, but the waiting in line for access would be quite annoying! Networking technologies have to ensure a feasible and stable method for allowing multiple people at access the service at the same time.

SOFDMA builds upon OFDMA which builds upon FDMA. SOFDMA is thus a specialized form of frequency division multiple access (FDMA). However, it is incompatible with the earlier technologies in some ways, and will require WiMAX ‘broadcasters’ to upgrade their equipments to support SOFDMA.

So, why SOFDMA, what does it do, and why is it important, that it was introduced despite compatibility issues.

Bandwidth, frequencies and data transfer

To transmit any kind of data it needs to first be modulated so that it can be carried over some kind of medium. Wired methods transmit data by sending electric signals of varying frequencies through the wire, which are modulated with the information to be sent. This modulation process involves the conversion of data which is composed of binary digits into a kind of electrical waveform.

In the modulation process, a base waveform or a carrier wave, is taken and modified with a pattern which is representative of the data being transferred. What quality of the carrier wave is modified depends on the modulation used. How much information a wave can carry depends greatly on the frequency of the waveform.

Different modulation schemes approach modulation in different ways in order to gain better and better efficiency. It is better to divide the same data and send it in parallel at multiple lower frequency bands than to transmit it over a single very high frequency band. The reason for this is simple, a single channel carrying data at 1 Mbps means that each binary digit will occur for just 1 micro-second! On the other hand, sending the same over 100 channels at 10 kbps would give us a much better duration per bit leading to easier detection.

The reason FDMA is superseded by OFDMA and OFDMA by SOFDMA is precisely because each improving technology brings with it better bandwidth efficiency, greater reach and better stability. The main concept used in all technologies remains frequency division, so let’s have a look at FDMA from which they evolved.

FDMA

FDM or frequency division multiplexing is a way of transmitting multiple streams of data along the same medium (wired or wireless) by using different frequency bands for each channel. By keeping the frequency bands far enough apart, we can ensure that each channel can be filtered out separately. We can compare this to a room filled with people speaking in different languages. Although you may be able to hear everything, you will only be able to understand the languages you know.

In reality, we can look at the example of an FM Radio, where different channels come at different frequencies. One signal may be allotted a central frequency of 98.1 MHz with a bandwidth of 200 kHz, meaning that it will broadcast at frequencies between 98.0 MHz and 98.2 MHz, while another may be allocated a 200 kHz bandwidth around the 98.7 MHz. As such, each frequency band is separated by sufficient margins so that they don’t overlap or interfere. On the receiving end, we can tune in the receiver to a particular frequency and filtering out the other signals, and boosting power at the selected frequency.

FDMA (Frequency Division Multiple Access), works by giving different users their own frequency bands, thus each user can communicate using a channel of their own.

This makes FDM quite inefficient, as a lot of bandwidth is wasted in the gaps used to separate different frequency bands. OFDM or Orthogonal Frequency Division Multiplexing packs in much more information by eliminating the requirement for these gap-bands.

OFDM and OFDMA

With OFDM (Orthogonal Frequency Division Multiplexing), sub-carriers in the signal wave can be overlapping without causing any interference. This is accomplished by making the sub-carriers in an OFDM exactly orthogonal to each other, meaning that while one is at its peak the others are all zero. The efficiency is greatly increased by this, as more data streams can now be transmitted using the same bandwidth.

OFDM spreads out the data among multiple carriers, each of which is modulated separately. As there is now larger number of carriers to modulate, the effective system throughput is increased. Also since a large no of frequencies is used, some of the issues surrounding wireless communication such as multi-path, signal cancellation, and spectral interference are reduced to a great extent.

However in OFDM each channel is assigned to a single user. To enable multiple user access it is additionally used with Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA), however each approach has its caveats.

OFDMA (Orthogonal Frequency Division Multiple Access) as its name suggests is designed for multiple user access. Unlike OFDM, OFDMA is capable of supporting multiple users on the same channel, by dividing each channel into sub-channel groups. Insofar that the channel assigned to any user will be optimized based on which will perform best for the current user’s location, environment and equipment.

In the original 802.16-2004 specification, OFDM-256 was used, which meant each channel would be composed of 256 sub-carriers and multiple user access available via TDMA (or FDMA). The 802.16e-2005 spec adds OFDMA 128/512/1024/2048, while the old OFDM-256 is kept for compatibility reasons.

What a difference an alphabet can make!

SOFDMA

SOFDMA is OFDMA but with scalability to enable its usage in a wider variety of scenarios, especially mobile. While the original 802.16-2004 standard was meant to enable wireless internet access to fixed locations such as homes and businesses, where a WiMAX receiver would be affixed outside a the building similar to a dish antenna, in 802.16e-2005 many specifications were added which would enable a mobile system to access the network.

SOFDMA is scalable in that it allows for changing the bandwidth allocation to channels from 1.25 MHz to 20 MHz. Smaller channels such as those to be used by mobile devices are optimized for lesser system complexity, while higher bandwidth channels such as those used by fixed point subscribers at home or in offices, are optimized for higher bandwidth. Even the modulation scheme is scalable such that a different encoding scheme can be depending on the quality of the signal.

Other 802.16e features

Many other features were also added in 802.16e-2005, to improve its mobility characteristics, an important one being MIMO (multiple-in multiple-out). Other than the mobility enhancements, there were also the addition of HARQ (Hybrid Automatic Repeat Request), and QoS (Quality of Service) improvements.

MIMO

To improve the range and speed characteristics, multiple antennae can be used at the transmitting and receiving station. This improves the maximum range of the transmission and the data throughput. Using this in WiMAX means that without increasing the output power of the transmitter, and while keeping the same bandwidth, they will be able to provide better performance to a longer range. This is of obvious benefit in mobile access, as it enables customers to get better access with wider area coverage, all without significant cost increase to the broadcaster.

QoS

Networks are used for a wide variety of communication, from internet gaming to bank transactions, each type of communication requiring different network characteristics. While we may consider out internet surfing quite important, there are many other protocols which are less resilient to network outages or interruptions. VoIP for example requires a constant guaranteed minimum rate of data flow for communication to persist. To guarantee that such time and delay critical communications are not interrupted, QoS protocols are employed to give them higher priority. Communications of different classes are given different priority. The QoS in 802.16e-2005 divides network communication into 5 classes:

1. Unsolicited Grant Service
2. Extended Real-time Polling Service
3. Real-time Polling Service
4. Non-real-time Polling Service
5. Best Effort

VoIP falls in the second class, and our favorite HTTP communication is sadly last, given only as much bandwidth as is left over in the end.

WiMax vs LtE

There is another war of standards coming up! LTE stands for Long Term Evolution, and it widely thought of as an upgrade path from the current GSM 3G / 3.5G standards. It has backing from a majority of the GSM providers, but unlike WiMAX it is not based on an open standard. Although LTE cannot compete with WiMAX in all its facilitations, in the mobile sphere they are both quite well matched. In fact they both use the same OFDM and MIMO technologies for communication. LTE has the upper hand in speeds right now, with the soon to come 802.16m standard, WiMAX may well overtake LTE. Unlike LTE for WiMAX mobile access is already available, and already starting to get implemented. As LTE cannot compete with WiMAX in all fronts, it is unlikely that LTE will replace WiMAX, but as far as the mobile space is concerned, the battle is ON!

Conclusion

Are we ready for internet as easily available as telephone signals? I believe we are, we have been good, and we deserve our candy. WiMAX is not the technology of the future; it is already something that is being implemented in many areas around the globe, a good example being WiBRO, which is an 802.16 implementation in South Korea.

802.16 holds a lot of promise for more than just internet, it is potentially a technology that can provide a wireless backbone for the telecommunications industry. Our homes are already starting to get wireless, WiMAX aims to make entire cities wireless, which if you live in India you can clearly sympathize with.

No more broken connection due to wiring faults, or overloaded trucks passing through narrow streets! Just like mobile phones have made the telecommunication an indispensible tool, and taken us from one landline per home to one phone per person, so does WiMAX hold the potential to someday make internet as personal and as available.