Fixed WiMAX Physical Layer
– Guard Time
– Subcarrier
– Bandwidth
– Channel Coding, Modulation and Bit Rates
– OFDMA
Wireless Broadband (WiBro)
Introduction of WiMAX
IEEE based mobile broadband
Wireless Metropolitan Area Network
The standardization under the IEEE 802.xx project should create networks of various sizes. Not only was WLAN specified under 802.11, but also WPAN (Wireless Personal Area Network) under 802.15. Here, for example, Bluetooth was included as the 802.15.1 standard.
Wireless Metropolitan Area Network (WMAN) began to be specified under the number 802.16. The goal was a standard that would enable data exchange from a transmission tower within a radius of perhaps 10 km. The driver here was the so-called wireless local loop, i.e. the so-called „last mile“ to the user. If a user does not have access to a wired data network such as DSL or a cable modem, wireless access should be enabled. Imagine that an antenna, perhaps even a directional antenna, is set up in a house or apartment. This can then exchange data with a radio tower and thus receive wireless broadband access. In a WMAN environment, such receivers are called Customer Premises Equipment (CPE).
WiMAX
After WLAN was a surprising success and was promoted by WiFi, a similar term for 802.16 was quickly created called Worldwide Interoperability for Microwave Access (WiMAX). A WiMAX Forum was founded in 2001 to organize interoperability of WiMAX devices, similar to the WiFi Alliance for WLAN. WiMAX received a special push from Intel. Intel had already started making WLAN an integral part of its PC business. They created the so-called Centrino platform for this purpose. An expansion towards WMAN was natural and part of Intel’s strategy that PCs should have easy access to the Internet everywhere, via WiFi or WiMAX.
Intel’s vision was that mobile Internet would be available via WiFi and WiMAX and not via mobile networks. These should be reserved for voice services.
Several versions of 802.16 were quickly specified in the early noughties. The first publication of the standard for WMAN in a fixed environment (fixed wireless) appeared in 2004. It allowed bit rates of 10 to 20 Mbit/s. Good for fast internet and significantly faster than W-CDMA and UMTS which was launched at the same time.
A little later, in 2005, a mobile version of WiMAX appeared under 802.16e. Like a cellular system, this allowed cell changes. This made WiMAX a “nomadic” standard. It not only worked in the house with a CPE, but also “on the go” from the PC or PDA. Intel was the first manufacturer to also offer a chip-set for 802.16e.
FIXED WIMAX PHYSICAL LAYER
After the success of 802.11 g, it is hardly surprising that WiMAX is also based on OFDM modulation. However, the air interface was designed to be very flexible. The following boundary conditions apply:
- Different frequency bands
- Different bandwidths
- Different ranges
OFDM-based transmission was therefore designed to be flexible. The only thing that is fixed for non monadic WiMAX is the size of the FFT with 256 points.
Guard Time
In contrast to WLAN, a WiMAX cell can be up to several kilometers long. This leads to multipath propagation and thus to interference between the symbols to be transmitted. With OFDM, these effects are smaller but not completely negligible, especially with short symbol times. That’s why a gap is introduced between two symbols, a guard time or cycle prefix (CP). The ratio of symbol length and guard time length is called G. G can be chosen depending on the situation. Possible values are 1/4, 1/8, 1/16 and 1/32.
Subcarrier
A 256 point FFT theoretically produces 256 subcarriers. As with WiFi, not all of these are used. The 27 or 28 carriers on the edge are not used and serve as “guard carriers” (protection carriers against neighboring bands). Furthermore, the carrier is not used at frequency 0. Eight carriers are used as pilot channels. This leaves 192 subcarriers for data transmission.
Bandwidth
Bandwidth depends on the sampling frequency of the OFDM symbols. The following applies: sampling frequency = n x bandwidth, where n is typically a value of 8/7. If you want a bandwidth of 20 MHz, the sampling frequency is calculated as 17.5 MHz. The subcarrier bandwidth is 20 MHz/256 = 78 kHz.
Channel Coding, Modulation and Bit Rate
As with HSDPA, WiMAX also uses the best channel coding method to date, a turbo encoder. Different coding rates can be generated by puncturing, namely 1/2, 2/3, 3/4 and 5/6.
As with 802.11 a, the following modulations are permitted to modulate the output signals of the iFFT: BPSK, QPSK, 16 QAM and 64 QAM. The modulation is of course chosen according to the channel quality. 64 QAM is only possible if there is very little noise, which is usually the case if you are close to the transmitter or have a good (directional) antenna for reception.
The data rates now result from the number of bits per OFDM symbol times the coding rate divided by the duration of the OFDM symbol. The table below shows the bit rates at 20 MHz with different encodings and modulations and variable guard times (G ratio). Under favorable circumstances, bit rates of over 70 Mbit/s can be achieved.
| BPSK | QPSK | QPSK | 16-QAM | 16-QAM | 64-QAM | 64-QAM | |
| G ratio | 1/2 | 1/4 | 3/4 | 1/2 | 3/4 | 2/3 | 3/4 |
| 1/32 | 8.31 | 16.62 | 24.94 | 33.25 | 49.87 | 66.43 | 74.81 |
| 1/16 | 8.07 | 16.13 | 24.20 | 32.27 | 48.40 | 64.47 | 72.61 |
| 1/8 | 7.62 | 15.24 | 22.86 | 30.48 | 45.71 | 60.89 | 68.57 |
| 1/4 | 6.86 | 13.71 | 20.57 | 27.43 | 41.14 | 54.80 | 61.71 |
OFDMA
Unlike in WLAN systems, it can be assumed that in a WMAN cell there are many users who want to use the channels at the same time. It is therefore possible to assign a subset of the existing subchannels to different users. This allows multiple users to share capacity like in CDMA or TDMA systems. This is called orthogonal frequency-division multiple access (OFDMA). With WiMAX, the 192 subchannels can be bundled into 16 channels, each of which has 12 subchannels. This means that 16 participants can be served at the same time.
Wireless Broadband (WiBro)
Korea was very progressive when it came to mobile communications in the early 2000s. In Korea, especially with CDMA2000, progress was often faster than in the USA. Nevertheless, the capacity and also the speed were not considered sufficient. In 2002, Korea released a 100 MHz band between 2.2 GHz and 2.3 GHz for wireless broadband services. They quickly worked on an OFDM based system that was similar to 802.16. Samsung was the main driver of the standardization called WiBro. A TDD system with a bandwidth of 10 MHz was targeted.
Initially, the development of WiBro and WiMAX ran parallel. However, Intel and Samsung, the main drivers of the standards, agreed in 2004 that there should be compatibility between WiBro and WiMax.
In 2005, a nomadic WiBro-compatible version of WiMax was created called 802.16e. In contrast to the fixed WiMAX, this standard is scalable via the number of subcarriers. (SOFDMA). This means you can operate different bandwidths with a fixed sampling rate or subchannel bandwidth of 10.94 kHz. This is done by using different lengths of the FFT.
- 128 points FFT = 1.25 MHz bandwidth
- 512 points FFT = 5 MHz bandwidth
- 1024 points FFT = 10 MHz bandwidth
- 2024 points FFT = 20 MHz bandwidth
Furthermore, functions and channels are defined for the standard that allow a handover from one cell to a new cell.
Introduction of WiMAX
Korea Telecom started WiBro/802.16e service back in 2005. Downlink speeds of 30-50 Mbit/s could be achieved (10 MHz bandwidth), an enormous number at that time.
As discussed, the semiconductor manufacturer Intel in particular had made it their strategy to implement WiMAX as the standard. Presumably in view of the enormous growth in mobile communications, Intel wanted to play a dominant role in fourth generation mobile communications. Intel was the largest semiconductor manufacturer and had a lot of money to invest.
In 2005, Intel announced that it had an agreement with Sprint to test WiMAX. Sprint had just merged with operator Nextel and was one of the major wireless carriers in the United States. Intel had already developed a WiMAX chip that would become part of the Intel PC chipset. In 2006, Intel invested $600 million in Clearwire to use WiMAX for wireless broadband. In the same year, Sprint announced that it would invest $3 billion in the new WiMAX network. The plan was to gain 100 million users in 2008. Meanwhile, WiMAX was adopted as the official IMT2000 standard in 2007. The standard therefore had a certain claim to the corresponding IMTS 2000 bands.
In 2008 there was a joint venture between Clearwire and Sprint/Nextel regarding WiMAX. At the same time, Mobile WiMAX went into operation in Baltimore. Sprint planned WiMAX in Atlanta, Charlotte, Chicago, Dallas, Fort Worth, Honolulu, Las Vegas, Philadelphia, Portland and Seattle. Boston, Houston, New York, San Francisco and Washington are scheduled to follow in 2010.
Intel is also investing heavily in operators worldwide to establish WiMAX. About $500 million alone for an operator in Taiwan. Intel even bought cellular frequencies to be able to operate WiMAX. In 2007, there were over 300 WiMAX field trials worldwide and 100 WiMAX operators offering mobile broadband via WiMAX.