WiMAX

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-pp. 257-258 Theoretical capacity (78 Mbit/s) and Realistic capacity (24.12 Mbit/s) of 802.16
  • Jeffrey G. Andrews, Arunabha Ghosh, Fundamentals of WiMAX: Understanding Broadband Wireless Networking, Prentice Hall, 2007


Project: Document due Tuesday, 11 December 2007

-Source of Alamouti's slides from Intel

Theoretical capacity

-Scalable OFDMA (SOFDMA) [3] is introduced in the IEEE 802.16e Amendment to support scalable channel bandwidths from 1.25 to 20 MHz.
-"The frequencies higher than 10 GHz are practical only for fixed line-of-sight (LOS) type services. Non-line of sight (NLOS) communications perform better when the frequencies of operation are kept under 10 GHz. The frequencies below 6 GHz have better propagation properties and are better suited for mobile communications because they most likely guarantee service to all the niches of the coverage area."
-"To approach the theoretical capacity of the system, WiMAX uses a combination of adaptive modulation schemes and coding ranging from ½ rate QPSK to 5/6 rate 64QAM. The amount of error correction applied to each transmission is adjustable and can be changed depending on the required QoS and based on the reliability of the link between each user and the base station."
-"The extensive flexibility introduced into the WiMAX standard also makes it harder to model correctly."

Channelization

-"Time Division Duplex (TDD) transmission scheme of the 802.16e standard will be considered. In the initial working group release, the standard supports 5 and 10 MHz bandwidth allocations for each radio frequency channel. The available channel bandwidth is made up of sub-carriers each of which can be modulated individually with information. WiMAX uses Orthogonal Frequency Division Multiple Access (OFDMA) to assign sub-carriers to different users. The number of sub-carriers available for assignment in the UL and DL are a function of the channel bandwidth, the frame size, and the UL/DL transmit ratio. In mobile WiMAX, the smallest unit of frequency-time allocation available is a slot which contains 48 data sub-carriers."
-"The sub-carriers comprising a slot can be made up of adjacent sub-carriers or can be allocated in a distributed fashion throughout the available carrier space. In general, distributed carrier allocations perform better in mobile environments, while adjacent sub carriers are better suited for fixed links. The number of slots assigned to a particular user per frame is a function of their data needs."
-The FFT size is a parameter that must be specified at synthesis time, but can change the guard interval at run time.

Doppler considerations

Computational Complexity

-Receiver complexity gor MIMO-CDMA grows exponentially with bandwidth and linearly for MIMO-OFDM
-"On Multivariate Communication Theory and Data Rate Multiplying Techniques for Multipath Chanels", PhD dissertaion, Greg Raleigh, December, 1998
  • Zhendong Luo, Hong Gao, Yuanan Liu, ["Adaptive Transmission With Linear Computational Complexity in MIMO-OFDM Systems"], IEEE Transactions on Communications, October 2007
-the total complexity of the enhanced mode is roughly 3K flops (K = subchannels x subcarriers)


Computational capability in mobile devices

Gbps Evolution

-Good graph on bandwidth/mobility evolution to 4G with WiMAX and mobile WiMAX
-Information on Siemens MIMO testbed for 1 Gbps wireless: "They used an RF channel with 100 MHz bandwidth, with 82 MHz occupied by the OFDM signal. The maximum data rate of 1 GBPS is achieved with 64 QAM modulation on all subcarriers."
-Convergence chart for LTE and 802.16 into 4G

Project: Presentation on Thursday, 8 November

WiMAX "OFDM and Wimax (4G) Networking"

-(4.3) "Fixed, nomadic, portable and mobile applications for 802.16-2004 and 802.16e WiMAX networks", WiMAX Forum White Paper, November 2005
-(4.4) "Can WiMAX Address Your Applications?", October 24, 2005
-(4.5) Carl Eklund, Roger B. Marks, Kenneth L. Stanwood and Stanley Wang, "IEEE Standard 802.16: A Technical Overview of the WirelessMAN™ Air Interface for Broadband Wireless Access", IEEE Communications Magazine, June 2002
-"Those backing the new spec plan to increase bandwidth by using larger MIMO antenna arrays"

Articles

Achieving 1Gbps

  • Martin Sauter, Communication Systems, John Wiley & Sons, Ltd, 2006
-5.10.2 MIMO: A typical MIMO system makes use of two or four paths, which requires two or four antennas respectively. In current systems, antenna designs are used which already incorporate two antennas to pick up horizontally and vertically polarized signals created by reflection and refraction to counter the multipath fading effect (polarized diversity).
-WiMAG and 3G LTE (Long Term Evolution) face the same hurdles, and are not imcompatible, SDR handsets might use both to advantage
-B3G (beyond 3G)
-ITU’s goals for IMT-Advanced appear quite bold: A multi-service platform capable of providing per-user bandwidths of 1 Gbps fixed-nomadic and 100 Mbps mobile.
-OFDMA is the core link technology for WiMAX and LTE 4G, but the performance gains must be built upon through an evolution more to do with how networks. The impact of the evolutionary shift to take advantage of the ‘spatial’ and architectural domain of wireless development will be to greatly increase bandwidth density while reducing costs. Suffice it to say that the shift is to a new evolutionary platform with all that this implies: An additional dimension of development that will deliver 3X-10X total network throughput improvement over cellular wireless.
-Compared to TDMA-based systems, it is known that OFDMA leads to a significant cell range extension on the uplink (from mobile stations to base station). This is due to the fact that the transmit power of the mobile station is concentrated in a small portion of the channel bandwidth and the signal-to-noise ratio (SNR) at the receiver input is increased. Cell range extension is also achievable on the downlink (from base station to mobile stations) by allocating more power to carrier groups assigned to distant users. Another interesting feature of OFDMA is that it eases the deployment of networks with a frequency reuse factor of 1, thus eliminating the need for frequency planning.
-The performance improvement that results from the use of diversity in wireless communications is well known and often exploited. On channels affected by Rayleigh fading, the BER is known to decrease proportionally to SNR^-d, where SNR designates the signal-to-noise ratio and d designates the system diversity obtained by transmitting the same symbol through d independently faded channels. Diversity is traditionally achieved by repeating the transmitted symbols in time, in frequency or using multiple antennas at the receiver. In the latter case, the diversity gain is compounded to the array gain, consisting of an increase in average receive SNR due to the coherent combination of received signals, which results in a reduction of the average noise power even in the absence of fading.
-4G promises very high capacity, and peak data rates in some configurations of more than 100 Megabits per second (Mbit/s) compared to about 15 Mbit/s for current wireless technologies. The first of the 4G standards being implemented is WiMAX, identified by Forbes magazine in 2006 as one of the top 10 technologies that will change the way we live.


Last printed: 4.6

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