Posted by: Paul DeBeasi
Introduction
My last blog post talked about the joint venture – called Clearwire – that received almost $15 billion from Google, Intel, Comcast, Time Warner, and Bright House Networks. According to joint venture announcement, Clearwire expects to dramatically enhance the speed in which customers access the Internet. What does it mean to “dramatically enhance the speed of Internet access”?
This blog post is the third in a five part WiMAX tutorial series, and focuses on WiMAX performance. It is based upon a series of articles that I wrote for Search Mobile Computing. The first part introduced WiMAX technology, applications, and terminology. The second part described WiMAX services. Follow-on blogs will discuss WiMAX security, and devices.
Important Performance Metrics
First, we must define what we mean by the term performance. Performance includes many factors. Two of the most visible metrics, from a user’s point of view, are download speed in megabits per second (Mbps), and upload speed in Mbps. Download speed has traditionally been the metric that most users care about because it affects the user’s web-browsing experience. However, the emergence of peer-to-peer networking and the growing need to upload user-generated content such as videos and pictures is creating the need for faster upload speeds.
Factors Affecting Performance
Next, we must draw a distinction between peak performance and average performance. Peak performance is the best possible performance that the technology can achieve under idealized conditions. The peak performance makes for a great headline, but users will never experience this lofty performance level. Conversely, average performance is the performance that users will typically experience using a deployed mobile service. Average performance can vary throughout a coverage area and is affected by many factors such as:
• Chanel bandwidth: Most mobile cellular systems are designed to operate with varying degrees of bandwidth. For example, a 10-megahertz (MHz)-wide channel provides twice as much bandwidth as a 5-MHz-wide channel. As channel bandwidth increases, the system can transmit more bits per second over the channel.
• Backhaul capacity: Many base stations do not have sufficient transmission capacity to transfer received wireless traffic into the core of the network, thus restricting individual user performance levels.
• Multiple input, multiple output (MIMO) antenna configuration: MIMO radio systems take advantage of multipath interference by transmitting unique data streams over the different paths. A 2 x 2 MIMO configuration, for example, uses two transmit antennas and can take advantage of two unique paths. The greater the number of antennas, the more bits per second the system can potentially transmit over the channel.
• Path loss: Path loss reduces the received signal and gets worse as the mobile device moves away from the base station. Weather conditions, such as rain, snow, and fog, can also increase path loss.
• Shadowing: Shadowing occurs when obstructions such as buildings and tress block the path of the wireless signal.
• Network load: Wireless is a shared medium so per-user performance in an individual base station sector will decrease as the number of users increases (cellular networks make use of sector antennas in order to improve frequency reuse).
• Mobility speed: The faster the mobile device moves the more difficult it is to maintain communication.
WiMAX Performance
Network operators deploy mobile technologies using varying amounts of channel bandwidth. In order to describe performance in a bandwidth-neutral way, throughput (Mb/s) is typically divided by the channel bandwidth (MHz). The resultant quantity provides the number of bits per second transmitted per cycle (b/s/Hz).
Figure 1 shows the peak and average sector throughput for WiMAX. Note that the average download speed is approximately 30 Mbps using a 20 MHz channel (similar to IEEE 802.11g average throughput). This means that there is approximately 30 Mbps of shared bandwidth available for download transmission per sector. For example, if there are 20 users connected to the network using the same sector, and if half of the users are downloading data at the same time, then each user will receive approximately 3 Mbps average download throughput. Note that WiMAX has sophisticated quality of service mechanisms that will help network operators equitably regulate per-user performance.
Figure 1: Mobile WiMAX Throughput (Source: Intel)
Over time, average performance will improve as the WiMAX network evolves. In addition, other technologies such as High Speed Packet Access (HSPA), Evolution-Data Optimized (EV-DO), and Long Term Evolution (LTE) will compete with mobile WIMAX to offer high speed Internet access.
Conclusion
Clearwire promises to dramatically enhance the speed of Internet access with their new mobile WiMAX service. Although many factors can affect performance, the service is expected to provide average download speeds of several mega-bits per second. Other technologies such as HSPA, EV-DO, and LTE will also compete with Mobile WiMAX.
Looking ahead to part 4
Next month we look at mobile WiMAX security. How safe is mobile WiMAX? More specifically, how does the technology provide authentication, data privacy and data integrity?
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