Developers of computing, entertainment and communications products have clear reasons to be concerned about the physical-layer standard used for home networks. Service providers around the world are now beginning to bundle services and content-including digital video and audio, Internet access and voice telephony-to the home. Wireless home networks, therefore, need enough network capacity and scalability to distribute this content to a variety of devices. Additionally, if this multimedia-rich content is to be distributed throughout the entire home, the market requires a robust technology that can withstand the home's challenging radio environment.
A look at existing 2.4-GHz wireless networking standards reveals that they fail to deliver the features and performance needed for multimedia home networks. For reasons ranging from data rate and capacity to interference susceptibility, the 2.4-GHz IEEE 802.11b, Bluetooth and HomeRF standards fall short. Fortunately, the 5-GHz IEEE 802.11a standard can provide the robust, high-speed physical links that wireless home networks require.
To appreciate the drawbacks of today's 2.4-GHz standards, it is important to understand the reasons the indoor wireless channel is one of the most difficult radio channels to maintain. First, the channel is subject to interference. Because 2.4 GHz is an unlicensed or "free" band, several types of systems may coexist within the band. Moreover, the resulting interference can be both narrowband and wideband depending upon the contentious systems, which in the home include devices like microwave ovens, Bluetooth-connected devices and digital cordless telephones. As indicated by its formal name-the Industrial, Scientific and Medical (ISM) band-this part of the 2.4-GHz band also serves a multitude of uses outside the home.
A second difficulty with indoor wireless links is a phenomenon known as multipath. In a real environment such as a home or office, there are several reflective surfaces, including walls, ceilings, furniture and people. A node, therefore, will receive a signal that is likely to be a combination of several signals from different directions with different strengths. This creates both time and frequency dispersion, which are referred to as delay spread and fading. The receiver, as a result, experiences locations within the environment where signals are very faint or non-existent (faded), and where the bit rate of the system becomes severely limited.
The final issue relates to the system's signal-to-noise ratio (S/N). The receiver, depending upon its sensitivity (minimum S/N), must have a certain level of signal above the noise in order to reliably receive the signal. The wireless medium is inherently a "noisy" one due to interference, fading effects, and other potentially coexisting systems within the 2.4-GHz band.
Before comparing 802.11b and 802.11a to reveal why the former falls short, a review of the 802.11 standard will prove helpful. Several similarities exist between 802.11 and 802.11b. Both 802.11 and 802.11b employ spread-spectrum methods, though they use two different approaches to spread the spectrum. Spread spectrum is a technique for using more bandwidth than the system really needs (spreading) for transmission, in order to reduce the impact of localized interferences on the system. Two basic techniques accomplish "spreading."
The 802.11 standard and HomeRF standard spread the spectrum by hopping from one frequency to another within the band. In a band ranging from 2.4 to 2.483 GHz, 802.11 specifies a total of 79 channels with 1-MHz spacing. The data rate of these channels is limited to 1 Mbit/s. Recent FCC rulings have extended the channel bandwidth to 5 MHz to allow HomeRF products to deliver data rates commensurate with 802.11b, around 10 Mbits/s. Unfortunately this does nothing to improve the reliability or robustness of the systems.
When using the frequency-hopping spread spectrum (FHSS) technique, any interfering signal that coincides with a frequency in the hopping sequence can cause an outage or degrade performance. Such an outage lasts only as long as it takes to hop to another channel, but even short outages can cause dropouts in multimedia signals.
The 802.11 standard does not specify the use of error-correcting codes to alleviate interference problems, so nearby devices such as cordless phones and microwave ovens commonly cause periodic outages or performance degradation. Additionally, 802.11 cannot completely overcome the multipath effect because the system has no ability to predict these fades and their location within the band.
The 802.11b standard occupies the same frequency band as 802.11, but uses three 22-MHz channels. This wider channel allows an 802.11b system to achieve a higher data rate compared to 802.11, at 11 vs. 1 Mbit/s, respectively.
In addition, 802.11b uses an alternate methodology known as direct sequence (DS) to spread the signal. In the direct sequence spread spectrum (DSSS) method, the baseband signal is multiplexed with a pseudo noise (PN) sequence, or code, that causes the baseband signal to spread across a broader band. Because of ongoing interference or fading, direct sequence is more susceptible to long- term outages than frequency hopping; the system cannot hop away from problem frequencies.
In fact, 802.11b methodology may not be a spread-spectrum methodology in the true sense. That is, the standard actually increases the data rate almost as much as it increases the channel bandwidth and thus exhibits a spreading factor that essentially is unity. In other words, 802.11b complies with FCC rules for spreading but uses the additional channel bandwidth principally to accomplish the higher data rate.
Full-rate 802.11b also specifies no error-correction coding (ECC), and this lack of coding means that interference and fading, both broadband and narrowband, can cause total signal loss. The scheme does not use frequency hopping, thereby lengthening the duration of outages and performance degradations.
The 802.11b standard uses either differential binary phase shift keying (DBPSK) for a 1-Msymbol/s rate or differential quadrature phase shift keying (DQPSK) for a 2-Msymbols/s rate. The sample rates correspond to data rates of 5.5 and 11 Mbits/s, respectively. An ECC option is available for 802.11b but cuts the data rates in half. As for future scalability, 802.11b data rates cannot be pushed any further without completely dropping the pretense of "spread spectrum." The standard would need more bits per Hz in modulation, heavier coding, equalization and other changes to increase the data rate significantly. It should be noted that 11 Mbits/s is not enough to distribute a single channel of HDTV. As such, 802.11b is unlikely to be a candidate for multimedia distribution within the home.
By comparison, the 5-GHz 802.11a standard offers much higher data rates than either the 802.11 or 802.11b standards, while providing a much more robust link than 802.11b. Part of 802.11a's advantage comes from its move up to a band of frequencies called the Unlicensed National Information Infrastructure (U-NII) band, which spans 5.15 to 5.35 GHz and 5.725 to 5.825 GHz. The lower 200 MHz of the band is used for in-building applications; the upper 100 MHz is typically used for building-to-building or campus-bridging systems. The upper band may also be used for broadband wireless Internet access to the home, which competes with DSL and cable modems.
Free of interference
The U-NII band was originally set aside for inexpensive computer networking in public schools. Since no other devices currently operate in this band, 802.11a systems enjoy an interference-free environment. Although the band is not legally restricted to local networking applications, its origins give it a strong impetus to remain relatively free of other wireless traffic.
At the same time, 802.11a's characteristics help prevent interference and fading problems. To begin with, 802.11a uses orthogonal frequency division multiplexing (OFDM) to achieve data rates as high as 54 Mbits/s in very noisy environments. OFDM is a preprocessing technique that attempts to prepare the signal ahead of transmission for the impact of delay spread. With the inherent challenge of delay spread, as data rates increase, the effects in indoor wireless environments become more pronounced and fundamentally limit reliable performance. Traditionally, this is resolved by using equalization, a post- processing technique. However, OFDM was developed to avoid the prohibitive implementation costs of equalization for consumer applications.
The 802.11a standard uses 20-MHz channels. Within those channels, it uses a set of subcarrier frequencies, each of which is orthogonal. Orthogonality means that the spectrum of each subcarrier can overlap with the adjacent subcarrier, but they can still be discriminated by the receiver.
The net effect of is a more efficient utilization of the 20-MHz channel in terms of the total number of subcarriers.
Achieving 54 Mbits/s
Each subcarrier is modulated individually, hence the bit rate of each subcarrier can be adapted to get the best system performance: More bits are put on the good subcarriers and fewer on the bad. Each subcarrier operates at a relatively low bit rate because of the inherent limitations of the wireless channel. Distributing the bit stream among the subcarriers and transmitting them in parallel achieve the 54 Mbits/s data rate. The effective throughput is thus the sum of the individual data rates on each of the subcarriers. Reducing the individual subcarrier bit rate effectively eliminates the effects of delay spread and achieves the 54 Mbits/s data rate.
Narrowband interference can still affect 802.11a systems because each subcarrier occupies a narrow frequency spectrum. However, with ECC on the subchannels, the receiver can recover data lost due to interference and fading. This ECC technique is not available on today's 2.4-GHz spread-spectrum systems. Consequently, the coded OFDM techniques of 802.11a are much more resistant to interference and multipath effects than are 802.11 (FH) or 802.11b (DS).
The 54-Mbits/s data rate offered by 802.11a is high enough to carry the traffic that can be expected in the foreseeable future, such as digital TV, MPEG-2 HDTV, MP3 streaming audio and multiple telephony and Internet connections. In addition, with extensions, the 802.11a standard could be upgraded to data rates of 108 Mbits/s, with backward compatibility to existing 802.11a functionality at 54 Mbits/s. Therefore, even further scalability is possible without affecting the backward compatibility of client devices.
The data rates of Bluetooth, 802.11, 802.11b and HomeRF, even with the recent extensions as allowed by the FCC, achieve no more than approximately 10 Mbits/s of system capacity. That is not enough to carry even a single channel of HDTV. Those systems do not provide the network capacity for the networked multimedia home.
While 802.11a traditionally has required expensive fabrication processes-such as gallium arsenide (GaAs), silicon-on-insulator (SOI), and silicon germanium (SiGe)-it is irrational to continue to develop products based on standards that cannot deliver what the market needs.
The 802.11a cost factor will not be an issue when an integrated 5-GHz transceiver solution becomes available in CMOS. This silicon is practical, using currently available CMOS technology, and will eliminate the expense of GaAs, SOI or SiGe technologies.
802.11a enables wireless networking products that exceed the performance levels of traditional standard Ethernet wireline solutions by up to a factor of five while simultaneously delivering the robustness of OFDM and U-NII band operation. Using a single, low-cost process technology such as CMOS for the entire signal processing functionality allows for extremely high levels of integration and commensurately lower costs. In fact, CMOS implementations can be delivered at prices equivalent to today's 802.11b technology.
A high-performance, cost-effective wireless solution will support location independence, mobile computing and multimedia applications. Only the 802.11a standard can concurrently address the issues of capacity, cost competitiveness, signal quality and interoperability. Products based on the IEEE 802.11a standard have the best chance of succeeding in the market, since they can deliver the multimedia-rich applications without compromising performance.