802.11n 100Mbps WLANs: Some Day

As if there isn't enough confusion out there concerning the alphabet
soup of wireless standards, the last thing we need is the VP of
Broadcom's enterprise computing division talking about products based on
standards that are about as mature as an eighth-grader. A Broadcom
spokesperson was subsequently quoted as stating the company had "no
plans to preemptively launch products that could possibly undermine the
strong standards-based industry that we've helped build."

So what's really going on here? The 802.11n standard, which calls for
WLANs with performance of at least 100 Mbps, is in its very early stages
of development and, given the immense complexities, I'll be shocked if
it is real before 2006. The technical hurdles are immense. Unlike
previous 802.11 standards, which have defined the system's physical
layer data rate, 11n defines performance as effective throughput. When
you account for overhead from interframe spacing, packet preambles, MAC
headers, contention and acknowledgments, today's 802.11 products have
typically achieved TCP throughput of about half the data rate (5.5 Mbps
for 11b and about 27 Mbps for 11a). Further, this roughly 4x improvement
in throughput must be achieved without decreasing effective range.
Applying Shannon's capacity limit theory using today's modulation
systems, the maximum possible data rate using existing 802.11 channels
is 133 Mbps. Even if the underlying MAC design were radically altered,
it would appear insufficient to achieve the stated goal of 100 Mbps
throughput.

One solution to this problem is to use multiple RF channels, thereby
doubling the available bandwidth. Both Broadcom and Atheros have used
this channel-bonding technique in proprietary implementations to boost
the performance of their systems. But it is far from ideal: Throughput
typically increases by less than 30 percent; it makes dense AP
deployment much more difficult; it reduces effective transmission range;
and it makes legacy system coexistence much more difficult. Overall,
it's a poor solution.

Most experts I have spoken with agree that newer, more spectrally
efficient wireless modulation technologies are far more preferable than
using larger channels. It's highly likely that some form of MIMO
(multiple-input multiple-output) technology will form the basis for
future 11n standards. But today's MIMO implementations, while promising,
fail to meet the 100 Mbps throughput standard, and they also can be
expected to suffer from other deficiencies normally associated with
first-generation implementations.