IEEE 802.11n is the new international standard for wireless Local Area Networks, incorporating new smart antenna technologies (MIMO - Multiple In and Multiple Out) permitting a 5x performance and 2x coverage improvement for WLANs. While this new technology is now becoming the de facto standard in consumer and enterprise networks, it has not yet made an appearance in outdoor, metropolitan scale networks derived from WiFi technology. Many of these same MIMO techniques are being incorporated in both WiMax and in LTE for cellular. Sadly neither is being produced in much volume as yet and fixed WiMax networks do not incorporate MIMO technology. There has been much dispute about whether the specifics of 802.11n designed for indoor networks would apply to outdoor networks and bring the economy of scale of 802.11 to outdoor applications. We (Novarum) decided to test the effects of .11n on outdoor performance. We found dramatic improvements in using indoor 802.11n technology outdoors - so much so that 802.11n has become the recommended baseline for new network deployments. First, let’s review the key pieces of technology incorporated in 802.11n and how it might affect outdoor performance.
In the course of Novarum’s Wireless Broadband Review in 2007 and 2008, we examined over 25 deployed WiFi networks (including all major vendors), 46 deployed 3G cellular networks and 4 fixed pre-WiMax networks. In the case of the WiFi networks, we noted the dramatic effects that client selection had on network performance, coverage and ultimately user satisfaction.
We both examined 802.11n clients against the installed multivendor base of 802.11g infrastructure and constructed our own testbed from early outdoor 802.11n components to evaluate the effect of 802.11n when deployed in the infrastructure itself.
It is important to recognize that in almost all outdoor WiFi networks, the client access uplink is the weakest link in the communication chain. Legacy 802.11b/b clients experience VERY high packet retry rates of between 100 and 1000% and there are often deep multi-path fades of between 10-30 dB within a few tens of feet. The WiFi protocol is VERY good at masking these effects - instead they are most commonly seen indirectly - by lower throughput and higher delay variance.
These effects are seen even for deployments of very high access node density of 50 nodes per square mile or more.
These deep fades and very high packet retry rates made mobility difficult, dramatically effect throughput, make packet delay variance so high as to make streaming media difficult and materially decrease the overall capacity of these networks.
The improvements that 802.11n provides outdoors astonished us - particularly for a technology that has been disparaged as inappropriate for outdoor deployment. Deploying IEEE 802.11n technology has dramatic effects outdoors - both with legacy systems and even more compellingly with green fields deployment.
Let’s summarize the facts of what we found in Novarum’s experiments:
|Maximal Ratio Combining||Receiver combines signal from multiple paths to maximize SNR. We can see a 3-4 dB receiver link budget improvement even to legacy clients.||Dramatically improved receiver signal strengths and dramatically reduced packet error rates. More reliable use of higher level encoding methods increasing link performance.|
|Transmit Beam-forming||Modulate phase and amplitude from multiple antennas to create phased antenna array pointing increased performance at the destination node. 7-8 dB gain possible with omnidirectional antennas||Decreased inteference, increased capacity, decreased deployment cost. Likely directional antenna performance with omnidirectional antennas substantially increasing network performance and decreasing deployment cost.|
|Spatial Multiplexing||Use the redundant paths created by multipath to increase throughput by transmitting data in parallel paths.||Probably not compelling outdoors since high SNR needed for parallel data paths. However, increasing reliability by taking advantage of multipath around deep fades.|
|Channel Bonding||20 and 40 MHz channels in both 2.4 and 5 GHz bands||20 MHz channels legacy compatible while 40 MHz channels double throughput, mostly useable in the 5 GHz band.|
|Protocol Improvements||Packet aggregation||Modest overhead reduction and performance improvement for streaming media and bulk transfers|
|Cost Reduction||Indoor WiFi network demand is for low cost, dual band (2.4 and 5 GHz) simultaneous radios at commodity prices||The availability of these dual band 3x3 MIMO chipsets drives the cost of multiradio outdoor units down|
- 100% throughput improvement of 802.11n WiFi clients with legacy 802.11g outdoor infrastructure;
- 100% throughput improvement of legacy 802.11g WiFi clients with new 802.11n outdoor infrastructure;
- 200% throughput improvement of 802.11n client with 802.11n outdoor infrastructure;
- Similar coverage of 802.11n clients and infrastructure in the 5.4 GHz band as for legacy 802.11g clients and infrastructure in the 2.4 GHz band - making the 5.4 GHz band useful for client access;
- 25% decrease in access latency and a dramatic improvement in latency variance;
- a low power 802.11n client has the same throughput and coverage as a high power 802.11g with 10x the power and antenna; and
- coverage to smartphones at low power and with poor antennas dramatically improves.
- 4-800% increase in system capacity and throughput
- 2-300% improvement in spectral efficiency through increased link budgets, reduced packet errors, increased modulation rates and improved fading performance
- effective client access to the 200 MHz of the 5.4 GHz band
- 802.11n clients dramatically improve legacy 802.11g networks and new 802.11n networks dramatically improve legacy 802.11g clients.
- Streaming media applications will perform as we expect and will be much easier to deploy.
- Better backbone designs by reducing the interference of the backbone mesh through beam-forming antennas rather than omnidirectional broadcast.
- Decreased deployment cost due to decreased node cost. Possibly dramatically.