Recent developments in fixed wireless technology have opened a much-needed way for cable operators to realize greater returns on investments in the commercial services business, provided they’re able to cut through a blizzard of confusing information to find the right wireless solution for each application.

With so much being written about fixed wireless trends and technology that is largely useless to cable operators, this is no small challenge. For example, there is a lot of noise about the grassroots explosion of broadband service wannabes who have made WiFi applications in so-called “neighborhood LANs” practically synonymous with fixed wireless in some people’s minds. Anybody who believes WiFi represents the state-of-the-art in fixed wireless is sure to dismiss the concept out of hand when it comes to finding a serious platform for delivering commercial class data, let alone voice.

Equally distracting is the noise coming from the opposite end of the tech perspective on fixed wireless, where a mind-numbing array of advanced techniques are each touted as the ultimate solution for providing commercial services over spectrum tiers outside the 802.11x WiFi zone. As it turns out, some of these solutions are, indeed, valid means of getting the job done, as demonstrated in various commercial deployments now underway here and there. But no one platform is optimal for all situations.

For cable operators, the challenge is finding the right platform for a specific set of conditions related to interference, transmission distances, bandwidth requirements, positioning of buildings and much more. This means cable operators must work with a supplier whose wireless product portfolio is broad enough to address a wide range of access scenarios. If a simple, low-cost approach can get the job done, as is the case in most situations, operators should have that option to choose from. If more advanced solutions are needed, then those should be readily at hand as well.

Cost-containment imperative

Notwithstanding the confusion surrounding fixed wireless, operators would be making a mistake if they were to dismiss the idea of using the technology to overcome an access problem that has made commercial services a tougher nut to crack than anticipated. When operators target a promising business neighborhood they often find that only a small number of the potential customers can be reached by their networks without incurring new capital expenses, which means the costs of marketing, connecting and servicing that neighborhood are spread over only a small segment of the potential customer base. Such inefficiencies often result in costs that are too high in proportion to the revenues an operator would earn, leaving many to wonder if all the money spent on putting together a business services unit was really worth it.

Use of fixed wireless access can alter the cost-to-revenue equation by delivering a much greater concentration of customers within a business neighborhood if two primary conditions are met:

  1. There is no reduction in quality of services offered over these connections.
  2. The costs of using the wireless platform are not significantly higher than the costs of connecting a customer who happens to be in close proximity to coaxial or fiber plant.

Fortunately, both of these conditions now have been met.

Several issues must be addressed by operators to ensure they are getting the right bang for the buck in choosing a fixed wireless system, starting with the question of whether the use of spectrum is going to be a cost item. If it is, that just about guarantees wireless will be a nonstarter, considering what it takes to compete in spectrum auctions at the FCC. But, fortunately, the commission has allocated three swaths of bandwidth adding up to a significant slice of spectrum in the 5 GHz tier for unlicensed, free use under U-NII (Unlicensed National Information Infrastructure) regulations. These allocations, at 5.15-5.25 GHz, 5.25-5.35 GHz and 5.725-5.825 GHz, have the advantage of opening spectrum that is free from widespread use, unlike the crowded unlicensed bands at 2.4 GHz and below, but is still low enough in the microwave range to support propagation at reasonable distances without much vulnerability to atmospheric absorption or low-density obstructions. The next question you must ask is whether the price to be paid for free access to spectrum is the risk of interference from other unlicensed users who might be operating in the same neighborhood. Interference is an especially onerous issue in point-to-multipoint (PMP) distribution of IP-based traffic, where a low proportion of RF interference can produce an untenable reduction in data rates. The severity of the potential problem in a PMP deployment increases with the angle of the beamwidth at the cell access point. And while point-to-point (P-P) transmissions are not nearly as susceptible to interference owing to the more focused area of coverage in such transmissions, the problem still needs to be addressed. How to beat interference

The solution to the interference issue lies in finding a cost-effective way to minimize the impact of unwanted signals while maximizing the number of users that can be served from each point of deployment. Many people push the envelope on modulation to maximize data throughput over any given slice of spectrum and then apply innovative means to mitigate the interference threat that attends packing a lot of data signals into a high order QAM (quadrature amplitude modulation) or even OFDM (orthogonal frequency division multiplexing) constellation.

There are interference-intensive situations where such solutions might make the most sense. For example, interference impact can be minimized by employing a system that transmits at high orders of modulation over many different 6-MHz channels, thereby giving the operator the option to transmit over frequency bands that are subject to the least amount of interference while still ensuring there is plenty of bandwidth to work with. The use of narrow channels at high levels of modulation also can be the optimal approach in situations calling for many P-P links. Where a platform using wider channels might be limited to deployment over three or four links owing to self-interference caused by re-use of spectrum within the system, the narrow-channel system can support as many as two dozen links.

Low-order modulation

In contrast, operators will find there are many, if not a majority, of situations where the best solution is to go with a low order of modulation in combination with other techniques that contribute to creating a highly interference-resistant, bandwidth-efficient environment. Nothing beats the resistance of a simple binary frequency shift keying (BFSK) signal, where the receiver only has to tell the difference between two phases of a sine wave to obtain a bit value of one or zero. Where a 16-QAM signal requires a carrier-to-noise ratio (C/N) of ~19 dB and a carrier-to-interference ratio (C/I) in the 12 to 14 dB range to achieve a bit error rate (BER) of 1 x 10-4 , a high-index BFSK signal can achieve the same BER at a C/I ratio in the single digits.

Another way to minimize interference is to ensure that undesired signals coming from other directions from the main path of the primary signal are received by the antenna at a low level in comparison to the primary signal. For instance, the front-to-back ratio in one working system is 20 dB, which, in conjunction with the low C/N capability of BFSK, allows an antenna receiving a primary signal at the weakest detectable level to sustain the connection at acceptable BER levels even when it is simultaneously hit from the opposite direction by a signal with an attenuating impact of as much as -60 dBm. These two methods combined have been proven in the field to be an effective method of getting good area coverage, which otherwise would be susceptible to both self-interference and interference from other systems.

Greater protection against the effects of interference can be achieved through a technique designed to prevent errors from triggering the TCP/IP mechanism that slows an IP network down on an assumption that packets are being dropped because of congestion. In the radio segment of IP networks, an interference-induced error can be detected as such with a mechanism at the receiver that inspects the transmitted radio data packets for errors, thereby triggering a request for retransmission from the sending radio and avoiding detection of the error at the TCP layer (Layer 4) of the protocol stack.

Another mechanism against interference is centralized control over transmissions to and from subscriber modules (SMs) to prevent proximately located access points (APs) and SMs from interfering with each other. This is done through transmission synchronization so APs and SMs never transmit at the same time, and SM antennas transmit and receive on individually assigned time slots in time division multiplexing (TDM) mode.

Maximizing bandwidth

With the robust levels of performance assured at very low cost through use of BFSK and the techniques discussed here, the issue then becomes bandwidth efficiency. This problem is made easier to address by the fact that a large amount of spectrum is available in the 5 GHz U-NII tiers. But, if an operator wants to to serve many dozens of locations from a single AP, other techniques must come into play if BFSK is to be the modulation of choice.

One of these is time division duplexing (TDD), which uses the same frequency band for upstream and downstream transmissions, thereby eliminating bandwidth-consuming guard bands that are used when two-way transmissions occur over separate bands. TDD also supports dynamic changes in bandwidth allocations for upstream and downstream communications.

Another key to bandwidth efficiency can be the use of more than one segment of the 5 GHz U-NII spectrum to serve customers. For example, in the system referenced here, with a six-sector antenna array providing 360 degree coverage over an area up to two miles in radius, radios operating at the 5.2 GHz and 5.8 GHz tiers can be co-located, effectively doubling the raw PMP throughput per antenna sector to 20 Mbps, which translates to 12.4 Mbps in effective throughput per each 60 degree segment of the serving area.

In a typical deployment, an operator might serve many locations through use of a high-power P-P antenna located at the HFC fiber node and connected directly to the headend router via a dedicated fiber or wavelength. This radio would connect to a building where a PMP access point is positioned to serve a cluster of nearby locations, and another P-P hop might connect still another PMP AP at another building, and so on. Because these P-P links can operate at higher power and are less vulnerable to interference, they can use higher orders of modulation than BFSK to achieve raw throughput on the order of 50 Mbps, thereby supporting a great number of users connected via the secondary PMP. (A secondary, BFSK-based P-P link also can be used at the AP in instances where high levels of dedicated bandwidth are required.)

In the example in the figure on page 34, 72 customers subscribe to services based on committed information rates ranging from symmetrical 56 kbps all the way to asymmetrical 15/1.5 Mbps. Four customers take services at tiers above 1 Mbps, while 11 are in the 500 kbps to 1 Mbps range, and the remaining 57 customers operate below 500 kbps.
The time has arrived for cable operators to examine the role fixed wireless can play in the commercial services business. In an environment where it can cost $100,000 to run an underground cable 30 feet, a fixed wireless solution can pay for itself in three months. The key to success is to have a choice of wireless solutions that address the full range of operating conditions.

Seton Kasmir is the founder, CTO and chairman of Viadux. Email him at [email protected].

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Figure: Fixed Wireless Example


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