Understanding the range of wireless systems

When you install a wireless router in your home, how far does it reach? How far can you be from the local cellphone tower and still carry on an uninterrupted conversation? What if instead of a voice call you have a data call, trying to browse the net or download some files? This distance is commonly referred to as the range of the wireless system.

Range is of course an important factor in characterizing and comparing the performance of different wireless systems. Having greater range is generally very beneficial, so we often see statements such as “WiMax has a far greater range than WiFi”, or “802.11n increases both the range and throughput of a wireless network”, or “xMax was able to demonstrate range orders of magnitude farther than other broadband technologies such as Wi-Fi.” What do these statements mean? Are they reliable? The answers, sadly, are “not much” and “not really”.

To understand what is going on here a brief explanation is needed. Consider a wireless system consisting of a transmitter at point A and a receiver at point B. The transmitter transmits an RF signal of a certain power. As this signal propagates from point A to point B only a small fraction of that power reaches the receiver. The ratio of these powers is referred to as the “pathloss” associated with the wireless transmission between these two locations.

For a wireless system to operate reliably the power of the signal at the receiver must be greater than some minimum value. The ratio of the transmit power to this minimum receive power equals the largest pathloss which the wireless system under consideration can handle. This pathloss as an important, well defined, and easily computed performance characteristic of any wireless system (This computation involves evaluating the “link budget” which I discussed in another post). As an example, WiFi systems typically can handle pathloss on the order of 110 – 120 dB. (As an aside, I assume here that the receiver only has thermal noise, but no interference. I will discuss the effect of interference in a separate posting).

Now the users of these systems are not interested in pathloss (and often have never heard the term). What they want to know is the range – how far does it reach? And therein lies the problem. A given pathloss can translate into very different ranges depending on the scenario. Consider for example a case where the tranmitter and receiver are located on high antenna towers with clear line of sight between them. Or better yet, assume that the antenna towers are on two different mountain tops with a valley in between. In this case a pathloss of 130dB may correspond, say, to a range of 30 miles. Next consider a case where the transmitter is located on a rooftop in a dense urban setting, and the receiver is in a device located on the first floor of an office building. In this case a pathloss of 130dB may correspond, say, to a fraction of a mile. If there happen to be some tall buildings or some natural obstruction between the transmitter and receiver, the range may reduce even further.

As an example, one sees statements such as “A general rule of thumb in home networking says that 802.11b and 802.11g WAPs and routers support a range of up to 150 feet (46 m) indoors and 300 feet (92 m) outdoors.” However, you can also read about the “New WiFi Record: 237 Miles” (It should be noted that to achieve this distance they used much better antennas than a standard WAP or router).

The point of this discussion is that the maximum pathloss that a system can handle is a much better performance indicator than the highly variable range figure. When you compare two systems, the first capable of operating with 120 dB pathloss and the second with 130 dB (10 times higher), you can be confident that the second will have a larger range than the first. Just how large will depend on the particular scenario. If on the other hand the literature describing system X claims that it has a range of 500 feet, which is twice larger than the range of 250 feet claimed by the competitor’s system Y, this claim has little or no meaning unless the two systems are being compared fairly in the same scenario. If the two range figures correspond to significantly different situations, system Y may well be the one which can handle a larger pathloss, and therefore have the larger range!

So whenever you read statements about the range of a wireless system, you should ask: Under what conditions was this range observed? What was the pathloss? If it is a comparative statement – were the two systems tested with the same pathloss? If not, what were the pathloss values. Unfortunately this information is (essentially) never given. Range, and other performance metrics, are routinely stated in a way that makes it impossible to properly evaluate their meaning. This makes it possible to make exaggerated and misleading claims of performance, without outright lying. WiMax can indeed transmit 75 MBPS over a distance of 30 miles, as was often claimed. However, you need a very unusual set of circumstances to make this happen (high gain antennas, free space propagation). The range in any practical scenario will be much much less. A system such as xMax can transmit a 3.67 Mbps signal using 35 milliwatts over a distance of 18 miles on the 902 MHz “ISM” band under ideal circumstances. Again, the range in any practical scenario will be much much less.

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3 responses to “Understanding the range of wireless systems

  1. Not only does range depend strongly on the circumstances, but it’s not always even a good thing. In a heavily loaded cellular network, too much range carries interference over great distances and makes it difficult to “reuse” the same channels for other calls in other
    areas. This is why mobile phone cell sites in urban areas are generally mounted on low buildings or towers, with directional, downtilted antennas.

    Cell sites are generally found on tall towers or mountaintops only in rural areas where coverage is more important than capacity.

  2. What you are saying is true, but in general it is desirable to have larger cells so as to decrease the number of required basestations. This seems to imply that larger range is generally desirable even in a cellular scenario. You can always reduce the range by turning the power down and that is where power control comes in, but that is another story. My comments really applied to the maximum range achievable by a given system.
    In any case, the full story is significantly more complicated and needs to be told in several installments. I will try to post explanations of what happens when there is interference (which I explicitly ignored here), discuss the difference between range and coverage, and so on …

  3. You’re quite right that you don’t want to build any more base stations than absolutely necessary, and range helps with that. But spectrum can also cost quite a bit of money, so it’s often cheaper to increase system capacity by building more base stations than by buying additional spectrum. As the number of cells goes up, the area served by each base station goes down — and the average mobile-to-cell range decreases — and at the system level each RF channel carries more calls.

    As a side benefit, the RF power that the mobiles must transmit to reach the closer base stations also goes down, increasing battery life and decreasing RF interference and any biological exposure problems.

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