UMTS/HSDPA uses a 5 MHz channel. It combines voice and data channels. For circuit voice it is similar to CDMA 1xRTT, and for fast data it is similar to CDMA EV-DO. However, it uses a 5 MHz channel and can therefore carry more traffic. Typical voice capacity in UMTS/HSDPA is 32-170 simultaneous calls per carrier. The highest data rate of 10.7 Mbps requires the same CINR as for the EV-DO 2.4 Mbps speed.
To put these differences in perspective, Figure 1 shows the differences in coverage areas and subscriber capacities of cells using GSM, CDMA 1xRTT, and UMTS. The assumed usage scenario is a dense office environment (150 square feet per subscriber) with medium user traffic (40 mErl, where an Erlang represents the total traffic volume of one hour, or 3600 seconds).
The area listed below represents the area where the base station would be 100 percent loaded. The number of antennas represents how many antennas are needed to cover this area assuming 20,000 square feet per antenna for voice coverage.
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Figure 1: Typical Coverage and subscriber capacities for GSM, CDMA, and UMTS.
Coverage and Interference Challenges
As mentioned previously, coverage and interference are two of the main factors affecting the quality of cellular service. The challenges of providing in-building coverage become even more difficult as providers roll out 3G data services such as EV-DO and HSDPA, because path loss attenuation increases with the frequency of the signal, particularly with respect to free space attenuation (objects typically attenuate signals at a more constant rate).
The 1900 MHz frequency band has higher attenuation than the 850 MHz band, so some mobile operators use the lower frequency band to get better coverage. (As a guideline, doubling the frequency reduces the link budget by 6 dB.)
Since the receiver in the base station or terminal needs a "cleaner" signal (equal or better CINR) for high data rates, it also requires a stronger signal because noise can be considered as an interferer. Higher frequencies attenuate more quickly, so if the protocol requires a 6 dB higher CINR ratio, for example, it reduces the maximum allowable path loss by at least 6 dB.
A 6 dB decrease of path loss is the equivalent of reducing the cell area by 50 percent. This has a significant consequence for high speed data rates since it requires a stronger signal (lower maximum path loss) as well as a better signal (higher CINR requirements).
The data in Figures 2, 3, and 4 show the CINR requirements for GSM, CDMA, and UMTS. As we have seen, the higher the data rate, the more stringent the requirements.
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Figure 2: GSM CINR requirements.
Note in Figure 2 that the difference in quality for the minimum and maximum protocol is about 23 dB, which means the maximum path loss is reduced by 23 dB. As a consequence, the area of a cell using high data-rate radios is about 6.2 percent as large as it would be for a cell using low data-rate radios, and the cell radius drops to about 25 percent of what it would be for a cell with low data rate radios.
Figure 3 shows the required CINR for different data rates with CDMA EV-DO. The CINR values include the coding gain.
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Figure 3: CDMA EV-DO CINR requirements.
Above 153 kbps, the signal requires a CINR greater than -5 dB and therefore would require frequency planning. Note the difference in CINR for the minimum protocol and maximum is about 22 dB, which means the maximum path loss is reduced by 22 dB.
As a consequence, the cell area of a cell using high data rate radios is about 7 percent of what it would be for a cell using low data rate radios, and the cell radius drops to about 26 percent of what it would be for a cell using low data rate radios.
Figure 4 shows the required CINR for different UMTS/HSPDA data rates. The CINR values include the coding gain.
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Figure 4: HSDPA/UMTS CINR requirements.
Above 480 Kbps, the signal requires a CINR greater than -5 dB and therefore would require frequency planning. Note the difference in CINR for the minimum and maximum protocol is about 25 dB, which means the maximum path loss is reduced by 25 dB. As a consequence, the area of a high data rate cell is about 4.8 percent of what it would be in a cell using low data rate radios, and the cell radius drops to about 22 percent of what it would be for a cell using low data rate radios.
Figure 5 summarizes the effect of lower path loss on cell radius and cell area for a typical indoor installation.
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Figure 5: Effect of path loss on radius and cell area.
If a network is designed for voice and low data rates, the antennas are farther apart and the interference is less of an issue. As a consequence, only small portions of the network will be able to offer high speed data. These high data rate areas can be as small as 5 percent of the voice area.
However, if the network is designed for high data rates, the antennas will be much closer together (up to 20 times more antennas). Interference is still an issue for an outdoor network. Frequency separation in CDMA or UMTS networks would be needed to achieve the required quality of the signals. For an indoor network, the interference level can be significantly reduced by using a DAS system. Therefore DAS systems may not require frequency separation.
In-building Technology
Essentially, an in-building system replicates the same type of coverage in an outdoor (macro) area in the more challenging indoor environment, where steel, concrete, furniture and equipment attenuate the signal much more quickly. This is accomplished with a distributed antenna system (DAS) that propagates the signal. There are two parts to the deployment process:
- Operators bring the cellular signal to the building, either by deploying a base station (BTS) in the building's equipment room or by adding a rooftop antenna and then placing a repeater in the equipment room. (Most DAS are now being deployed with dedicated base stations because the cost of smaller BTS continues to decline, and BTS provide dedicated capacity.)
- Operators and/or the building owner install a DAS to distribute the signals from the repeater or BTS throughout the building.
The BTS technology used for in-building coverage is the same as it is for macro cells, although the units are typically smaller. However, there are three types of DAS in use: passive, active, and hybrid. Each type of DAS has unique performance characteristics that impact the system's ability to provide in-building coverage.
Next week, we'll compare various in-building cellular systems and see how they stand up to the technical challenges of cellular communications.
About the author
Stefan Scheinert is the Chief Technical Officer of LGC Wireless. He has more than 25 years' senior management experience in the design, development, and commercialization of innovative mobile communications products, and holds a Masters Degree in Electrical Engineering from Germany's University of Braunschweig. He can be reached at [email protected].
