Link Budget and Penetration  

The link budget is used to determine the maximum allowable path loss for a BALANCED LINK. The purpose is to ensure that the CPE can talk back to the base station.

The higher the frequency the faster the rate of attenuation, the less distance covered. Remember

Path Loss = 32.45 + 20*log D(km) + 20*log F(Mhz)

WiMAX is not like CDMA with RW and no of RWs available to use. There is no concept of RW per radio in WiMAX. In WiMAX, it is all about Service Flows that can be supported in each radio. The larger the number of SF created in a radio channel, the smaller each SF bandwidth will become. To counter this, WiMAX support packet priority and CIR /MIR to allocate this valueable resource accordingly (bandwidth) per SF.

64QAM means higher throughput but needs good SNR to achieve. BPSK is less demanding on SNR and give you lowest throughput. This is the modulation usually used for acquisition of the radio link to ensure the best chance of getting a 2way comms with the BS. For BPSK operation at non LOS condition (one brick wall blockage), the range could be 3-4km. General rule of thumb is, a single brick wall gives around 10dB attenuation, metalise window could give as much as 20dB attenuation. So if you can get 64QAM operating at -70dBm, shuting a metalise window could drop you down to BPSK operation.

1) Yes, you do have a link budget for each modulation scheme, more specifically you would create a link budget for the Pilot/Preamble, UL/DL MAP channels and then the UL/DL traffic channels with the most robust modulation scheme. Choose the weakest link, and the MAPL from that will define you cell range.

I recall a WiMAX Forum document with example link budget in this regard (I beleive the title of the document contains "Part 1: Technical description").

2) I'm guessing you mean 3.5 and5.5 Ghz compared to the GSM 850/900/1800/1900 MHz bands. No doubt will you have much lower in-building coverage/penetration with the higher WiMAX bands compared to the GSM bands ...

At this stage you will seldom see any real link longer than, say, 5km, and in this range a link budget is nicely established by power control. Everything more than that is usually some experimental thing, and there is really no point of rubbing in the 75Mbps at 30km any more - it washes off.
For any successful wireless business you simply MUST have most of your users at QUAM 64 3/4 (or better :). To squeeze the most of it, you must have some surplus juice (for power control), lowest possible installation (for better reuse – steeper signal decay), cheap installations, and no antenna tilting. Then it is perfect.
To do that, your footprint gets small, and in case of indoor it gets soooo small that WiFi becomes equally viable solution.

Link budget 2.doc


[get this widget]

AddThis Social Bookmark Button