## WiMAX Receiver Sensitivity

I have some problem to understand the Sensitivity Calcutaion in IEEE 802.16e-2005.

For the OFDM PHY, at page 351 of the standard (Receiver Requirements), the sensitivity is scaled by the number of active subchannel (for example in downlink i can use only one among 16 channels).

In the OFDMA part (page 646, Receiver sensitivity) in the expression appear Nused*(Fs/Nfft), where I understand that Nused are all subcarrier except DC subcarrier.

So, there is a difference in the calculation. For the OFDMA the expression rapresent the minimum received power that guaranted a BER of 10^-6, in the case of the end user in uplink used all the available channels.

This is the worse case, infact the subcanalization has the following advantage:

1) the trasmitter power is concentrated in a limited bandwidth, so this can increase the coverage

2) the Bandwith, in the expression of sensitivity, is smaller, so the minimum received power is less that in the case of all sub-channel allocated to one user.

Can anyone explain me the difference?

Receiver sensitivity is a factor of bandwidth as follows:

Receiver sensitivity = -174 + 10log BW + NF of receive amp

so, the narrower the bandwidth the lower the noise, hense the lower the receive thereshold for narrower BW.

The Standard 802.16e (I refer to OFDMA) specify:

Rss=-114+ SNRrx - 10log(Repetition Factor) + 10log( FS x Nused / Nfft) + ImpLoss + NF

Where, in according to the definition of "Nused", the term "Fs x Nused / Nfft" is practicaly the bandwidth occupied by all sub-channels.

So, this is the minimum sensitivity for a user that use all available subchannels (in OFDMA). But if a MS use only one subchannel, the BW is narrow hense the receive thereshold is lower.

If I use this expression for sensitivity calculation I have the worst case results, because the expression don't take in account the effect of subchanalization.

Is this true?

Maybe, I think I can take in account the canalizazion effect as a Gain in the link budget (One gain for the power concentration in a sub-channel, and one gain to compensate the fact that the sensitivity, in reality, is better).

But my problem is: "how can I foreseen the canalization gain if I don't know how many sub-channels the scheduler of the BS allocate for each user?"

Download the 802.16e standard (if you haven't already) and do a "find" on the key words "Link Budget". You will find your answers here.

I started to try to describe Link Budgets and Path Balance but they do a better job than I do.

"Receiver sensitivity = -174 + 10log BW + NF of receive amp"

What's NF? Is this equation valid for single carrier modulation?

-174 is the thermal noise floor, 10log BW applies to bandwidths, run a couple of exercises, try 200 khz, then 1.25 Mhz then 5 Mhz, etc.

NF is the noise figure of the receive amp. BTS amps run around 5-7 db, smaller cellphone amps run higher.

Receiver threshold is the amount of receive signal required to obtain a certain throughput at 1 x 10-6

What about this equation Rss= SNR-10log(BW/Rb) + Nw +Nf

Nw:thermal noise floor ; Rb: data rate (b/s)

is it equivalent, there is an extra term (SNR+logRb).

An other question: the required SNR have to be calculated or is given, what's the formula if yes?

The signal part of the equation depends on the strength of the recieve signal. The noise part is dependent of the bandwidth of the receive filter. The next factor to understand is the interference, because the determining factor for throughput will be decide by the type of modulation and coding, and that is determined by the Signal to Noise + Interference sometimes written CINR for Carrier to Interference & Noise Ratio or C/N+I.

I was referred to this equation just to have relation between the range and the throughput, actually I forgot the source.

Also I’m working with the following formula:

Rss=-174+10 log(BW(Hz))+SNR+NF+10log(Nsubchannels)

Referring to an example of BL attached. Also I use these values of SNR:

Modulation coding rate SNR Rx

BPSK ½ 6.4

QPSK ½ 9.4

QPSK ¾ 11.2

16-QAM ½ 16.4

16-QAM ¾ 18.2

64-QAM 2/3 22.7

64-QAM ¾ 24.4

Are those values valid for all bandwidths (exactly 25MHz)?

Other question, what are the typical values of antenna gain? directive=17 dBi; omnidirectionnel=0 dBi what about sectoriel?

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