WiMAX questions
with reference to yousuf001's questions...
1) OFDM is implemented on OFDM256 and OFDM1024 in 802.16d and 802.16e respectively.
2) WiMAX is better for a number of reasons. OFDM is but one of the reasons why it works brilliantly in a non-line-of-sight mode. In fact, in our tests, standing under the transmitter, we got about 4 times better throughput by aiming the CPE into an obstacle/building etc in order to capture the bounce. The Quality of service component built into WiMAX is also very capable. It supports best effort, real time polling, non real time polling and continuous grant. Effectively it means the operator can set up service streams for you inside you service, so that you will say always have a 56kbps for VoIP regardless of other traffic on your pipe. The technology itself is TDMA which meas that the inherent problems of cell breathing experienced by CDMA technologies is simply not present. Most of the newer equipment becoming available is TDD as opposed to the FDD stuff from earlier. It makes more sense in a spectrum restricted market such as ours and furthermore it allows for dynamic scaling of uplink versus downlink.
3) The speed is achieved mainly because of two reasons. Firstly because of the way in which WiMAX allows you to select your channel bandwith. 802.16d profiles are normally in 3.5MHx increments with 7 MHz being very popular. This is compared to UMTS/WCDMA which operates in 5MHz bands (not stackable to make the pipe wider). 802.16e mobile WiMAX uses 5 MHz bands as well, but again you can choose to use 10MHz and double your throughput. Furthermore you are looking at a much better modulation scheme. 3G supports only QPSK, and with HSDPA you get 16QAM on top of that. HSUPA as far as I know does not do anything for the modulation scheme even on the uplink. On WiMAX you get 64 QAM,whcih brings you very close to 4bps/Hz, which is very high considering you are still only using one antenna element. MIMO in 802.16e will bring the spectral efficiency up to almost 10bps/Hz but then again, 3G/HSPA will also have this so involving antenna trickery does not really make for a fair comparison. Also the link budget on WiMAX (especially mobile WiMAX) is significantly better that for 3G due to smart technologies such as sub-channelization on the uplink.
4) Answer is pretty much contained in the above - also remember it is horses for courses. WiAMX was initially developed to be a broadband wireless technology whereas 3G started as a voice carrier with continuous evolution towards data carrying abililty.
Hope this helps...
