WiMax RF Transport Layer in Frequency Division Duplex (FDD)

The antennas and filters provide noise and interference rejection to the front end amplifier. The aggregate performance of these devices is comparable through both up and down links. The hub antenna will typically cover a sector ranging from 45 to 120 degrees in azimuth. The customer premise electronics (CPE) antenna will have a gain ranging from about 7dBi to 25dBi. Higher gain CPE antennas tend to be larger and prohibitively expensive.

The spatial filtering of the CPE antenna provides a degree of interference rejection not available in a cellular system. An externally mounted WiMax CPE antenna can consistently provide between 25dB and 40dB of additional isolation between cells. In contrast, cellular phones have omni directional antennas that provide no isolation between cell site locations. Hence the isolation is purely due to the frequency filtering and the propagation characteristics.

The cost constraints on the hub are not as stringent as those on the CPE. Therefore higher Q filters and higher performance antenna can be employed at the hub end. Because line power is generally available at both the CPE and hub, both the hub and CPE amplifiers can operate in a linear region. Therefore, spectrally efficient modulations such as versions of OFDM with QAM are used. The level of the modulations (i.e. QPSK through 64-QAM) will be determined by the acceptable bit error rate (BER) and the signal to noise ratio (SNR) in a sparsely deployed system or Carrier to Interference (C/I) in a densely deployed system.

For simplicity let us assume that we can increase our N-QAM level by a factor of 4 for every 7dB improvement in SNR. This corresponds to a doubling of the data rate with the same 7dB improvement. The effect of improving C/I is more modulation and error correction dependent, but similar trends hold. Hence, reducing interference increases data capacity so long as we are well above the receiver threshold. Consequently, we can potentially double the revenue generating potential of the base station if we choose RF hardware that improves the C/I by 7dB.

In their most basic configuration, cellular phone systems are sectorized into three-cell configurations. The motivation for this is frequency reuse. In a single cell, the frequency band might be broken into three equal sub-bands with one sub band per sector. This in itself does not provide any frequency reuse over an omni configuration. However, when the same scheme is used in a multiple cell configuration, the result becomes a reuse of three. The cost of the additional hub equipment required for the sectorization is small in comparison to the over all system cost.

This scheme keeps the C/I ratio “good enough” for phone traffic (CDMA or GMSK) over the bulk of the multicells coverage area and hence maximizes the capacity, which in this case corresponds to the number of users. The cell phone modulations allow the transmitters to work in a saturated mode that increase the power amplifiers efficiency and thus prolong battery life. The tradeoff to these import features is spectrally efficient. GSM’s GMSK modulation can have a "spectral efficiency" of up to approximately 1 bit per symbol or 1 bit per hertz of bandwidth while 64QAM over OFDM might have a spectral efficiency of 6 bits per symbol. The word "approximately" is used because there are several different ways to measure the bandwidth of a signal.

The method used for GMSK signals in GSM is to find the bandwidth that contains about 99% of the radio signal power. Gaussian Minimum Shift Keying (GMSK) was developed specifically for GSM by the COST (Council on Science and Technology and later ETSI). GMSK is a type of minimum frequency shift (MFS) modulation that achieves an approximately optimum compromise between the amount of power out of the desired bandwidth of the modulated carrier signal (only about 1% for the type used in GSM) and still allows the binary data to be accurately demodulated from the received radio signal in the presence of noise. GMSK is one of a number of well thought-out modulations appropriate for mobile phone systems. While great for mobile telephony, it is not the modulation of choice for broadband data due to the low spectral efficiency.