My mistake: Given an equal allocation of spectrum, all mobile systems have higher down-link compared to up-link capacity due to the lower antenna gain, number of antennas, and lower power of small and portable devices. SC-FDMA used for the up-link in 3G-LTE helps to reduce PAPR, peak-to-average power ratio, which lowers the power required of subscriber units and somewhat increases range and number of subscribers supported. However, as capacity per user is increased more base stations, nodes, or remote stations (different network device architecture terminology is used between 802.16/WiMAX and 3GPP LTE) are required which also reduces the distance between network devices and subscriber units. That generally results in higher signal levels which allows use of higher order modulation rates and more symmetrical operating conditions between the up-link and down-link. In other words, the lower power and lower gain and fewer number of antennas of small mobile devices is less of a disadvantage as the distance is shortened.
Where the designs are relatively quickly heading is towards the use of adaptive modulation as I have been predicting: Both 802.16m and LTE-Advanced are looking (somewhat over each others shoulder, so to speak) at using an adaptive scheme that uses OFDMA, SC-FDMA or a proposed modulation variation of OFDMA depending on current channel conditions.
For example: If a user starts out from their home using a mobile device that is communicating to a WiMAX or LTE femtocell that is connected to Comcast or Time Warner high speed cable or fiber optic, the unit would use 64 QAM OFDMA connection. When the user moves out to the patio the unit may determine to keep the connection but shift to a lower modulation rate or to shift to the wide area network. The connection may be made to a local remote station mounted on the side of a building or lamp post, for example. Because the signal level from the mobile device must travel farther, rather than require it to ramp up the power level, the remote station decides, in a hand-shake series of communications with teh mobile unit, to switch to SC-FDMA.
Design schemes being actively considered include using collaborative Co-MIMO, virtual MIMO V-MIMO, and multiple user MIMO and multiple radios or individually modulated signal chains within the same device. This is particularly considered for base stations and remote stations/nodes where more antennas and processing power can more easily be accommodated. However, some think that it is only a matter of time before low power mobile devices will be able to support higher order antenna arrays and multiple signal processing channels/radios needed to be the active participants in these higher order MIMO-AAS methods.
The design constraints of OFDMA are being gotten around and taken advantage of through the use of more advanced network architecture and smart antenna system schemes such that the low power and longer range advantages of SC-FDMA become more narrowly defined and countered but higher bandwidth and lower power methods achieved by improved signaling and smart antenna methods.
If not for the IPR licensing considerations, the two groups might easily derive at adaptive modulation methods that are flexible and allow advances to take place while defining the needed core compatibility requirements.
There is a tremendous amount of R&D activity taking place in the smart antenna, network architecture and various adaptive radio methods that are advancing performance and self-configuration of wireless networks. This combines to take wireless to a 'whole 'nother level'. |
