Since both G* and Iridium use LEO satellite systems, what accounts for the "quality of transmission" advantage G* holds over Iridium
The quick summary in three words: Better System Design
The Globalstar system uses a significantly more sophisticated communications system design than Iridium.
Among the issues:
Globalstar uses a Qualcomm variable-rate vocoder with a maximum transmission rate of 9600 bits per second. Iridium uses a vocoder with a fixed transmission rate of 2400 bits per second. Fixed 2400 bps vocoders just don't sound very good.
Lower data rates are good for system capacity. The fewer bits per second that one user consumes, the more phone calls the system can handle. Of course higher rate vocoders in general sound better. This is an example of a system design compromise.
Iridium engineers chose to get a low transmission rate by using a low data rate (2400bps) vocoder that doesn't sound very good. Globalstar engineers chose a variable rate vocoder that provides a low average transmission rate (and therefore retains system capacity) while at the same time is able to use more bits to encode those portions of speech which need more bits in order to sound good.
Globalstar's communication system is a CDMA, rather than TDMA, system, which allows the Globalstar system to do several things that Iridium cannot do. The combined effect of many of these design decisions is increased capacity and better voice quality. Some examples follow.
The Globalstar system uses satellite diversity. That means that the signal from your phone may be relayed back to the gateway not just by one satellite, but by several satellites in parallel, whenever multiple satellites are in view. (In temperate latitudes you most often do have more than one satellite in view.) One implication of this is that when the transmission path to one satellite is interrupted (by a tree, or a building, or whatever) there is no interruption in the received signal.
Of course, when a satellite or beam goes out of view, the handoff from one beam to another or one satellite to another is a soft handoff, meaning that when you are in the transmission region between two beams you use both beams at once for a period of time. Iridium, on the other hand, can do only hard handoffs. Soft handoffs in Globalstar have all the advantages that the soft handoffs in the terrestrial CDMA systems have.
Because the satellites are always moving, the transmission paths between each satellite (actually each antenna beam of each satellite) and each phone are always changing.
The system must adapt in real time. If you don't adapt quickly enough, you may drop some data frames (because you were supplying not enough power), or have reduced system capacity (because you were supplying more power than needed). Dropped data frames can easily be heard as distortion or momentary interruptions in voice.
The use of CDMA signals makes it easy for the Globalstar system to continuously and precisely adjust the strength of the signal being sent to each phone. This allows the system to more carefully optimize the use of power, which is a precious and limited resource in any satellite system. (Because power is a limited resource, how carefully you can allocate power has a significant impact on system capacity.)
Globalstar uses an advanced beam to beam and satellite to satellite handoff mechanism which allows the system to hand off (from beam to beam or satellite to satellite) the forward link (to the phone) and return link (from the phone) signals independently. This allows the Globalstar system to more carefully optimize the transmission paths between satellites and phones. Iridium has no such flexibility.
I could go on, but you probably get the picture.
It is interesting to note that the Globalstar system sounds better and has more capacity, in spite of the fact that the Iridium system uses more complex and more expensive satellites. For example, Iridium satellites have 48 antenna beams each, whereas Globalstar satellites have 16 antenna beams each. A 48 beam satellite is a lot more complex than a 16 beam satellite! Of course many others have written about the fact that the Iridium satellites carry the additional complexity of switching-in-the-sky and satellite-to-satellite crosslinks. |
