Channel capacity in a TDMA system is fixed and indisputable. Each channel carries a finite number of "slots", and you can never accommodate a new caller once each of those slots is filled. Spectral efficiency varies from one technology to another, but computing a precise number is still a contentious issue. For example, GSM provides 8 slots in a channel 200 kHz wide, while iDEN provides 3 slots in a channel only 25 kHz wide. GSM therefore consumes 25 kHz per user, while IS-136 consumes only 8.333 kHz per user. When Direct Connect is used on iDEN, 6 users can be stuffed into a single channel, thus only 4.166 kHz is consumer per user. There is also a new 6:1 interconnect CODEC coming for iDEN which will allow 6 phone users per channel.
One would be sorely tempted to proclaim that iDEN has 3 to 6 times the capacity of GSM. In a one-cell system this is certainly true, but once we start deploying multiple cells and channel reuse the situation becomes more complex. Due to GSM's better error management and frequency hopping the interference of a co-channel site is greatly reduced. This allows frequencies to be reused more frequently without a degradation in the overall quality of the service.
Capacity is measured in "calls per cell per MHz". An GSM system using N=4 reuse (this means you have 4 different sets of frequencies to spread out around town) the figure is 5.0 We get an efficiency value of 6.6 for N=3. Unfortunately I could not find any figures for iDEN systems, but based on similar figures released for the IS-136 system we can expect efficiency values of 6.0 to 10.0.
Computing this figure for CDMA requires that certain assumptions are made. Formulas have been devised, and using very optimistic assumptions CDMA can provide a whopping 45 users per cell per MHz. However, when using more pessimistic (and perhaps more realistic) assumptions, the value is 12. That still gives CDMA an almost 2:1 advantage over GSM, but questionable advantage over a well-implement iDEN system.
One would be sorely tempted to proclaim that iDEN has 3 to 6 times the capacity of GSM. In a one-cell system this is certainly true, but once we start deploying multiple cells and channel reuse the situation becomes more complex. Due to GSM's better error management and frequency hopping the interference of a co-channel site is greatly reduced. This allows frequencies to be reused more frequently without a degradation in the overall quality of the service.
Capacity is measured in "calls per cell per MHz". An GSM system using N=4 reuse (this means you have 4 different sets of frequencies to spread out around town) the figure is 5.0 We get an efficiency value of 6.6 for N=3. Unfortunately I could not find any figures for iDEN systems, but based on similar figures released for the IS-136 system we can expect efficiency values of 6.0 to 10.0.
Computing this figure for CDMA requires that certain assumptions are made. Formulas have been devised, and using very optimistic assumptions CDMA can provide a whopping 45 users per cell per MHz. However, when using more pessimistic (and perhaps more realistic) assumptions, the value is 12. That still gives CDMA an almost 2:1 advantage over GSM, but questionable advantage over a well-implement iDEN system.
Tidak ada komentar:
Posting Komentar