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Your question is properly. In fact, Spread Spectrum techiques are born for Military use, due to the important features: 1) Narrow-Bandwidth Jamming Immunity, 2) Unrelevability (with the classical Scanners) and 3) Undecifrability (the carrier hope pseudocasually in frequeency).
The cost is an high band expansion! Naturally, in military application is not the need to share the resources so this problem is not a problem!
Hi
I found some points when i talked to one of my friend. so I am not shure about my last reply.
paolon1340 is right about military application of spread spectrum systems. but CDMA systems are different and have civil application for example in mobile communications. so pmonon question is actually right.
CDMA certainly increases bandwidth if your consider a single user only. However, in a commercial application, the same bandwidth is available for a lot of users, with each using a different spreading code. So it is not completely justifed to call CDMA bandwidth inefficient.
In addition, as it has been already mentioned, the robustness against narrow band interference is improved
-b
Spread spectrum consimes large BW but as mentioned above its minly for military application (aganist jammers)
CDMA is a multiuser system. Although the user uses a larger BW, all users share the same BW
example for illustration:
if BW of user is X and when spreaded it becomes 5X
lets say CDMA system can afford 10 users (with acceptble MAI) so all the 10 users will use the same 5X which is much better than FDMA in which the 10 users will need 10X
From each user's perspective, the spectral efficiency is rather low; however, in CDMA systems, many user share the same frequency band and the aggregate spectral efficency is boosted.
Nevertheless, there is debate if CDMA is superior to TDMA or FDMA. Someone advocates CDMA, but others believe they are comparable. Based on practical system parameters, the calculation supports the latter. (Ref: Mischa Schwartz, “Mobile Wireless Communications,” Cambridge University Press 2005.)
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