A CDMA system can be either code limited or interference limited.
For an interference limited system, every user has a code, but only uses it when active, this is referred to as a soft capacity system . The more users active in the system, the more codes that are used. However as more codes are used the signal to interference (S/I) ratio will drop and the bit error rate (BER) will go up for all users.
CDMA requires tight power control as it suffers from far-near effect. In other words, a user close to the base station transmitting with the same power as a user farther away will drown the latter’s signal. All signals must have more or less equal power at the receiver.
Rake receivers can be used to improve signal reception. Time delayed versions (a chip or more delayed) of the signal (multipath signals) can be collected and used to make bit level decisions.
Soft handoffs can be used. Mobiles can switch base stations without switching carriers. Two base stations receive the mobile signal and the mobile is receiving from two base stations (one of the rake receivers is used to listen to other signals).
Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal using a special code
coexistence of several signals without dynamic coordination
Techniques: Direct Sequence, Frequency Hopping
Operation of SS detection at receiver interference spread signal signal spread interference f f power power P f i) P f ii) sender P f iii) P f iv) user signal broadband interference narrowband interference
SS and Fading frequency channel quality 1 2 3 4 5 6 narrow band signal guard space 2 2 2 2 2 frequency channel quality 1 spread spectrum narrowband channels spread spectrum channels
if a node moves without changing its IP address it will be unable to receive its packets,
if a node changes its IP address it will have to terminate and restart its ongoing connections everytime it moves to a new network area (new network prefix).
Mobile IP is a routing protocol with a very specific purpose.
Mobile IP is a network layer solution to node mobility in the Internet.
Mobile IP is not a complete solution to mobility, changes to the transport protocols need to be made for a better solution (i.e., the transport layers are unaware of the mobile node’s point of attachment and it might be useful if, e.g., TCP knew that a wireless link was being used!).
typically wrong in wireless networks, here we often have packet loss due to transmission errors
furthermore, mobility itself can cause packet loss, if e.g. a mobile node roams from one access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit to the old access point and forwarding from old to new access point is not possible for some reason
The performance of unmodified (i.e., as is) TCP degrades severely
note that TCP cannot be changed fundamentally due to the large base of installation in the fixed network, TCP for mobility has to remain compatible
the basic TCP mechanisms keep the whole Internet together