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Differentiate between: a.) CDMA and FDMA b.) CDMA and TDMAa.) CDMA and FDMA
CDMA
- Same frequency is used by every user and simultaneous transmission occurs
- Every narrowband signal is multiplied by wideband spreading signal, usually known as codeword
- Every user has a separate pseudo-codeword, i.e., orthogonal to others
- Only the desired codeword is detected by the receivers and others appear as noise
- It is mandatory for the receivers to know about the transmitter’s codeword
FDMA
- When the channel is not in use, it sits simply idle
- Bandwidth of Channel is relatively narrow (30 KHz), known as narrowband system
- Little or no equalization is needed for spreading symbol time
- Analog links are suitable for FDMA
- Framing or synchronization bits are not needed for continuous transmission
- Tight filtering is needed to minimize interference
- Combined with FDD for duplexing
b.) CDMA and TDMA
CDMA
- Power limited system
- While people talking, random noise band playing occurs
- Conversation need to be extracted from the background din
- GP is high when people speak different languages, which is easier to distinguish individual speakers
- It is difficult for distinguishing individuals, when GP is low.
- The system performance will be degraded for every user when the number of users increases.
- Fading would be reduced with wide frequency spectrum
- Need to have separate multipath signals with different delays by “chip” unit.
TDMA
- Receiving or transmission is allowed for only one user in a given slot
- All slots are assigned cyclically
- The transmission is non-continuous
- It is essential to use digital data and modulation
- Data rate overhead is between 20% – 30%
- Overhead tradeoffs are size of data payload and latency
- Multiple users are shared with single carrier frequency
- Handoff is made simpler by using non-continuous transmission
- All slots are assigned on demand
- Due to reduced inter user interference, the power control is less stringent.
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