radiowaves

FDM:

– frequency division multiplexing

– it is an analog multiplexing technique that combines low bandwidth analog channels into one large bandwidth analog channel

TDM:

– time division multiplexing

– digital multiplexing technique for combining several low-rate digital channels into one high-rate digital channel

Duplexing means sending and receiving simultaneously.

FDD:

– two bands of frequencies for each user

-forward band, BS to mobile and reverse band mobile to BS

TDD:

-use different time slots for forward and reverse link

-forward time slot from BS to mobile and reverse time slot from mobile to BS

Multiple Access:

– allow multiple users to share a common resource in a managed and effective way

TDMA, FDMA, CDMA divide channel into smaller pieces ( time slots, frequency and code)

Spread Spectrum:

– spread the energy of an information bit over a bandwidth which is much larger than the information bit rate.

Direct Sequence Spread Spectrum:

– each bit in original signal is represented by multiple bits in the transmitted signal using a spreading code called chip

– the chip rate is n times greater than the data rate

source signal —>modulator———>spread signal

chip generator ———>modulator

Frequency Hopping Spread Spectrum:

– uses M different carrier frequencies that are modulated by a single source signal

– a pseudorandom code generator creates k-bit pattern for every hopping period (Tc)

– the frequency table uses the pattern to find the frequency to be used for the particular hopping period, and passes it to the frequency synthesizer

source signal —–> modulator———->spread signal

pseudorandom code generator———–>modulator

– slow frequency hopping occurs when the time to hop a new frequency ┬áis greater than or equal to the duration of a signal element, Tc>= Ts

– fast frequency hopping occurs when the time to hop a new frequency Tc is less than the duration of a signal element, Tc < Ts

CDMA:

– Advantage: high immunity against interference and jamming, doesnot require synchronization between sending and receiving stations

– disadvantage: system is complicated, performance degrade with increase in no. of users, due to limited no. of orthogonal codes.