Tag Archives: FDMA

CDMA vs OFDMA

LTE, WCDMA, WiMAX, , , , ,
Property CDMA OFDMA
1. Channel bandwidth Full system bandwidth Variable system bandwidth to accommodate users with different data rates, 1.25, 2.50, 5.00, 10.00, 15.00 and 20.00 MHz, actual transmission bandwidth is a bit lower than this
2. Frequency-selective scheduling Not possible A key advantage of OFDMA, although it requires accurate real-time feedback of channel conditions from receiver to transmitter
3. Symbol period Very short—inverse of the system bandwidth Very long—defined by subcarrier spacing and independent of system bandwidth
4. Equalization Complicated time domain equalization Simple frequency domain equalization
5. Resistance to mulitpath Rake receiver can combine various multipath components Highly resistant to multipath due to insertion of cyclic prefix (CP)
6. Suitability for MIMO MIMO is not suited to a wideband frequency selective channel MIMO is suited to the independent narrowband flat fading channels that the subcarriers provide
7. Resistance to narrowband interference Resistant to narrow band interference Some subcarriers to be affected by narrowband interference
8. Separation of users Scrambling and orthogonal spreading codes Frequency and time although scrambling and spreading can be added as well

Reference: Agilent 3GPP Long Term Evolution System Overview, Product Development and Test Challenges Application Note.

CDMA vs FDMA Capacity (Mbps)

Capacity, WCDMA, ,

We saw previously that the channel capacity of a WCDMA system is severely limited by the Multiple Access Interference (MAI). Now let us consider the case where the 5MHz channel is divided equally among 20 users such that each user has a bandwidth of 250kHz. Keeping the transmit power for the narrow band signal the same as the wideband signal the signal to noise ratio would improve tremendously (since there is no MAI and the noise power is reduced by a factor of 20). Thus each narrowband channel would have a capacity of 2.99Mbps giving a combined capacity of 59.83Mbps for the 5MHz channel. The main drawback of an FDMA system is that the capacity of each narrowband channel is fixed at 2.99Mbps. However the capacity of a WCDMA user is dependent upon the number of users varying from 38Mbps for a single user to 5Mbps for two users (10Mbps combined) to 370kbps for 20 users (7.4Mbps combined). The higher capacity of FDMA when combined with dynamic allocation of channels builds the case for OFDMA based techniques.

Note: For the narrowband case the signal power is -80dBm and noise power is -116dBm resulting in an SNR of 36dB.