Tag Archives: WCDMA

Inside Qualcomm Snapdragon S4

We have previously looked at the antennas inside a cell phone. Now we look at another important component of a cell phone; the mobile station modem (MSM). One of the most popular MSM in cell phones today is the Qualcomm Snapdragon S4. The details of this MSM are given in the table below.

Qualcomm Snapdragon S4
Qualcomm Snapdragon S4

As can be seen from the above table this small chipset (can easily fit on a fingertip) packs a punch as far as processing power is concerned. It supports a number of wireless standards from GSM/GPRS to LTE and from CDMA 2000 to TD-SCDMA. One of its close competitors is the NVIDIA Tegra 3 which has four ARM Cortex A9 cores (compared to Snapdragon’s two).

Qualcomm Snapdragon – S4

CDMA vs OFDMA

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.

WCDMA Uplink Capacity (N-pole)

The uplink capacity of a WCDMA cell also known as the pole capacity is given as:

N=(W/R)/((Eb/Nt)*v*(1+a))

where

W is the spreading bandwidth fixed at 3.84MHz

R is the radio access bearer bit rate e.g. 12.2kbps

Eb/Nt is the energy per bit to noise power spectral density ratio e.g. 5dB

v is the voice activity factor which depends upon the vocoder, channel coding and actual application e.g. 0.5

a is the other-cell to in-cell interference ratio e.g. 0.65

Using the above values the pole capacity of the WCDMA cell is calculated as 120. In the case of a mobile UE (3km/hr) the required Eb/Nt may be as high as 12dB resulting in pole capacity of 24. The actual capacity is obtained by multiplying the pole capacity with network loading factor which maybe taken as 0.75 in this example resulting in an uplink capacity of 18.