Category 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.

CDMA vs FDMA Capacity (Mbps)

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.

 

WCDMA Capacity (Mbps)

The capacity of any wireless communication channel is given by the well known Shannon Capacity Theorem:

C=B*log2(1+SNR)

or

C=B*log2(1+P/(NoB))

where C is the capacity of the channel in bits/sec, P in the noise power in Watts, No is the noise power spectral density in Watts/Hz and B is the channel bandwidth in Hz. It is obvious that the channel capacity increases with increase in signal power. However, the relationship with bandwidth is a bit complicated. The increase in bandwidth decreases the SNR (keeping the signal power and noise power spectral density same). Therefore the capacity does not increase linearly with bandwidth.

Single User Capacity

It is assumed that the signal power is -80dBm and the noise power spectral density is -170dBm/Hz.

Let us now consider the case of a WCDMA system where the noise component consists of the AWGN noise as well Multiple Access Interference (MAI). The above capacity formula is then modified as:

C=B*log2(1+SINR)

or

C=B*log2(1+P/(NoB+(U-1)*P))

where U is the number of users.

WCDMA Capacity

It is assumed that WCDMA system has a bandwidth of 5MHz, signal power of -80dBm and noise power spectral density of -170dBm/Hz. It is observed that the capacity of the WCDMA system decreases exponentially with increase in number of users (all users are assumed to have equal power at the receiver). The capacity of a 5MHz channel drops from about 38Mbps for a single user to about 370kbps for 20 users (the combined capacity of 20 users is 7.4Mbps which is much lesser than the capacity of a single user in AWGN only).

Note:

1. In some references the signal power is adjusted by the processing gain but I think that this is not correct for capacity calculations because in that case the bandwidth should be adjusted as well.

2. The capacity in fading environments would be less than AWGN capacity. However, multi-antenna systems allow for higher capacities.