How does CDMA work? • Each bit (zero or one) is spread into N smaller pulses/chips (a series of zeros and ones). • The receiver which knows the spread pattern (code) will be able to recover the original bits. • Other receivers which do not know the code will only get small ripples (noise).
Advantages of CDMA over TDMA • Can easily handle both voice and data. • May have larger capacity when data are transmitted. • Is more robust against frequency selective fading and ISI due to multipath propagation. • Uses soft handoff that improves quality. • Uses elaborated power control that saves battery life for MS. • Provides better privacy.
Disadvantages of CDMA • DSSS is more complex than techniques used in TDMA/FDMA. • Power control in CDMA is more complicated. • The bandwidth obtained by each user is limited due to spread spectrum. (The signal will occupy a large bandwidth but the actual spectrum is only a fraction of it. It is fine for voice and low data speed applications but not for 4G.)
Handle both voice and data • More flexible format. Multimedia applications require both voice and data support, but data and voice communications have very different characteristics. Voice can tolerate errors, but not delays nor interruptions, and data are the opposite. • TDMA/FDMA systems were designed for voice, each user is given a fixed channel, which is good for voice, but not efficient for data. • With CDMA users are separated by codes. Data transmission does not occupy a fixed bandwidth, as in TDMA/FDMA. Both voice and data can be transmitted efficiently.
Larger capacity • Potentially larger capacity (more users can communicate simultaneously) • In TDMA/FDMA, each user needs a separate channel. Thus the capacity is determined by the number of channels, i.e., the number of frequencies and time slots available. • In CDMA, users are separated by different codes. The number of available codes in CDMA far exceeds the number of cannels in TDMA/FDMA. Thus it has a potential to handle a large number of users. • In reality the capacity is restricted by the interference (noise) generated by users. Increasing the number of users will gradually reduce the quality (larger noise). (You can compare CDMA with 802.11). • If users don’t use the medium all the time (e.g., they are just reading e-mail), CDMA will allow much more users to communicate simultaneously. In other words, CDMA will use the resource (the radio spectrum) more efficiently.
Robust against fading • Provides larger spread spectrum, thus more robust against noise bursts and multipath frequency selective fading • GSM bandwidth = 200 kHz • IS-95 bandwidth = 1.25 MHz • W-CDMA (3G) bandwidth = 10MHz
Use of RAKE receiver • RAKE receiver can use the signals from different paths to get one stronger signal. Signals from different paths arrive at the receiver at different time instances. Longer paths create longer delays. • The RAKE receiver is designed to combine those signals to get one strong signal.
RAKE receiver • Multipath reception in CDMA • Chip rate: 1.25 Mcps, symbol rate: 4,800 Sps • Can resolve multipath components 1/1.25 Mcps = 800 ms apart. • A multipath spread of up to 1/4800 bps = 2.08 ms cannot cause ISI.
Soft handoff • In TDMA when a MS moves from the coverage area of one BS to that of another, handoff takes place, and the user may experience bad reception and hear several clicks. • In CDMA, since two adjacent cells may use the same frequency, a MS at the edge of the coverage area of one BS may communicate simultaneously with two or more BSs. It selects the BS that provides the best signal. The transition from one BS to another (handoff) is not abrupt, as in TDMA, and provides better quality.
Better power control • CDMA’s capacity is determined by the total noise generated by users. • Power control is essential because if there were no power control the MS that were very close to the BS would generate very strong signal and thus very large interference. • Good power control reduces the power emitted by an MS that is close to the BS. Thus the noise levels generated by all MSs will be comparable. • This in turn reduces power consumption of an MS, and low power consumption is an important feature for mobile devices.