In SDMA, a cell is divided into narrow sectors each being served by a directional antenna called as spot beam antennas. These antennas may operate on the same set of frequencies since they are spatially separated. i.e. the underlying principle by which the spot beam smart antennas serve the users can be either TDMA, FDMA or CDMA.

SDMA techniques use smart antennas to establish such narrow beamwidth. A smart antenna is an antenna with a digital signal processing capability to transmit and receive in an adaptive and spatially sensitive manner. Some examples of smart antennas are the following:

1. Sectored antennas: The simplest form of smart antenna is the sectored antenna. This antenna provides significant capacity gains simply by dividing the service area of each Base station into more than three angular sections with a significant amount of isolation between them.
2. Switched beam antennas: These are the next type of smart antennas. These antennas have a number of fixed beams that cover 360 degree and are typically narrower than sectored antennas. The receiver selects the beam that provides the best signal and interference reduction.
3. Adaptive antennas: The most advanced example is the adaptive antenna. This antenna dynamically adjusts its pattern to minimize the effects of noise, interference and multipath. With adaptive antenna , there is one beam for each user.

There are many advantages of smart antennas in Mobile applications,

• Greater range: These antennas are highly directional. Hence they have larger gains and provide a stronger signal strength for the same transmit power.
• Better building penetration: Greater signal strength and increased transmitter gain enhances in-building penetration.
• Less sensitive to power control errors: SDMA provides better isolation to different user signals. Hence, this advantage.
• More sensitive to traffic hotspots: In areas such as airports and railways, user density can become high and directional antennas allow one or a small number of base stations to serve these areas better.

SDMA improves system capacity by allowing greater spectrum reuse, through

i) Minimization of the effects of interference and,

ii) Increasing signal strength for both the user terminal and the Base Station.

Till now four different multiple access have been discussed which are FDMA, TDMA, CDMA and SDMA. Practical systems are usually hybrid of two or more of these multiple access techniques. The hybrid approach is applied due to the following reasons,

• To provide a reasonable growth strategy
• To reduce the complexity of the overall system
• To be backward compatible with the existing system

Final adoption of the multiple access strategy will depend upon many factors, such as economic considerations, number of users, resources available etc.. For example, the use of CDMA will not be justified in a single or few user system.

Comparison of FDMA, TDMA, CDMA, SSMA

A comparison on various multiple access techniques is presented in following section with respect to various parameters.

1. Modulation:

   • FDMA: Relies on bandwidth efficient modulation. A higher order modulation is required to achieve higher throughput.
   • TDMA: Relies on bandwidth efficient modulation. A higher order modulation is required to achieve higher throughput.
   • CDMA: A simple modulation like QPSK is used.
   • SDMA: The choice of modulation strategy and the use of SDMA are independent.

2. Diversity:

   • FDMA: Multiple transmitters, receivers or both are needed to obtain diversity. This adds to the system cost.
   • TDMA: Same as FDMA. However, can use frequency hopping to obtain diversity.
   • CDMA: Rake receivers are used to implement frequency diversity.
   • SDMA: Orthogonal coding improves diversity with multiple transmit antennas.

3. User terminal complexity:

   • FDMA: Simple
   • TDMA: Medium complexity
   • CDMA: Core complexity
   • SDMA: Additional complexity due to smart antennas

4. Handover:

   • FDMA: Hard
   • TDMA: Hard
   • CDMA: Soft
   • SDMA: Potentially soft

5. System complexity:

   • FDMA: Large number of simple components
   • TDMA: Reduced number of channel units
   • CDMA: Large number of complex interacting components
   • SDMA: Additional complexity related to antennas

6. Fading channel sensitivity:

   • FDMA: Typically narrowband, hence undergoes flat fading, no diversity is required and the signal is simple to track
   • TDMA: Typically wideband, hence may undergo frequency selective fading and will need equalizers.
   • CDMA: Frequency selective fading is observed. Special receivers called RAKE receivers are used.
   • SDMA: Reduced multipath

7. Bandwidth efficiency:

   • FDMA: For single cell systems, more bandwidth efficient than CDMA. Efficiency is based on modulation and channel spacing.Once frequency reuse and mofulation is fixed, the throughput becomes fixed and cannot be changed.
   • TDMA: Same as FDMA
   • CDMA: More multicell systems, more bandwidth efficient than other techniques.
   • SDMA: Depends on antenna resolution

8. Synchronization:

   • FDMA: Symbol timing is used to synchronize MS to BS.
   • TDMA: Symbol timing and slot timing are used.
   • CDMA: Chip timing is used for synchronization.
   • SDMA: SDMA has additional synchronization-like requirement related to location of the MS.

9. Flexibilty:

   • FDMA: Least flexible, fixed data rate
   • TDMA: Data rate can be varied in discrete steps.
   • CDMA: Data rate will depend on the length of the code and chip rate. Hence, can provide a variety of data rates without affecting signal propagation.
   • SDMA: Totally depends on the underlying principle used under SDMA.

10. Voice and data integration:

    • FDMA: Possible, but may require revisions to the system.
    • TDMA: Makes use of voice inactivity periods to transmit data. Voice and data transmission is straightforward using multiple slots
    • CDMA: Can easily integrate voice and data ith multicode transmission.
    • SDMA: Totally depends on the underlying principle used under SDMA.

11. Evolution: Means evolving or growing from a small system to a large system as number of users increase.

    • FDMA: Easiest with FDMA approach.
    • TDMA: The system can easily evolve through the addition of more TDMA channels with an FDMA overlay.
    • CDMA: Requires a large initial bandwidth. Hence, there is large startup cost everytime the system is evolved.
    • SDMA: This approach makes use of smart antennas only on Base Station side. So evolution can be smooth, since any changes brought on BS do not affect the mobile terminals.