frequency in Software Creator Code 39 Full ASCII in Software frequency

frequency use none none generator toembed none on none Web service code Channel K Channel 3 Channel 2 Channel 1 time Figure 14.4: none for none Code-Division Multiple Access. was given in (13.

47) as SIR = 3G , (K 1). where K is th e number of users and G 128 is the ratio of spread bandwidth to signal bandwidth. In IS-95 the uplink channel is assigned 1.25 MHz of spectrum.

Thus, the bandwidth of the information signal prior to spreading is Bs 1.25 106 /128 = 9.765 KHz.

Neglecting noise, if the required SINR on a channel is 10 dB, how many users can the CDMA uplink support How many could be supported within the same total bandwidth for an FDMA system Solution: To determine how many users can be supported, we invert the SIR expression to get K 256 3G +1= + 1 = 39.4, SIR 20 1.25 106 = 128, 9.

765 103. and since K must be an integer, the system can support 39 users. In FDMA we have K= so the total system bandwidth of 1.25 MHz can support 128 channels of 9.765 KHz.

This calculation implies that FDMA is three times more ef cient than non-orthogonal CDMA under the standard Gaussian assumption for code cross-correlation (FDMA is even more ef cient under different assumptions about the code cross correlation). But in fact, IS-95 typically supports 64 users on the uplink and downlink by allowing variable voice compression rates depending on interference and channel quality and taking advantage of the fact that interference is not always present (called a voice-activity factor). While this makes CDMA less ef cient than FDMA for a single cell, cellular systems have channel reuse, which can be done more ef ciently in CDMA than in FDMA, as discussed in more detail in 15.

2.. 14.2.4 Space-Division Space-divisio n multiple access (SDMA) uses direction (angle) as another dimension in signal space, which can be channelized and assigned to different users. This is generally done with directional antennas, as shown in Figure 14.5.

Orthogonal channels can only be assigned if the angular separation between users exceeds the angular resolution of the directional antenna. If directionality is obtained using an antenna array, precise angular resolution requires a very large array, which may be impractical for the base station or access point and is certainly infeasible in small user terminals. In practice SDMA is often implemented using sectorized antenna arrays, discussed in 10.

8. In these arrays the 360o angular range is divided into N sectors. There is high directional gain in each sector and little interference between sectors.

TDMA or FDMA is used to channelize users within a sector. For mobile users SDMA must adapt as user angles change or, if directionality is achieved via sectorized antennas, then a user must be handed off to a new sector when it moves out of its original sector..

n an Channel 2 Figure 14.5: Space-Division Multiple Access. 14.2.5 Hybrid Techniques Many systems none for none use a combination of different multiple access schemes to allocate signaling dimensions. OFDMA can be combined with tone hopping to improve frequency diversity [9]. DSSS can be combined with FDMA to break the system bandwidth into subbands.

In this hybrid method different users are assigned to different subbands with their signals spread across the subband bandwidth. Within a subband, the processing gain is smaller than it would be over the entire system bandwidth, so interference and ISI rejection is reduced. However, this technique does not require contiguous spectrum between subbands, and also allows more exibility in spreading user signals over different size subbands depending on their requirements.

Another hybrid method combines DS-CDMA with FH-CDMA so that the carrier frequency of the spread signal is hopped over the available bandwidth. This reduces the near-far effect since the interfering users change on each hop. Alternatively, TDMA and FH can be combined so that a channel with deep fading or interference is only used on periodic hops, so that the fading and interference effects can be mitigated by error correction coding.

This idea is used in the GSM standard, which combines FH with its TDMA scheme to reduce the effect of strong interferers in other cells. 429. There has bee n much discussion, debate, and analysis about the relative performance of different multiple access techniques for current and future wireless systems, e.g. [8, 9, 10, 11, 12, 13].

While analysis and general conclusions can be made for simple system and channel models, it is dif cult to come up with a de nitive answer as to the best technique for a complex multiuser system under a range of typical operating conditions. Moreover, simplifying assumptions must be made to perform a comparative analysis or simulation study, and these assumptions can bias the results in favor of one particular scheme. As with most engineering design questions, the choice of which multiple access technique to use will depend on the system requirements and characteristics along with cost and complexity constraints.

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